How to add new elements to an array

Question

I have the following code   String   where  where append ContactsContract Contacts HAS PHONE NUMBER     1    where append ContactsContract Contacts IN VISIBLE GROUP     1      Those two appends are not compiling  How would that work correctly

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You can simply do this   System arraycopy initialArray  0  newArray  0  initialArray length

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Apache Commons Lang has  T   t   ArrayUtils add  initialArray  newitem      it returns a new array  but if you re really working with arrays for some reason  this may be the ideal way to do this

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If one really want to resize an array you could do something like this   String   arr     a    b    c    System out println Arrays toString arr        Output is   a  b  c   arr   Arrays copyOf arr  10      new size will be 10 elements arr 3     d   arr 4     e   arr 5     f    System out println Arrays toString arr       Output is   a  b  c  d  e  f  null  null  null  null

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I m not that experienced in Java but I have always been told that arrays are static structures that have a predefined size   You have to use an ArrayList or a Vector or any other dynamic structure

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Use a List lt String gt   such as an ArrayList lt String gt   It s dynamically growable  unlike arrays  see  Effective Java 2nd Edition  Item 25  Prefer lists to arrays    import java util            List lt String gt  list   new ArrayList lt String gt     list add  1    list add  2    list add  3    System out println list      prints   1  2  3     If you insist on using arrays  you can use java util Arrays copyOf to allocate a bigger array to accomodate the additional element  This is really not the best solution  though   static  lt T gt  T   append T   arr  T element        final int N   arr length      arr   Arrays copyOf arr  N   1       arr N    element      return arr     String   arr      1    2    3     System out println Arrays toString arr       prints   1  2  3   arr   append arr   4    System out println Arrays toString arr       prints   1  2  3  4     This is O N  per append  ArrayList  on the other hand  has O 1  amortized cost per operation   See also   Java Tutorials Arrays   An array is a container object that holds a fixed number of values of a single type  The length of an array is established when the array is created  After creation  its length is fixed   Java Tutorials The List interface

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There are many ways to add an element to an array  You can use a temp List to manage the element and then convert it back to Array or you can use the java util Arrays copyOf and combine it with generics for better results   This example will show you how   public static  lt T gt  T   append2Array T   elements  T element        T   newArray   Arrays copyOf elements  elements length   1       newArray elements length    element       return newArray      To use this method you just need to call it like this   String   numbers   new String    one    two    three    System out println Arrays toString numbers    numbers   append2Array numbers   four    System out println Arrays toString numbers      If you want to merge two array you can modify the previous method like this   public static  lt T gt  T   append2Array T   elements  T   newElements        T   newArray   Arrays copyOf elements  elements length   newElements length       System arraycopy newElements  0  newArray  elements length  newElements length        return newArray      Now you can call the method like this   String   numbers   new String    one    two    three    String   moreNumbers   new String    four    five    six    System out println Arrays toString numbers    numbers   append2Array numbers  moreNumbers   System out println Arrays toString numbers      As I mentioned  you also may use List objects  However  it will require a little hack to cast it safe like this   public static  lt T gt  T   append2Array Class lt T   gt  clazz  List lt T gt  elements  T element        elements add element       return clazz cast elements toArray         Now you can call the method like this   String   numbers   new String    one    two    three    System out println Arrays toString numbers    numbers   append2Array String   class  Arrays asList numbers    four    System out println Arrays toString numbers

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You need to use a Collection List   You cannot re-dimension an array

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Adding new items to String array   String   myArray   new String     x    y        Convert array to list List lt String gt  listFromArray   Arrays asList myArray       Create new list  because  List to Array always returns a fixed-size list backed by the specified array  List lt String gt  tempList   new ArrayList lt String gt  listFromArray   tempList add  z       Convert list back to array String   tempArray   new String tempList size     myArray   tempList toArray tempArray

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Size of array cannot be modified  If you have to use an array  you can use   System arraycopy src  srcpos  dest  destpos  length

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you can create a arraylist  and use Collection addAll   to convert the string array to your arraylist

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There is no method append   on arrays   Instead as already suggested a List object can service the need for dynamically inserting elements eg   List lt String gt  where   new ArrayList lt String gt     where add ContactsContract Contacts HAS PHONE NUMBER     1    where add ContactsContract Contacts IN VISIBLE GROUP     1      Or if you are really keen to use an array   String   where   new String        ContactsContract Contacts HAS PHONE NUMBER     1       ContactsContract Contacts IN VISIBLE GROUP     1       but then this is a fixed size and no elements can be added

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If you would like to store your data in simple array like this   String   where   new String 10     and you want to add some elements to it like numbers please us StringBuilder which is much more efficient than concatenating string   StringBuilder phoneNumber   new StringBuilder    phoneNumber append  1    phoneNumber append  2    where 0    phoneNumber toString      This is much better method to build your string and store it into your  where  array

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As tangens said  the size of an array is fixed  But you have to instantiate it first  else it will be only a null reference   String   where   new String 10     This array can contain only 10 elements  So you can append a value only 10 times  In your code you re accessing a null reference  That s why it doesnt work  In order to have a  dynamically growing collection  use the ArrayList

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There is another option which i haven t seen here and which doesn t involve  complex  Objects or Collections   String   array1   new String    one    two    String   array2   new String    three    String   array   new String array1 length   array2 length   System arraycopy array1  0  array  0  array1 length   System arraycopy array2  0  array  array1 length  array2 length

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String   source   new String      a    b    c    d     String   destination   new String source length   2   destination 0      bin sh   destination 1     -c   System arraycopy source  0  destination  2  source length    for  String parts   destination      System out println parts

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It s also possible to pre-allocate large enough memory size  Here is a simple stack implementation  the program is supposed to output 3 and 5   class Stk       static public final int STKSIZ   256      public int   info   new int STKSIZ       public int sp   0     stack pointer     public void push int value            info sp      value          class App       public static void main String   args            Stk stk   new Stk            stk push 3           stk push 5           System out println stk info 0            System out println stk info 1

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The size of an array can t be modified  If you want a bigger array you have to instantiate a new one   A better solution would be to use an ArrayList which can grow as you need it  The method ArrayList toArray  T   a   gives you back your array if you need it in this form   List lt String gt  where   new ArrayList lt String gt     where add  ContactsContract Contacts HAS PHONE NUMBER   1     where add  ContactsContract Contacts IN VISIBLE GROUP   1       If you need to convert it to a simple array     String   simpleArray   new String  where size      where toArray  simpleArray      But most things you do with an array you can do with this ArrayList  too      iterate over the array for  String oneItem   where                   get specific items where get  1

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I ve made this code  It works like a charm   public String   AddToStringArray String   oldArray  String newString        String   newArray   Arrays copyOf oldArray  oldArray length 1       newArray oldArray length    newString      return newArray      I hope you like it

[java] How to add new elements to an array?

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