How can I concatenate two arrays in Java

Question

I need to concatenate two String arrays in Java   void f String   first  String   second        String   both           What is the easiest way to do this

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You could try converting it into a Arraylist and use the addAll method then convert back to an array   List list   new ArrayList Arrays asList first      list addAll Arrays asList second      String   both   list toArray

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Using only Javas own API    String   join String      arrays         calculate size of target array   int size   0    for  String   array   arrays        size    array length            create list of appropriate size   java util List list   new java util ArrayList size         add arrays   for  String   array   arrays        list addAll java util Arrays asList array              create and return final array   return list toArray new String size        Now  this code ist not the most efficient  but it relies only on standard java classes and is easy to understand  It works for any number of String    even zero arrays

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Using only Javas own API    String   join String      arrays         calculate size of target array   int size   0    for  String   array   arrays        size    array length            create list of appropriate size   java util List list   new java util ArrayList size         add arrays   for  String   array   arrays        list addAll java util Arrays asList array              create and return final array   return list toArray new String size        Now  this code ist not the most efficient  but it relies only on standard java classes and is easy to understand  It works for any number of String    even zero arrays

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How about simply      public static class Array        public static  lt T gt  T   concat T      arrays            ArrayList lt T gt  al   new ArrayList lt T gt             for  T   one   arrays              Collections addAll al  one           return  T    al toArray arrays 0  clone               And just do Array concat arr1  arr2   As long as arr1 and arr2 are of the same type  this will give you another array of the same type containing both arrays

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public static String   toArray String      object       List lt String gt  list new ArrayList lt  gt         for  String   i   object            list addAll Arrays asList i              return list toArray new String list size

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public String   concat String      arrays        int length   0      for  String   array   arrays            length    array length            String   result   new String length       int destPos   0      for  String   array   arrays            System arraycopy array  0  result  destPos  array length           destPos    array length            return result

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Here s an adaptation of silvertab s solution  with generics retrofitted   static  lt T gt  T   concat T   a  T   b        final int alen   a length      final int blen   b length      final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      NOTE  See Joachim s answer for a Java 6 solution  Not only does it eliminate the warning  it s also shorter  more efficient and easier to read

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Here s an adaptation of silvertab s solution  with generics retrofitted   static  lt T gt  T   concat T   a  T   b        final int alen   a length      final int blen   b length      final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      NOTE  See Joachim s answer for a Java 6 solution  Not only does it eliminate the warning  it s also shorter  more efficient and easier to read

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The easiest way i could find is as following      List allFiltersList   Arrays asList regularFilters   allFiltersList addAll Arrays asList preFiltersArray    Filter   mergedFilterArray    Filter    allFiltersList toArray

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An easy  but inefficient  way to do this  generics not included    ArrayList baseArray   new ArrayList Arrays asList array1    baseArray addAll Arrays asList array2    String concatenated      String     baseArray toArray new String baseArray size

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Here is what worked for me   String   data null  String   data2 null  ArrayList lt String gt  data1   new ArrayList lt String gt     for int i 0  i lt 2 i         data2   input readLine   split          data1 addAll Arrays asList data2       data  data1 toArray new String data1 size

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The Functional Java library has an array wrapper class that equips arrays with handy methods like concatenation   import static fj data Array array       and then  Array lt String gt  both   array first  append array second      To get the unwrapped array back out  call  String   s   both array

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I tested below code and worked ok  Also I m using library  org apache commons lang ArrayUtils  public void testConcatArrayString        String   a   null      String   b   null      String   c   null      a   new String     1   2   3   4   5        b   new String     A   B   C   D   E         c    String    ArrayUtils addAll a  b       if c  null           for int i 0  i lt c length  i                 System out println  c      i 1             c i                        Regards

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A generic static version that uses the high performing System arraycopy without requiring a  SuppressWarnings annotation   public static  lt T gt  T   arrayConcat T   a  T   b        T   both   Arrays copyOf a  a length   b length       System arraycopy b  0  both  a length  b length       return both

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Non Java 8 solution   public static int   combineArrays int   a  int   b            int   c   new int a length   b length            for  int i   0  i  lt  a length  i                  c i    a i                      for  int j   0  k   a length  j  lt  b length  j    k                  c k    b j                      return c

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Or with the beloved Guava   String   both   ObjectArrays concat first  second  String class     Also  there are versions for primitive arrays    Booleans concat first  second  Bytes concat first  second  Chars concat first  second  Doubles concat first  second  Shorts concat first  second  Ints concat first  second  Longs concat first  second  Floats concat first  second

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You can try this   public static Object   addTwoArray Object   objArr1  Object   objArr2       int arr1Length   objArr1  null  amp  amp  objArr1 length gt 0 objArr1 length 0      int arr2Length   objArr2  null  amp  amp  objArr2 length gt 0 objArr2 length 0      Object   resutlentArray   new Object arr1Length arr2Length        for int i 0 j 0 i lt resutlentArray length i             if i 1 lt  arr1Length               resutlentArray i  objArr1 i            else              resutlentArray i  objArr2 j               j                         return resutlentArray      U can type cast your array

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This should be one-liner   public String    concatenate  final String array1    final String array2          return Stream concat Stream of array1   Stream of array2   toArray String    new

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The Functional Java library has an array wrapper class that equips arrays with handy methods like concatenation   import static fj data Array array       and then  Array lt String gt  both   array first  append array second      To get the unwrapped array back out  call  String   s   both array

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This one works only with int but the idea is generic  public static int   junta int   v  int   w     int   junta   new int v length   w length    for  int i   0  i  lt  v length  i                      junta i    v i      for  int j   v length  j  lt  junta length  j          junta j    w j - v length

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The Functional Java library has an array wrapper class that equips arrays with handy methods like concatenation   import static fj data Array array       and then  Array lt String gt  both   array first  append array second      To get the unwrapped array back out  call  String   s   both array

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Look at this elegant solution  if you need other type than char  change it    private static void concatArrays char   destination  char      sources        int currPos   0      for  char   source   sources            int length   source length          System arraycopy source  0  destination  currPos  length           currPos    length            You can concatenate a every count of arrays

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I found a one-line solution from the good old Apache Commons Lang library  ArrayUtils addAll T    T      Code   String   both   ArrayUtils addAll first  second

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Another way to think about the question  To concatenate two or more arrays  one have to do is to list all elements of each arrays  and then build a new array  This sounds like create a List lt T gt  and then calls toArray on it  Some other answers uses ArrayList  and that s fine  But how about implement our own  It is not hard   private static  lt T gt  T   addAll final T   f  final T   o       return new AbstractList lt T gt               Override         public T get int i                return i gt  f length   o i - f length    f i                       Override         public int size                 return f length   o length                   toArray f       I believe the above is equivalent to solutions that uses System arraycopy  However I think this one has its own beauty

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You can try this method which concatenates multiple arrays   public static  lt T gt  T   concatMultipleArrays T      arrays       int length   0     for  T   array   arrays             length    array length          T   result    T    Array newInstance arrays getClass   getComponentType    length        length   0     for  int i   0  i  lt  arrays length  i               System arraycopy arrays i   0  result  length  arrays i  length         length    arrays i  length           return result

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How about    public String   combineArray  String       strings        List lt String gt  tmpList   new ArrayList lt String gt         for  int i   0  i  lt  strings length  i            tmpList addAll Arrays asList strings i         return tmpList toArray new String tmpList size

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I ve recently fought problems with excessive memory rotation  If a and or b are known to be commonly empty  here is another adaption of silvertab s code  generified too    private static  lt T gt  T   concatOrReturnSame T   a  T   b        final int alen   a length      final int blen   b length      if  alen    0            return b            if  blen    0            return a            final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      Edit  A previous version of this post stated that array re-usage like this shall be clearly documented  As Maarten points out in the comments it would in general be better to just remove the if statements  thus voiding the need for having documentation  But then again  those if statements were the whole point of this particular optimization in the first place  I ll leave this answer here  but be wary

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You can append the two arrays in two lines of code   String   both   Arrays copyOf first  first length   second length   System arraycopy second  0  both  first length  second length     This is a fast and efficient solution and will work for primitive types as well as the two methods involved are overloaded   You should avoid solutions involving ArrayLists  streams  etc as these will need to allocate temporary memory for no useful purpose   You should avoid for loops for large arrays as these are not efficient  The built in methods use block-copy functions that are extremely fast

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Import java util     String array1       bla   bla    String array2       bla   bla     ArrayList lt String gt  tempArray   new ArrayList lt String gt  Arrays asList array1    tempArray addAll Arrays asList array2    String array3     films toArray new String 1       size will be overwritten if needed   You could replace String by a Type Class of your liking  Im sure this can be made shorter and better  but it works and im to lazy to sort it out further

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I think the best solution with generics would be      This for non primitive types    public static  lt T gt  T   concatenate  T      elements         T   C   null      for  T   element  elements            if  element  null  continue          if  C  null  C    T    Array newInstance element getClass   getComponentType    element length           else C   resizeArray C  C length element length            System arraycopy element  0  C  C length-element length  element length              return C            as far as i know  primitive types do not accept generics     http   stackoverflow com questions 2721546 why-dont-java-generics-support-primitive-types    for primitive types we could do something like this        public static int   concatenate  int      elements       int   C   null      for  int   element  elements            if  element  null  continue          if  C  null  C   new int element length           else C   resizeArray C  C length element length            System arraycopy element  0  C  C length-element length  element length             return C     private static  lt T gt  T resizeArray  T array  int newSize        int oldSize               java lang reflect Array getLength array       Class elementType               array getClass   getComponentType        Object newArray               java lang reflect Array newInstance                      elementType  newSize       int preserveLength   Math min oldSize  newSize       if  preserveLength  gt  0          System arraycopy array  0                  newArray  0  preserveLength       return  T  newArray

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If you use this way so you no need to import any third party class   If you want concatenate String  Sample code for concate two String Array  public static String   combineString String   first  String   second           int length   first length   second length          String   result   new String length           System arraycopy first  0  result  0  first length           System arraycopy second  0  result  first length  second length           return result          If you want concatenate Int  Sample code for concate two Integer Array  public static int   combineInt int   a  int   b           int length   a length   b length          int   result   new int length           System arraycopy a  0  result  0  a length           System arraycopy b  0  result  a length  b length           return result          Here is Main method      public static void main String   args                 String    first     a    b    c                String    second     d    e                 String    joined   combineString first  second               System out println  concatenated String array       Arrays toString joined                 int   array1    101 102 103 104               int   array2    105 106 107 108               int   concatenateInt   combineInt array1  array2                System out println  concatenated Int array       Arrays toString concatenateInt                         We can use this way also

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You could try converting it into a Arraylist and use the addAll method then convert back to an array   List list   new ArrayList Arrays asList first      list addAll Arrays asList second      String   both   list toArray

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The easiest way i could find is as following      List allFiltersList   Arrays asList regularFilters   allFiltersList addAll Arrays asList preFiltersArray    Filter   mergedFilterArray    Filter    allFiltersList toArray

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concatenates a series of arrays compact  fast and type-safe with lambda   SafeVarargs public static  lt T gt  T   concat  T      arrays       return  Stream of  arrays   reduce    arr1  arr2   - gt          T   rslt   Arrays copyOf  arr1  arr1 length   arr2 length          System arraycopy  arr2  0  rslt  arr1 length  arr2 length          return  rslt            orElse  null           returns null when called without argument  eg  example with 3 arrays   String   a   new String      a    b    c    d     String   b   new String      e    f    g    h     String   c   new String      i    j    k    l      concat  a  b  c         a  b  c  d  e  f  g  h  i  j  k  l        probably the only generic and type-safe way      adapted   Number   array1     1  2  3    Number   array2     4 0  5 0  6 0    Number   array   concat  array1  array2         1  2  3  4 0  5 0  6 0

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Another way with Java8 using Stream    public String   concatString String   a  String   b        Stream lt String gt  streamA   Arrays stream a       Stream lt String gt  streamB   Arrays stream b       return Stream concat streamA  streamB  toArray String    new

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ArrayList lt String gt  both   new ArrayList Arrays asList first    both addAll Arrays asList second     both toArray new String 0

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You can try this   public static Object   addTwoArray Object   objArr1  Object   objArr2       int arr1Length   objArr1  null  amp  amp  objArr1 length gt 0 objArr1 length 0      int arr2Length   objArr2  null  amp  amp  objArr2 length gt 0 objArr2 length 0      Object   resutlentArray   new Object arr1Length arr2Length        for int i 0 j 0 i lt resutlentArray length i             if i 1 lt  arr1Length               resutlentArray i  objArr1 i            else              resutlentArray i  objArr2 j               j                         return resutlentArray      U can type cast your array

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This is a converted function for a String array   public String   mergeArrays String   mainArray  String   addArray        String   finalArray   new String mainArray length   addArray length       System arraycopy mainArray  0  finalArray  0  mainArray length       System arraycopy addArray  0  finalArray  mainArray length  addArray length        return finalArray

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This works  but you need to insert your own error checking   public class StringConcatenate        public static void main String   args               Create two arrays to concatenate and one array to hold both         String   arr1   new String    s   t   r   i   n   g            String   arr2   new String    s   t   r   i   n   g            String   arrBoth   new String arr1 length arr2 length               Copy elements from first array into first part of new array         for int i   0  i  lt  arr1 length  i                 arrBoth i    arr1 i                         Copy elements from second array into last part of new array         for int j   arr1 length j  lt  arrBoth length j                 arrBoth j    arr2 j-arr1 length                         Print result         for int k   0  k  lt  arrBoth length  k                 System out print arrBoth k                          Additional line to make your terminal look better at completion          System out println               It s probably not the most efficient  but it doesn t rely on anything other than Java s own API

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void f String   first  String   second        String   both   new String first length second length       for int i 0 i lt first length i            both i    first i       for int i 0 i lt second length i            both first length   i    second i       This one works without knowledge of any other classes libraries etc  It works for any data type  Just replace String with anything like int double or char  It works quite efficiently

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Here is the code by AbacusUtil   String   a     a    b    c    String   b     1    2    3    String   c   N concat a  b      c     a    b    c    1    2    3       N concat      is null-safety  a   null  c   N concat a  b      c     1    2    3

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If you d like to work with ArrayLists in the solution  you can try this   public final String    f final String    first  final String    second           Assuming non-null for brevity      final ArrayList lt String gt  resultList   new ArrayList lt String gt  Arrays asList first        resultList addAll new ArrayList lt String gt  Arrays asList second         return resultList toArray new String  resultList size

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I found I had to deal with the case where the arrays can be null     private double   concat   double  a double  b       if  a    null  return b      if  b    null  return a      double   r   new double a length b length       System arraycopy a  0  r  0  a length       System arraycopy b  0  r  a length  b length       return r     private double   copyRest  double  a  int start       if  a    null  return null      if  start  gt  a length return null      double  r   new double a length-start       System arraycopy a start r 0 a length-start        return r

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Here is what worked for me   String   data null  String   data2 null  ArrayList lt String gt  data1   new ArrayList lt String gt     for int i 0  i lt 2 i         data2   input readLine   split          data1 addAll Arrays asList data2       data  data1 toArray new String data1 size

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Or with the beloved Guava   String   both   ObjectArrays concat first  second  String class     Also  there are versions for primitive arrays    Booleans concat first  second  Bytes concat first  second  Chars concat first  second  Doubles concat first  second  Shorts concat first  second  Ints concat first  second  Longs concat first  second  Floats concat first  second

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This one works only with int but the idea is generic  public static int   junta int   v  int   w     int   junta   new int v length   w length    for  int i   0  i  lt  v length  i                      junta i    v i      for  int j   v length  j  lt  junta length  j          junta j    w j - v length

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This works  but you need to insert your own error checking   public class StringConcatenate        public static void main String   args               Create two arrays to concatenate and one array to hold both         String   arr1   new String    s   t   r   i   n   g            String   arr2   new String    s   t   r   i   n   g            String   arrBoth   new String arr1 length arr2 length               Copy elements from first array into first part of new array         for int i   0  i  lt  arr1 length  i                 arrBoth i    arr1 i                         Copy elements from second array into last part of new array         for int j   arr1 length j  lt  arrBoth length j                 arrBoth j    arr2 j-arr1 length                         Print result         for int k   0  k  lt  arrBoth length  k                 System out print arrBoth k                          Additional line to make your terminal look better at completion          System out println               It s probably not the most efficient  but it doesn t rely on anything other than Java s own API

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Wow  lot of complex answers here including some simple ones that depend on external dependencies  how about doing it like this   String    arg1   new String  a   b   c    String    arg2   new String  x   y   z     ArrayList lt String gt  temp   new ArrayList lt String gt     temp addAll Arrays asList arg1    temp addAll Arrays asList arg2    String    concatedArgs   temp toArray new String arg1 length arg2 length

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I think the best solution with generics would be      This for non primitive types    public static  lt T gt  T   concatenate  T      elements         T   C   null      for  T   element  elements            if  element  null  continue          if  C  null  C    T    Array newInstance element getClass   getComponentType    element length           else C   resizeArray C  C length element length            System arraycopy element  0  C  C length-element length  element length              return C            as far as i know  primitive types do not accept generics     http   stackoverflow com questions 2721546 why-dont-java-generics-support-primitive-types    for primitive types we could do something like this        public static int   concatenate  int      elements       int   C   null      for  int   element  elements            if  element  null  continue          if  C  null  C   new int element length           else C   resizeArray C  C length element length            System arraycopy element  0  C  C length-element length  element length             return C     private static  lt T gt  T resizeArray  T array  int newSize        int oldSize               java lang reflect Array getLength array       Class elementType               array getClass   getComponentType        Object newArray               java lang reflect Array newInstance                      elementType  newSize       int preserveLength   Math min oldSize  newSize       if  preserveLength  gt  0          System arraycopy array  0                  newArray  0  preserveLength       return  T  newArray

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Here s an adaptation of silvertab s solution  with generics retrofitted   static  lt T gt  T   concat T   a  T   b        final int alen   a length      final int blen   b length      final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      NOTE  See Joachim s answer for a Java 6 solution  Not only does it eliminate the warning  it s also shorter  more efficient and easier to read

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Here s my slightly improved version of Joachim Sauer s concatAll  It can work on Java 5 or 6  using Java 6 s System arraycopy if it s available at runtime  This method  IMHO  is perfect for Android  as it work on Android  lt 9  which doesn t have System arraycopy  but will use the faster method if possible     public static  lt T gt  T   concatAll T   first  T      rest        int totalLength   first length      for  T   array   rest          totalLength    array length            T   result      try         Method arraysCopyOf   Arrays class getMethod  copyOf   Object   class  int class         result    T    arraysCopyOf invoke null  first  totalLength         catch  Exception e           Java 6   Android  gt   9 way didn t work  so use the  traditional  approach       result    T    java lang reflect Array newInstance first getClass   getComponentType    totalLength         System arraycopy first  0  result  0  first length             int offset   first length      for  T   array   rest          System arraycopy array  0  result  offset  array length         offset    array length            return result

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I use next method to concatenate any number of arrays of the same type using java 8  public static  lt G gt  G   concatenate IntFunction lt G   gt  generator  G       arrays        int len   arrays length      if  len    0            return generator apply 0         else if  len    1            return arrays 0             int pos   0      Stream lt G gt  result   Stream concat Arrays stream arrays pos    Arrays stream arrays   pos         while  pos  lt  len - 1            result   Stream concat result  Arrays stream arrays   pos               return result toArray generator      usage   concatenate String    new  new String    quot one quot    new String    quot two quot    new String    quot three quot      or  concatenate Integer    new  new Integer   1   new Integer   2   new Integer   3

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In Haskell you can do something like that  a  b  c      d  e  to get  a  b  c  d  e   These are Haskell lists concatenated but that d very nice to see a similar operator in Java for arrays  Don t you think so   That s elegant  simple  generic and it s not that difficult to implement   If you want to  I suggest you to have a look at Alexander Hristov s work in his Hacking the OpenJDK compiler  He explains how to modify javac source to create a new operator  His example consists in defining a      operator where i    j   Math pow i  j   One could take that example to implement an operator that concatenates two arrays of same type   Once you do that  you are bound to your customized javac to compile your code but the generated bytecode will be understood by any JVM  Of course  you can implement your own array concatenatation method at your source level  there are many examples on how to do it in the other answers    There are so many useful operators that could be added  this one would be one of them

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A type independent variation  UPDATED - thanks to Volley for instantiating T     SuppressWarnings  unchecked   public static  lt T gt  T   join T     arrays         final List lt T gt  output   new ArrayList lt T gt          for T   array   arrays            output addAll Arrays asList array               return output toArray  T   Array newInstance          arrays 0  getClass   getComponentType    output size

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public int   mergeArrays int    a  int    b        int    merged   new int a length   b length       int i   0  k   0  l   a length      int j   a length  gt  b length   a length   b length      while i  lt  j            if k  lt  a length                merged k    a k               k                      if  l - a length   lt  b length                merged l    b l - a length               l                      i              return merged

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I found a one-line solution from the good old Apache Commons Lang library  ArrayUtils addAll T    T      Code   String   both   ArrayUtils addAll first  second

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Non Java 8 solution   public static int   combineArrays int   a  int   b            int   c   new int a length   b length            for  int i   0  i  lt  a length  i                  c i    a i                      for  int j   0  k   a length  j  lt  b length  j    k                  c k    b j                      return c

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A simple variation allowing the joining of more than one array   public static String   join String     arrays         final List lt String gt  output   new ArrayList lt String gt          for String   array   arrays            output addAll Arrays asList array               return output toArray new String output size

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Please forgive me for adding yet another version to this already long list  I looked at every answer and decided that I really wanted a version with just one parameter in the signature  I also added some argument checking to benefit from early failure with sensible info in case of unexpected input    SuppressWarnings  unchecked   public static  lt T gt  T   concat T      inputArrays      if inputArrays length  lt  2        throw new IllegalArgumentException  inputArrays must contain at least 2 arrays           for int i   0  i  lt  inputArrays length  i          if inputArrays i     null          throw new IllegalArgumentException  inputArrays     i      is null                 int totalLength   0     for T   array   inputArrays        totalLength    array length         T   result    T    Array newInstance inputArrays 0  getClass   getComponentType    totalLength      int offset   0     for T   array   inputArrays        System arraycopy array  0  result  offset  array length        offset    array length         return result

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In Java 8   public String   concat String   arr1  String   arr2       Stream lt String gt  stream1   Stream of arr1       Stream lt String gt  stream2   Stream of arr2       Stream lt String gt  stream   Stream concat stream1  stream2       return Arrays toString stream toArray String    new

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public String   concat String      arrays        int length   0      for  String   array   arrays            length    array length            String   result   new String length       int destPos   0      for  String   array   arrays            System arraycopy array  0  result  destPos  array length           destPos    array length            return result

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I found a one-line solution from the good old Apache Commons Lang library  ArrayUtils addAll T    T      Code   String   both   ArrayUtils addAll first  second

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Using only Javas own API    String   join String      arrays         calculate size of target array   int size   0    for  String   array   arrays        size    array length            create list of appropriate size   java util List list   new java util ArrayList size         add arrays   for  String   array   arrays        list addAll java util Arrays asList array              create and return final array   return list toArray new String size        Now  this code ist not the most efficient  but it relies only on standard java classes and is easy to understand  It works for any number of String    even zero arrays

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Another way with Java8 using Stream    public String   concatString String   a  String   b        Stream lt String gt  streamA   Arrays stream a       Stream lt String gt  streamB   Arrays stream b       return Stream concat streamA  streamB  toArray String    new

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concatenates a series of arrays compact  fast and type-safe with lambda   SafeVarargs public static  lt T gt  T   concat  T      arrays       return  Stream of  arrays   reduce    arr1  arr2   - gt          T   rslt   Arrays copyOf  arr1  arr1 length   arr2 length          System arraycopy  arr2  0  rslt  arr1 length  arr2 length          return  rslt            orElse  null           returns null when called without argument  eg  example with 3 arrays   String   a   new String      a    b    c    d     String   b   new String      e    f    g    h     String   c   new String      i    j    k    l      concat  a  b  c         a  b  c  d  e  f  g  h  i  j  k  l        probably the only generic and type-safe way      adapted   Number   array1     1  2  3    Number   array2     4 0  5 0  6 0    Number   array   concat  array1  array2         1  2  3  4 0  5 0  6 0

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This is a converted function for a String array   public String   mergeArrays String   mainArray  String   addArray        String   finalArray   new String mainArray length   addArray length       System arraycopy mainArray  0  finalArray  0  mainArray length       System arraycopy addArray  0  finalArray  mainArray length  addArray length        return finalArray

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How about    public String   combineArray  String       strings        List lt String gt  tmpList   new ArrayList lt String gt         for  int i   0  i  lt  strings length  i            tmpList addAll Arrays asList strings i         return tmpList toArray new String tmpList size

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This is probably the only generic and type-safe way   public class ArrayConcatenator lt T gt        private final IntFunction lt T   gt  generator       private ArrayConcatenator IntFunction lt T   gt  generator            this generator   generator             public static  lt T gt  ArrayConcatenator lt T gt  concat IntFunction lt T   gt  generator            return new ArrayConcatenator lt  gt  generator              public T   apply T   array1  T   array2            T   array   generator apply array1 length   array2 length           System arraycopy array1  0  array  0  array1 length           System arraycopy array2  0  array  array1 length  array2 length           return array            And the usage is quite concise   Integer   array1     1  2  3    Double   array2     4 0  5 0  6 0    Number   array   concat Number    new  apply array1  array2      requires static import   Invalid array types are rejected   concat String    new  apply array1  array2      error concat Integer    new  apply array1  array2      error

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Using the Java API   String   f String   first  String   second        List lt String gt  both   new ArrayList lt String gt  first length   second length       Collections addAll both  first       Collections addAll both  second       return both toArray new String both size

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Another way to think about the question  To concatenate two or more arrays  one have to do is to list all elements of each arrays  and then build a new array  This sounds like create a List lt T gt  and then calls toArray on it  Some other answers uses ArrayList  and that s fine  But how about implement our own  It is not hard   private static  lt T gt  T   addAll final T   f  final T   o       return new AbstractList lt T gt               Override         public T get int i                return i gt  f length   o i - f length    f i                       Override         public int size                 return f length   o length                   toArray f       I believe the above is equivalent to solutions that uses System arraycopy  However I think this one has its own beauty

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You can try this method which concatenates multiple arrays   public static  lt T gt  T   concatMultipleArrays T      arrays       int length   0     for  T   array   arrays             length    array length          T   result    T    Array newInstance arrays getClass   getComponentType    length        length   0     for  int i   0  i  lt  arrays length  i               System arraycopy arrays i   0  result  length  arrays i  length         length    arrays i  length           return result

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String    both   new ArrayList lt String gt     addAll Arrays asList first    addAll Arrays asList second      toArray new String 0

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If you use this way so you no need to import any third party class   If you want concatenate String  Sample code for concate two String Array  public static String   combineString String   first  String   second           int length   first length   second length          String   result   new String length           System arraycopy first  0  result  0  first length           System arraycopy second  0  result  first length  second length           return result          If you want concatenate Int  Sample code for concate two Integer Array  public static int   combineInt int   a  int   b           int length   a length   b length          int   result   new int length           System arraycopy a  0  result  0  a length           System arraycopy b  0  result  a length  b length           return result          Here is Main method      public static void main String   args                 String    first     a    b    c                String    second     d    e                 String    joined   combineString first  second               System out println  concatenated String array       Arrays toString joined                 int   array1    101 102 103 104               int   array2    105 106 107 108               int   concatenateInt   combineInt array1  array2                System out println  concatenated Int array       Arrays toString concatenateInt                         We can use this way also

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If you d like to work with ArrayLists in the solution  you can try this   public final String    f final String    first  final String    second           Assuming non-null for brevity      final ArrayList lt String gt  resultList   new ArrayList lt String gt  Arrays asList first        resultList addAll new ArrayList lt String gt  Arrays asList second         return resultList toArray new String  resultList size

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Wow  lot of complex answers here including some simple ones that depend on external dependencies  how about doing it like this   String    arg1   new String  a   b   c    String    arg2   new String  x   y   z     ArrayList lt String gt  temp   new ArrayList lt String gt     temp addAll Arrays asList arg1    temp addAll Arrays asList arg2    String    concatedArgs   temp toArray new String arg1 length arg2 length

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I ve recently fought problems with excessive memory rotation  If a and or b are known to be commonly empty  here is another adaption of silvertab s code  generified too    private static  lt T gt  T   concatOrReturnSame T   a  T   b        final int alen   a length      final int blen   b length      if  alen    0            return b            if  blen    0            return a            final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      Edit  A previous version of this post stated that array re-usage like this shall be clearly documented  As Maarten points out in the comments it would in general be better to just remove the if statements  thus voiding the need for having documentation  But then again  those if statements were the whole point of this particular optimization in the first place  I ll leave this answer here  but be wary

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Please forgive me for adding yet another version to this already long list  I looked at every answer and decided that I really wanted a version with just one parameter in the signature  I also added some argument checking to benefit from early failure with sensible info in case of unexpected input    SuppressWarnings  unchecked   public static  lt T gt  T   concat T      inputArrays      if inputArrays length  lt  2        throw new IllegalArgumentException  inputArrays must contain at least 2 arrays           for int i   0  i  lt  inputArrays length  i          if inputArrays i     null          throw new IllegalArgumentException  inputArrays     i      is null                 int totalLength   0     for T   array   inputArrays        totalLength    array length         T   result    T    Array newInstance inputArrays 0  getClass   getComponentType    totalLength      int offset   0     for T   array   inputArrays        System arraycopy array  0  result  offset  array length        offset    array length         return result

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You can append the two arrays in two lines of code   String   both   Arrays copyOf first  first length   second length   System arraycopy second  0  both  first length  second length     This is a fast and efficient solution and will work for primitive types as well as the two methods involved are overloaded   You should avoid solutions involving ArrayLists  streams  etc as these will need to allocate temporary memory for no useful purpose   You should avoid for loops for large arrays as these are not efficient  The built in methods use block-copy functions that are extremely fast

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A solution 100  old java and without System arraycopy  not available in GWT client for example    static String   concat String      arrays        int length   0      for  String   array   arrays            length    array length            String   result   new String length       int pos   0      for  String   array   arrays            for  String element   array                result pos    element              pos                        return result

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This should be one-liner   public String    concatenate  final String array1    final String array2          return Stream concat Stream of array1   Stream of array2   toArray String    new

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Should do the trick  This is assuming String   first and String   second  List lt String gt  myList   new ArrayList lt String gt  Arrays asList first    myList addAll new ArrayList lt String gt  Arrays asList second     String   both   myList toArray new String myList size

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Look at this elegant solution  if you need other type than char  change it    private static void concatArrays char   destination  char      sources        int currPos   0      for  char   source   sources            int length   source length          System arraycopy source  0  destination  currPos  length           currPos    length            You can concatenate a every count of arrays

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An easy  but inefficient  way to do this  generics not included    ArrayList baseArray   new ArrayList Arrays asList array1    baseArray addAll Arrays asList array2    String concatenated      String     baseArray toArray new String baseArray size

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public int   mergeArrays int    a  int    b        int    merged   new int a length   b length       int i   0  k   0  l   a length      int j   a length  gt  b length   a length   b length      while i  lt  j            if k  lt  a length                merged k    a k               k                      if  l - a length   lt  b length                merged l    b l - a length               l                      i              return merged

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String    both   new ArrayList lt String gt     addAll Arrays asList first    addAll Arrays asList second      toArray new String 0

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Object   mixArray String   a  String   b  String   s1   a  String   s2   b  Object   result  List lt String gt  input   new ArrayList lt String gt     for  int i   0  i  lt  s1 length  i          input add s1 i      for  int i   0  i  lt  s2 length  i          input add s2 i      result   input toArray    return result

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I just discovered this question  sorry very late  and saw a lot of answers that were too far away  using certain libraries  using the feature of converting data from an array to a stream and back to an array and so on  But  we can just use a simple loop and the problem is done public String   concat String   firstArr String   secondArr             if both is empty just return         if firstArr length  0  amp  amp  secondArr length  0 return new String 0            String   res   new String firstArr length secondArr length           int idxFromFirst 0             loop over firstArr  idxFromFirst will be used as starting offset for secondArr         for int i 0 i lt firstArr length i                 res i    firstArr i               idxFromFirst                         loop over secondArr  with starting offset idxFromFirst  the offset track from first array          for int i 0 i lt secondArr length i                 res idxFromFirst i  secondArr i                      return res         Thats it all  right  he didnt say he care about the order or anything  This should be the easiest way of it

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public static String   toArray String      object       List lt String gt  list new ArrayList lt  gt         for  String   i   object            list addAll Arrays asList i              return list toArray new String list size

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A generic static version that uses the high performing System arraycopy without requiring a  SuppressWarnings annotation   public static  lt T gt  T   arrayConcat T   a  T   b        T   both   Arrays copyOf a  a length   b length       System arraycopy b  0  both  a length  b length       return both

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Every single answer is copying data and creating a new array  This is not strictly necessary and is definitely NOT what you want to do if your arrays are reasonably large  Java creators already knew that array copies are wasteful and that is why they provided us System arrayCopy   to do those outside Java when we have to   Instead of copying your data around  consider leaving it in place and draw from it where it lies  Copying data locations just because the programmer would like to organize them is not always sensible      I have arrayA and arrayB  would like to treat them as concatenated    but leave my damn bytes where they are  Object accessElement   int index          if   index  lt  0   throw new ArrayIndexOutOfBoundsException               is reading from the head part       if   index  lt  arrayA length             return arrayA  index            is reading from the tail part       if   index  lt    arrayA length   arrayB length               return arrayB  index - arrayA length         throw new ArrayIndexOutOfBoundsException          index too large

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Should do the trick  This is assuming String   first and String   second  List lt String gt  myList   new ArrayList lt String gt  Arrays asList first    myList addAll new ArrayList lt String gt  Arrays asList second     String   both   myList toArray new String myList size

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Using Java 8  streams you can write the following function   private static String   concatArrays final String      arrays        return Arrays stream arrays            flatMap Arrays  stream            toArray String    new

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Using the Java API   String   f String   first  String   second        List lt String gt  both   new ArrayList lt String gt  first length   second length       Collections addAll both  first       Collections addAll both  second       return both toArray new String both size

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Using the Java API   String   f String   first  String   second        List lt String gt  both   new ArrayList lt String gt  first length   second length       Collections addAll both  first       Collections addAll both  second       return both toArray new String both size

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In Haskell you can do something like that  a  b  c      d  e  to get  a  b  c  d  e   These are Haskell lists concatenated but that d very nice to see a similar operator in Java for arrays  Don t you think so   That s elegant  simple  generic and it s not that difficult to implement   If you want to  I suggest you to have a look at Alexander Hristov s work in his Hacking the OpenJDK compiler  He explains how to modify javac source to create a new operator  His example consists in defining a      operator where i    j   Math pow i  j   One could take that example to implement an operator that concatenates two arrays of same type   Once you do that  you are bound to your customized javac to compile your code but the generated bytecode will be understood by any JVM  Of course  you can implement your own array concatenatation method at your source level  there are many examples on how to do it in the other answers    There are so many useful operators that could be added  this one would be one of them

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Here a possible implementation in working code of the pseudo code solution written by silvertab    Thanks silvertab   public class Array       public static  lt T gt  T   concat T   a  T   b  ArrayBuilderI lt T gt  builder          T   c   builder build a length   b length         System arraycopy a  0  c  0  a length         System arraycopy b  0  c  a length  b length         return c           Following next is the builder interface    Note  A builder is necessary because in java it is not possible to do   new T size    due to generic type erasure   public interface ArrayBuilderI lt T gt        public T   build int size       Here a concrete builder implementing the interface  building a Integer array   public class IntegerArrayBuilder implements ArrayBuilderI lt Integer gt         Override    public Integer   build int size          return new Integer size            And finally the application   test    Test public class ArrayTest       public void array concatenation           Integer a     new Integer   0 1         Integer b     new Integer   2 3         Integer c     Array concat a  b  new IntegerArrayBuilder           assertEquals 4  c length         assertEquals 0   int c 0          assertEquals 1   int c 1          assertEquals 2   int c 2          assertEquals 3   int c 3

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Here s a simple method that will concatenate two arrays and return the result   public  lt T gt  T   concatenate T   a  T   b        int aLen   a length      int bLen   b length        SuppressWarnings  unchecked       T   c    T    Array newInstance a getClass   getComponentType    aLen   bLen       System arraycopy a  0  c  0  aLen       System arraycopy b  0  c  aLen  bLen        return c      Note that it will not work with primitive data types  only with object types   The following slightly more complicated version works with both object and primitive arrays  It does this by using T instead of T   as the argument type   It also makes it possible to concatenate arrays of two different types by picking the most general type as the component type of the result   public static  lt T gt  T concatenate T a  T b        if   a getClass   isArray       b getClass   isArray              throw new IllegalArgumentException               Class lt   gt  resCompType      Class lt   gt  aCompType   a getClass   getComponentType        Class lt   gt  bCompType   b getClass   getComponentType         if  aCompType isAssignableFrom bCompType             resCompType   aCompType        else if  bCompType isAssignableFrom aCompType             resCompType   bCompType        else           throw new IllegalArgumentException               int aLen   Array getLength a       int bLen   Array getLength b         SuppressWarnings  unchecked       T result    T  Array newInstance resCompType  aLen   bLen       System arraycopy a  0  result  0  aLen       System arraycopy b  0  result  aLen  bLen                return result      Here is an example   Assert assertArrayEquals new int     1  2  3    concatenate new int     1  2    new int     3      Assert assertArrayEquals new Number     1  2  3f    concatenate new Integer     1  2    new Number     3f

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Using Java 8  streams you can write the following function   private static String   concatArrays final String      arrays        return Arrays stream arrays            flatMap Arrays  stream            toArray String    new

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I ve recently fought problems with excessive memory rotation  If a and or b are known to be commonly empty  here is another adaption of silvertab s code  generified too    private static  lt T gt  T   concatOrReturnSame T   a  T   b        final int alen   a length      final int blen   b length      if  alen    0            return b            if  blen    0            return a            final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      Edit  A previous version of this post stated that array re-usage like this shall be clearly documented  As Maarten points out in the comments it would in general be better to just remove the if statements  thus voiding the need for having documentation  But then again  those if statements were the whole point of this particular optimization in the first place  I ll leave this answer here  but be wary

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Here is the code by AbacusUtil   String   a     a    b    c    String   b     1    2    3    String   c   N concat a  b      c     a    b    c    1    2    3       N concat      is null-safety  a   null  c   N concat a  b      c     1    2    3

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Here a possible implementation in working code of the pseudo code solution written by silvertab    Thanks silvertab   public class Array       public static  lt T gt  T   concat T   a  T   b  ArrayBuilderI lt T gt  builder          T   c   builder build a length   b length         System arraycopy a  0  c  0  a length         System arraycopy b  0  c  a length  b length         return c           Following next is the builder interface    Note  A builder is necessary because in java it is not possible to do   new T size    due to generic type erasure   public interface ArrayBuilderI lt T gt        public T   build int size       Here a concrete builder implementing the interface  building a Integer array   public class IntegerArrayBuilder implements ArrayBuilderI lt Integer gt         Override    public Integer   build int size          return new Integer size            And finally the application   test    Test public class ArrayTest       public void array concatenation           Integer a     new Integer   0 1         Integer b     new Integer   2 3         Integer c     Array concat a  b  new IntegerArrayBuilder           assertEquals 4  c length         assertEquals 0   int c 0          assertEquals 1   int c 1          assertEquals 2   int c 2          assertEquals 3   int c 3

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Every single answer is copying data and creating a new array  This is not strictly necessary and is definitely NOT what you want to do if your arrays are reasonably large  Java creators already knew that array copies are wasteful and that is why they provided us System arrayCopy   to do those outside Java when we have to   Instead of copying your data around  consider leaving it in place and draw from it where it lies  Copying data locations just because the programmer would like to organize them is not always sensible      I have arrayA and arrayB  would like to treat them as concatenated    but leave my damn bytes where they are  Object accessElement   int index          if   index  lt  0   throw new ArrayIndexOutOfBoundsException               is reading from the head part       if   index  lt  arrayA length             return arrayA  index            is reading from the tail part       if   index  lt    arrayA length   arrayB length               return arrayB  index - arrayA length         throw new ArrayIndexOutOfBoundsException          index too large

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The Functional Java library has an array wrapper class that equips arrays with handy methods like concatenation   import static fj data Array array       and then  Array lt String gt  both   array first  append array second      To get the unwrapped array back out  call  String   s   both array

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A type independent variation  UPDATED - thanks to Volley for instantiating T     SuppressWarnings  unchecked   public static  lt T gt  T   join T     arrays         final List lt T gt  output   new ArrayList lt T gt          for T   array   arrays            output addAll Arrays asList array               return output toArray  T   Array newInstance          arrays 0  getClass   getComponentType    output size

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Object   obj     hi   there    Object   obj2    im   fine   what abt u    Object   obj3   new Object obj length obj2 length    for int i  0 i lt obj3 length i        obj3 i     i lt obj length  obj i  obj2 i-obj length

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void f String   first  String   second        String   both   new String first length second length       for int i 0 i lt first length i            both i    first i       for int i 0 i lt second length i            both first length   i    second i       This one works without knowledge of any other classes libraries etc  It works for any data type  Just replace String with anything like int double or char  It works quite efficiently

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ArrayList lt String gt  both   new ArrayList Arrays asList first    both addAll Arrays asList second     both toArray new String 0

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Object   obj     hi   there    Object   obj2    im   fine   what abt u    Object   obj3   new Object obj length obj2 length    for int i  0 i lt obj3 length i        obj3 i     i lt obj length  obj i  obj2 i-obj length

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This is probably the only generic and type-safe way   public class ArrayConcatenator lt T gt        private final IntFunction lt T   gt  generator       private ArrayConcatenator IntFunction lt T   gt  generator            this generator   generator             public static  lt T gt  ArrayConcatenator lt T gt  concat IntFunction lt T   gt  generator            return new ArrayConcatenator lt  gt  generator              public T   apply T   array1  T   array2            T   array   generator apply array1 length   array2 length           System arraycopy array1  0  array  0  array1 length           System arraycopy array2  0  array  array1 length  array2 length           return array            And the usage is quite concise   Integer   array1     1  2  3    Double   array2     4 0  5 0  6 0    Number   array   concat Number    new  apply array1  array2      requires static import   Invalid array types are rejected   concat String    new  apply array1  array2      error concat Integer    new  apply array1  array2      error

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It s possible to write a fully generic version that can even be extended to concatenate any number of arrays  This versions require Java 6  as they use Arrays copyOf    Both versions avoid creating any intermediary List objects and use System arraycopy   to ensure that copying large arrays is as fast as possible   For two arrays it looks like this   public static  lt T gt  T   concat T   first  T   second      T   result   Arrays copyOf first  first length   second length     System arraycopy second  0  result  first length  second length     return result      And for a arbitrary number of arrays     1  it looks like this   public static  lt T gt  T   concatAll T   first  T      rest      int totalLength   first length    for  T   array   rest        totalLength    array length        T   result   Arrays copyOf first  totalLength     int offset   first length    for  T   array   rest        System arraycopy array  0  result  offset  array length       offset    array length        return result

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I found I had to deal with the case where the arrays can be null     private double   concat   double  a double  b       if  a    null  return b      if  b    null  return a      double   r   new double a length b length       System arraycopy a  0  r  0  a length       System arraycopy b  0  r  a length  b length       return r     private double   copyRest  double  a  int start       if  a    null  return null      if  start  gt  a length return null      double  r   new double a length-start       System arraycopy a start r 0 a length-start        return r

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I use next method to concatenate any number of arrays of the same type using java 8  public static  lt G gt  G   concatenate IntFunction lt G   gt  generator  G       arrays        int len   arrays length      if  len    0            return generator apply 0         else if  len    1            return arrays 0             int pos   0      Stream lt G gt  result   Stream concat Arrays stream arrays pos    Arrays stream arrays   pos         while  pos  lt  len - 1            result   Stream concat result  Arrays stream arrays   pos               return result toArray generator      usage   concatenate String    new  new String    quot one quot    new String    quot two quot    new String    quot three quot      or  concatenate Integer    new  new Integer   1   new Integer   2   new Integer   3

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A simple variation allowing the joining of more than one array   public static String   join String     arrays         final List lt String gt  output   new ArrayList lt String gt          for String   array   arrays            output addAll Arrays asList array               return output toArray new String output size

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A solution 100  old java and without System arraycopy  not available in GWT client for example    static String   concat String      arrays        int length   0      for  String   array   arrays            length    array length            String   result   new String length       int pos   0      for  String   array   arrays            for  String element   array                result pos    element              pos                        return result

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Yet another answer for algorithm lovers   public static String   mergeArrays String   array1  String   array2        int totalSize   array1 length   array2 length     Get total size     String   merged   new String totalSize      Create new array        Loop over the total size     for  int i   0  i  lt  totalSize  i              if  i  lt  array1 length     If the current position is less than the length of the first array  take value from first array             merged i    array1 i      Position in first array is the current position          else    If current position is equal or greater than the first array  take value from second array              merged i    array2 i - array1 length      Position in second array is current position minus length of first array             return merged    Usage   String   array1str   new String    a    b    c    d     String   array2str   new String    e    f    g    h    i    String   listTotalstr   mergeArrays array1str  array2str   System out println Arrays toString listTotalstr      Result    a  b  c  d  e  f  g  h  i

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Here s my slightly improved version of Joachim Sauer s concatAll  It can work on Java 5 or 6  using Java 6 s System arraycopy if it s available at runtime  This method  IMHO  is perfect for Android  as it work on Android  lt 9  which doesn t have System arraycopy  but will use the faster method if possible     public static  lt T gt  T   concatAll T   first  T      rest        int totalLength   first length      for  T   array   rest          totalLength    array length            T   result      try         Method arraysCopyOf   Arrays class getMethod  copyOf   Object   class  int class         result    T    arraysCopyOf invoke null  first  totalLength         catch  Exception e           Java 6   Android  gt   9 way didn t work  so use the  traditional  approach       result    T    java lang reflect Array newInstance first getClass   getComponentType    totalLength         System arraycopy first  0  result  0  first length             int offset   first length      for  T   array   rest          System arraycopy array  0  result  offset  array length         offset    array length            return result

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Here s an adaptation of silvertab s solution  with generics retrofitted   static  lt T gt  T   concat T   a  T   b        final int alen   a length      final int blen   b length      final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      NOTE  See Joachim s answer for a Java 6 solution  Not only does it eliminate the warning  it s also shorter  more efficient and easier to read

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Using Stream in Java 8   String   both   Stream concat Arrays stream a   Arrays stream b                          toArray String    new     Or like this  using flatMap   String   both   Stream of a  b  flatMap Stream  of                         toArray String    new     To do this for a generic type you have to use reflection    SuppressWarnings  unchecked   T   both   Stream concat Arrays stream a   Arrays stream b   toArray      size - gt   T    Array newInstance a getClass   getComponentType    size

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I tested below code and worked ok  Also I m using library  org apache commons lang ArrayUtils  public void testConcatArrayString        String   a   null      String   b   null      String   c   null      a   new String     1   2   3   4   5        b   new String     A   B   C   D   E         c    String    ArrayUtils addAll a  b       if c  null           for int i 0  i lt c length  i                 System out println  c      i 1             c i                        Regards

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Yet another answer for algorithm lovers   public static String   mergeArrays String   array1  String   array2        int totalSize   array1 length   array2 length     Get total size     String   merged   new String totalSize      Create new array        Loop over the total size     for  int i   0  i  lt  totalSize  i              if  i  lt  array1 length     If the current position is less than the length of the first array  take value from first array             merged i    array1 i      Position in first array is the current position          else    If current position is equal or greater than the first array  take value from second array              merged i    array2 i - array1 length      Position in second array is current position minus length of first array             return merged    Usage   String   array1str   new String    a    b    c    d     String   array2str   new String    e    f    g    h    i    String   listTotalstr   mergeArrays array1str  array2str   System out println Arrays toString listTotalstr      Result    a  b  c  d  e  f  g  h  i

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Here s a simple method that will concatenate two arrays and return the result   public  lt T gt  T   concatenate T   a  T   b        int aLen   a length      int bLen   b length        SuppressWarnings  unchecked       T   c    T    Array newInstance a getClass   getComponentType    aLen   bLen       System arraycopy a  0  c  0  aLen       System arraycopy b  0  c  aLen  bLen        return c      Note that it will not work with primitive data types  only with object types   The following slightly more complicated version works with both object and primitive arrays  It does this by using T instead of T   as the argument type   It also makes it possible to concatenate arrays of two different types by picking the most general type as the component type of the result   public static  lt T gt  T concatenate T a  T b        if   a getClass   isArray       b getClass   isArray              throw new IllegalArgumentException               Class lt   gt  resCompType      Class lt   gt  aCompType   a getClass   getComponentType        Class lt   gt  bCompType   b getClass   getComponentType         if  aCompType isAssignableFrom bCompType             resCompType   aCompType        else if  bCompType isAssignableFrom aCompType             resCompType   bCompType        else           throw new IllegalArgumentException               int aLen   Array getLength a       int bLen   Array getLength b         SuppressWarnings  unchecked       T result    T  Array newInstance resCompType  aLen   bLen       System arraycopy a  0  result  0  aLen       System arraycopy b  0  result  aLen  bLen                return result      Here is an example   Assert assertArrayEquals new int     1  2  3    concatenate new int     1  2    new int     3      Assert assertArrayEquals new Number     1  2  3f    concatenate new Integer     1  2    new Number     3f

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I found a one-line solution from the good old Apache Commons Lang library  ArrayUtils addAll T    T      Code   String   both   ArrayUtils addAll first  second

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Just wanted to add  you can use System arraycopy too   import static java lang System out  import static java lang System arraycopy  import java lang reflect Array  class Playground        SuppressWarnings  unchecked       public static  lt T gt T   combineArrays T   a1  T   a2            T   result    T    Array newInstance a1 getClass   getComponentType    a1 length a2 length           arraycopy a1 0 result 0 a1 length           arraycopy a2 0 result a1 length a2 length           return result            public static void main String    args            String monthsString    JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC           String   months   monthsString split     lt    G  3              String daysString    SUNMONTUEWEDTHUFRISAT           String   days   daysString split     lt    G  3              for  String m   months                out println m                     out println                  for  String d   days                out println d                     out println                 String   results   combineArrays months  days           for  String r   results                out println r                     out println

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Using only Javas own API    String   join String      arrays         calculate size of target array   int size   0    for  String   array   arrays        size    array length            create list of appropriate size   java util List list   new java util ArrayList size         add arrays   for  String   array   arrays        list addAll java util Arrays asList array              create and return final array   return list toArray new String size        Now  this code ist not the most efficient  but it relies only on standard java classes and is easy to understand  It works for any number of String    even zero arrays

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Object   mixArray String   a  String   b  String   s1   a  String   s2   b  Object   result  List lt String gt  input   new ArrayList lt String gt     for  int i   0  i  lt  s1 length  i          input add s1 i      for  int i   0  i  lt  s2 length  i          input add s2 i      result   input toArray    return result

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Here s a simple method that will concatenate two arrays and return the result   public  lt T gt  T   concatenate T   a  T   b        int aLen   a length      int bLen   b length        SuppressWarnings  unchecked       T   c    T    Array newInstance a getClass   getComponentType    aLen   bLen       System arraycopy a  0  c  0  aLen       System arraycopy b  0  c  aLen  bLen        return c      Note that it will not work with primitive data types  only with object types   The following slightly more complicated version works with both object and primitive arrays  It does this by using T instead of T   as the argument type   It also makes it possible to concatenate arrays of two different types by picking the most general type as the component type of the result   public static  lt T gt  T concatenate T a  T b        if   a getClass   isArray       b getClass   isArray              throw new IllegalArgumentException               Class lt   gt  resCompType      Class lt   gt  aCompType   a getClass   getComponentType        Class lt   gt  bCompType   b getClass   getComponentType         if  aCompType isAssignableFrom bCompType             resCompType   aCompType        else if  bCompType isAssignableFrom aCompType             resCompType   bCompType        else           throw new IllegalArgumentException               int aLen   Array getLength a       int bLen   Array getLength b         SuppressWarnings  unchecked       T result    T  Array newInstance resCompType  aLen   bLen       System arraycopy a  0  result  0  aLen       System arraycopy b  0  result  aLen  bLen                return result      Here is an example   Assert assertArrayEquals new int     1  2  3    concatenate new int     1  2    new int     3      Assert assertArrayEquals new Number     1  2  3f    concatenate new Integer     1  2    new Number     3f

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How about simply      public static class Array        public static  lt T gt  T   concat T      arrays            ArrayList lt T gt  al   new ArrayList lt T gt             for  T   one   arrays              Collections addAll al  one           return  T    al toArray arrays 0  clone               And just do Array concat arr1  arr2   As long as arr1 and arr2 are of the same type  this will give you another array of the same type containing both arrays

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Another one based on SilverTab s suggestion  but made to support x number of arguments and not require Java 6  It is also not generic  but I m sure it could be made generic   private byte   concat byte      args        int fulllength   0      for  byte   arrItem   args                fulllength    arrItem length            byte   retArray   new byte fulllength       int start   0      for  byte   arrItem   args                System arraycopy arrItem  0  retArray  start  arrItem length           start    arrItem length            return retArray

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Using Stream in Java 8   String   both   Stream concat Arrays stream a   Arrays stream b                          toArray String    new     Or like this  using flatMap   String   both   Stream of a  b  flatMap Stream  of                         toArray String    new     To do this for a generic type you have to use reflection    SuppressWarnings  unchecked   T   both   Stream concat Arrays stream a   Arrays stream b   toArray      size - gt   T    Array newInstance a getClass   getComponentType    size

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Here s a simple method that will concatenate two arrays and return the result   public  lt T gt  T   concatenate T   a  T   b        int aLen   a length      int bLen   b length        SuppressWarnings  unchecked       T   c    T    Array newInstance a getClass   getComponentType    aLen   bLen       System arraycopy a  0  c  0  aLen       System arraycopy b  0  c  aLen  bLen        return c      Note that it will not work with primitive data types  only with object types   The following slightly more complicated version works with both object and primitive arrays  It does this by using T instead of T   as the argument type   It also makes it possible to concatenate arrays of two different types by picking the most general type as the component type of the result   public static  lt T gt  T concatenate T a  T b        if   a getClass   isArray       b getClass   isArray              throw new IllegalArgumentException               Class lt   gt  resCompType      Class lt   gt  aCompType   a getClass   getComponentType        Class lt   gt  bCompType   b getClass   getComponentType         if  aCompType isAssignableFrom bCompType             resCompType   aCompType        else if  bCompType isAssignableFrom aCompType             resCompType   bCompType        else           throw new IllegalArgumentException               int aLen   Array getLength a       int bLen   Array getLength b         SuppressWarnings  unchecked       T result    T  Array newInstance resCompType  aLen   bLen       System arraycopy a  0  result  0  aLen       System arraycopy b  0  result  aLen  bLen                return result      Here is an example   Assert assertArrayEquals new int     1  2  3    concatenate new int     1  2    new int     3      Assert assertArrayEquals new Number     1  2  3f    concatenate new Integer     1  2    new Number     3f

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Just wanted to add  you can use System arraycopy too   import static java lang System out  import static java lang System arraycopy  import java lang reflect Array  class Playground        SuppressWarnings  unchecked       public static  lt T gt T   combineArrays T   a1  T   a2            T   result    T    Array newInstance a1 getClass   getComponentType    a1 length a2 length           arraycopy a1 0 result 0 a1 length           arraycopy a2 0 result a1 length a2 length           return result            public static void main String    args            String monthsString    JANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDEC           String   months   monthsString split     lt    G  3              String daysString    SUNMONTUEWEDTHUFRISAT           String   days   daysString split     lt    G  3              for  String m   months                out println m                     out println                  for  String d   days                out println d                     out println                 String   results   combineArrays months  days           for  String r   results                out println r                     out println

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Using the Java API   String   f String   first  String   second        List lt String gt  both   new ArrayList lt String gt  first length   second length       Collections addAll both  first       Collections addAll both  second       return both toArray new String both size

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I just discovered this question  sorry very late  and saw a lot of answers that were too far away  using certain libraries  using the feature of converting data from an array to a stream and back to an array and so on  But  we can just use a simple loop and the problem is done public String   concat String   firstArr String   secondArr             if both is empty just return         if firstArr length  0  amp  amp  secondArr length  0 return new String 0            String   res   new String firstArr length secondArr length           int idxFromFirst 0             loop over firstArr  idxFromFirst will be used as starting offset for secondArr         for int i 0 i lt firstArr length i                 res i    firstArr i               idxFromFirst                         loop over secondArr  with starting offset idxFromFirst  the offset track from first array          for int i 0 i lt secondArr length i                 res idxFromFirst i  secondArr i                      return res         Thats it all  right  he didnt say he care about the order or anything  This should be the easiest way of it

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Import java util     String array1       bla   bla    String array2       bla   bla     ArrayList lt String gt  tempArray   new ArrayList lt String gt  Arrays asList array1    tempArray addAll Arrays asList array2    String array3     films toArray new String 1       size will be overwritten if needed   You could replace String by a Type Class of your liking  Im sure this can be made shorter and better  but it works and im to lazy to sort it out further

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In Java 8   public String   concat String   arr1  String   arr2       Stream lt String gt  stream1   Stream of arr1       Stream lt String gt  stream2   Stream of arr2       Stream lt String gt  stream   Stream concat stream1  stream2       return Arrays toString stream toArray String    new

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It s possible to write a fully generic version that can even be extended to concatenate any number of arrays  This versions require Java 6  as they use Arrays copyOf    Both versions avoid creating any intermediary List objects and use System arraycopy   to ensure that copying large arrays is as fast as possible   For two arrays it looks like this   public static  lt T gt  T   concat T   first  T   second      T   result   Arrays copyOf first  first length   second length     System arraycopy second  0  result  first length  second length     return result      And for a arbitrary number of arrays     1  it looks like this   public static  lt T gt  T   concatAll T   first  T      rest      int totalLength   first length    for  T   array   rest        totalLength    array length        T   result   Arrays copyOf first  totalLength     int offset   first length    for  T   array   rest        System arraycopy array  0  result  offset  array length       offset    array length        return result

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I ve recently fought problems with excessive memory rotation  If a and or b are known to be commonly empty  here is another adaption of silvertab s code  generified too    private static  lt T gt  T   concatOrReturnSame T   a  T   b        final int alen   a length      final int blen   b length      if  alen    0            return b            if  blen    0            return a            final T   result    T    java lang reflect Array              newInstance a getClass   getComponentType    alen   blen       System arraycopy a  0  result  0  alen       System arraycopy b  0  result  alen  blen       return result      Edit  A previous version of this post stated that array re-usage like this shall be clearly documented  As Maarten points out in the comments it would in general be better to just remove the if statements  thus voiding the need for having documentation  But then again  those if statements were the whole point of this particular optimization in the first place  I ll leave this answer here  but be wary

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Another one based on SilverTab s suggestion  but made to support x number of arguments and not require Java 6  It is also not generic  but I m sure it could be made generic   private byte   concat byte      args        int fulllength   0      for  byte   arrItem   args                fulllength    arrItem length            byte   retArray   new byte fulllength       int start   0      for  byte   arrItem   args                System arraycopy arrItem  0  retArray  start  arrItem length           start    arrItem length            return retArray

[java] How can I concatenate two arrays in Java?

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