How do I join two lists in Java

Question

Conditions  do not modifiy the original lists  JDK only  no external libraries  Bonus points for a one-liner or a JDK 1 3 version   Is there a simpler way than   List lt String gt  newList   new ArrayList lt String gt     newList addAll listOne   newList addAll listTwo

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Another Java 8 one-liner   List lt String gt  newList   Stream of listOne  listTwo                               flatMap Collection  stream                               collect Collectors toList       As a bonus  since Stream of   is variadic  you may concatenate as many lists as you like   List lt String gt  newList   Stream of listOne  listTwo  listThree                               flatMap Collection  stream                               collect Collectors toList

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You can create your generic Java 8 utility method to concat any number of lists    SafeVarargs public static  lt T gt  List lt T gt  concat List lt T gt     lists        return Stream of lists  flatMap List  stream  collect Collectors toList

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You can create your generic Java 8 utility method to concat any number of lists    SafeVarargs public static  lt T gt  List lt T gt  concat List lt T gt     lists        return Stream of lists  flatMap List  stream  collect Collectors toList

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Here is a java 8 solution using two lines   List lt Object gt  newList   new ArrayList lt  gt     Stream of list1  list2  forEach newList  addAll     Be aware that this method should not be used if   the origin of newList is not known and it may already be shared with other threads the stream that modifies newList is a parallel stream and access to  newList is not synchronized or threadsafe   due to side effect considerations   Both of the above conditions do not apply for the above case of joining two lists  so this is safe   Based on this answer to another question

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Java 8 version with support for joining by object key   public List lt SomeClass gt  mergeLists final List lt SomeClass gt  left  final List lt SomeClass gt  right  String primaryKey        final Map lt Object  SomeClass gt  mergedList   new LinkedHashMap lt  gt          Stream concat left stream    right stream             map someObject - gt  new Pair lt Object  SomeClass gt  someObject getSomeKey    someObject            forEach pair- gt  mergedList put pair getKey    pair getValue           return new ArrayList lt  gt  mergedList values

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If you want to do this statically you can the following   The examples uses 2 EnumSets in natural-order    Enum-order  A  B and joins then in an ALL list   public static final EnumSet lt MyType gt  CATEGORY A   EnumSet of A 1  A 2   public static final EnumSet lt MyType gt  CATEGORY B   EnumSet of B 1  B 2  B 3    public static final List lt MyType gt  ALL                  Collections unmodifiableList                    new ArrayList lt MyType gt  CATEGORY A size     CATEGORY B size                                               addAll CATEGORY A                         addAll CATEGORY B

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No way near one-liner  but I think this is the simplest   List lt String gt  newList   new ArrayList lt String gt  l1   newList addAll l2    for String w newList          System out printf   s    w

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public static  lt T gt  List lt T gt  merge  Nonnull final List lt T gt     list           calculate length first     int mergedLength   0      for  List lt T gt  ts   list          mergedLength    ts size               final List lt T gt  mergedList   new ArrayList lt  gt  mergedLength        for  List lt T gt  ts   list          mergedList addAll ts              return mergedList

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In Java 8   List lt String gt  newList   Stream concat listOne stream    listTwo stream                                  collect Collectors toList

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public class TestApp            param args     public static void main String   args        System out println  Hi        Set lt List lt String gt  gt  bcOwnersList   new HashSet lt List lt String gt  gt         List lt String gt  bclist   new ArrayList lt String gt         List lt String gt  bclist1   new ArrayList lt String gt         List lt String gt  object   new ArrayList lt String gt         object add  BC11        object add  C2        bclist add  BC1        bclist add  BC2        bclist add  BC3        bclist add  BC4        bclist add  BC5        bcOwnersList add bclist       bcOwnersList add object        bclist1 add  BC11        bclist1 add  BC21        bclist1 add  BC31        bclist1 add  BC4        bclist1 add  BC5         List lt String gt  listList  new ArrayList lt String gt         for List lt String gt  ll   bcOwnersList           listList    List lt String gt   CollectionUtils union listList CollectionUtils intersection ll  bclist1                for List lt String gt  lists   listList           test    List lt String gt   CollectionUtils union test  listList               for Object l   listList           System out println l toString               System out println bclist contains  BC

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Found this question looking to concatenate arbitrary amount of lists  not minding external libraries  So  perhaps it will help someone else   com google common collect Iterables concat     Useful if you want to apply the same logic to a number of different collections in one for

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The smartest in my opinion           param smallLists     return one big list containing all elements of the small ones  in the same order      public static  lt E gt  List lt E gt  concatenate  final List lt E gt      smallLists        final ArrayList lt E gt  bigList   new ArrayList lt E gt         for  final List lt E gt  list  smallLists                bigList addAll list             return bigList

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One of your requirements is to preserve the original lists   If you create a new list and use addAll    you are effectively doubling the number of references to the objects in your lists   This could lead to memory problems if your lists are very large   If you don t need to modify the concatenated result  you can avoid this using a custom list implementation   The custom implementation class is more than one line  obviously   but using it is short and sweet   CompositeUnmodifiableList java   public class CompositeUnmodifiableList lt E gt  extends AbstractList lt E gt         private final List lt E gt  list1      private final List lt E gt  list2       public CompositeUnmodifiableList List lt E gt  list1  List lt E gt  list2            this list1   list1          this list2   list2              Override     public E get int index            if  index  lt  list1 size                  return list1 get index                     return list2 get index-list1 size                 Override     public int size             return list1 size     list2 size              Usage   List lt String gt  newList   new CompositeUnmodifiableList lt String gt  listOne listTwo

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A little shorter would be   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Off the top of my head  I can shorten it by one line   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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You can do a oneliner if the target list is predeclared    newList   new ArrayList lt String gt  list1   addAll list2

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Another Java 8 one-liner   List lt String gt  newList   Stream of listOne  listTwo                               flatMap Collection  stream                               collect Collectors toList       As a bonus  since Stream of   is variadic  you may concatenate as many lists as you like   List lt String gt  newList   Stream of listOne  listTwo  listThree                               flatMap Collection  stream                               collect Collectors toList

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I can t improve on the two-liner in the general case without introducing your own utility method  but if you do have lists of Strings and you re willing to assume those Strings don t contain commas  you can pull this long one-liner   List lt String gt  newList   new ArrayList lt String gt  Arrays asList  listOne toString   subString 1  listOne length   - 1           listTwo toString   subString 1  listTwo length   - 1   split            If you drop the generics  this should be JDK 1 4 compliant  though I haven t tested that   Also not recommended for production code  -

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We can join 2 lists using java8 with 2 approaches       List lt String gt  list1   Arrays asList  S    T        List lt String gt  list2   Arrays asList  U    V      1  Using concat        List lt String gt  collect2   Stream concat list1 stream    list2 stream    collect toList         System out println  collect2       collect2      collect2    S  T  U  V    2  Using flatMap         List lt String gt  collect3   Stream of list1  list2  flatMap Collection  stream  collect toList         System out println  collect3       collect3      collect3    S  T  U  V

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This is simple and just one line  but will add the contents of listTwo to listOne  Do you really need to put the contents in a third list   Collections addAll listOne  listTwo toArray

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You could use the Apache commons-collections library   List lt String gt  newList   ListUtils union list1  list2

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Probably not simpler  but intriguing and ugly   List lt String gt  newList   new ArrayList lt String gt        addAll listOne   addAll listTwo          Don t use it in production code

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In Java 8  the other way    List lt   gt  newList    Stream of list1  list2  flatMap List  stream  collect Collectors toList

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Here is a java 8 solution using two lines   List lt Object gt  newList   new ArrayList lt  gt     Stream of list1  list2  forEach newList  addAll     Be aware that this method should not be used if   the origin of newList is not known and it may already be shared with other threads the stream that modifies newList is a parallel stream and access to  newList is not synchronized or threadsafe   due to side effect considerations   Both of the above conditions do not apply for the above case of joining two lists  so this is safe   Based on this answer to another question

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You can do a oneliner if the target list is predeclared    newList   new ArrayList lt String gt  list1   addAll list2

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The smartest in my opinion           param smallLists     return one big list containing all elements of the small ones  in the same order      public static  lt E gt  List lt E gt  concatenate  final List lt E gt      smallLists        final ArrayList lt E gt  bigList   new ArrayList lt E gt         for  final List lt E gt  list  smallLists                bigList addAll list             return bigList

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We can join 2 lists using java8 with 2 approaches       List lt String gt  list1   Arrays asList  S    T        List lt String gt  list2   Arrays asList  U    V      1  Using concat        List lt String gt  collect2   Stream concat list1 stream    list2 stream    collect toList         System out println  collect2       collect2      collect2    S  T  U  V    2  Using flatMap         List lt String gt  collect3   Stream of list1  list2  flatMap Collection  stream  collect toList         System out println  collect3       collect3      collect3    S  T  U  V

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Slightly simpler   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Probably not simpler  but intriguing and ugly   List lt String gt  newList   new ArrayList lt String gt        addAll listOne   addAll listTwo          Don t use it in production code

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Off the top of my head  I can shorten it by one line   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Not simpler  but without resizing overhead   List lt String gt  newList   new ArrayList lt  gt  listOne size     listTwo size     newList addAll listOne   newList addAll listTwo

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If you want to do this statically you can the following   The examples uses 2 EnumSets in natural-order    Enum-order  A  B and joins then in an ALL list   public static final EnumSet lt MyType gt  CATEGORY A   EnumSet of A 1  A 2   public static final EnumSet lt MyType gt  CATEGORY B   EnumSet of B 1  B 2  B 3    public static final List lt MyType gt  ALL                  Collections unmodifiableList                    new ArrayList lt MyType gt  CATEGORY A size     CATEGORY B size                                               addAll CATEGORY A                         addAll CATEGORY B

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Here s an approach using streams and java 8 if your lists have different types and you want to combine them to a list of another type   public static void main String   args        List lt String gt  list2   new ArrayList lt  gt         List lt Pair lt Integer  String gt  gt  list1   new ArrayList lt  gt          list2 add  asd        list2 add  asdaf        list1 add new Pair lt  gt  1   werwe         list1 add new Pair lt  gt  2   tyutyu          Stream stream   Stream concat list1 stream    list2 stream          List lt Pair lt Integer  String gt  gt  res    List lt Pair lt Integer  String gt  gt   stream              map item - gt                    if  item instanceof String                        return new Pair lt  gt  0  item                                     else                       return new Pair lt  gt    Pair lt Integer  String gt  item  getKey      Pair lt Integer  String gt  item  getValue                                                   collect Collectors toList

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I can t improve on the two-liner in the general case without introducing your own utility method  but if you do have lists of Strings and you re willing to assume those Strings don t contain commas  you can pull this long one-liner   List lt String gt  newList   new ArrayList lt String gt  Arrays asList  listOne toString   subString 1  listOne length   - 1           listTwo toString   subString 1  listTwo length   - 1   split            If you drop the generics  this should be JDK 1 4 compliant  though I haven t tested that   Also not recommended for production code  -

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I can t improve on the two-liner in the general case without introducing your own utility method  but if you do have lists of Strings and you re willing to assume those Strings don t contain commas  you can pull this long one-liner   List lt String gt  newList   new ArrayList lt String gt  Arrays asList  listOne toString   subString 1  listOne length   - 1           listTwo toString   subString 1  listTwo length   - 1   split            If you drop the generics  this should be JDK 1 4 compliant  though I haven t tested that   Also not recommended for production code  -

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A little shorter would be   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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import java util AbstractList  import java util List           The   code ConcatList  is a lightweight view of two   code List s      lt p gt     This implementation is  lt em gt not lt  em gt  thread-safe even though the underlying lists can be          param  lt E gt                the type of elements in this list     public class ConcatList lt E gt  extends AbstractList lt E gt             The first underlying list         private final List lt E gt  list1          The second underlying list         private final List lt E gt  list2                  Constructs a new   code ConcatList  from the given two lists                  param list1                   the first list         param list2                   the second list             public ConcatList final List lt E gt  list1  final List lt E gt  list2            this list1   list1          this list2   list2              Override     public E get final int index            return getList index  get getListIndex index                Override     public E set final int index  final E element            return getList index  set getListIndex index   element               Override     public void add final int index  final E element            getList index  add getListIndex index   element               Override     public E remove final int index            return getList index  remove getListIndex index                Override     public int size             return list1 size     list2 size                Override     public boolean contains final Object o            return list1 contains o     list2 contains o               Override     public void clear             list1 clear            list2 clear                          Returns the index within the corresponding list related to the given index                  param index                   the index in this list                 return the index of the underlying list             private int getListIndex final int index            final int size1   list1 size            return index  gt   size1   index - size1   index                        Returns the list that corresponds to the given index                  param index                   the index in this list                 return the underlying list that corresponds to that index             private List lt E gt  getList final int index            return index  gt   list1 size     list2   list1

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You can do a oneliner if the target list is predeclared    newList   new ArrayList lt String gt  list1   addAll list2

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I m not claiming that it s simple  but you mentioned bonus for one-liners  -   Collection mergedList   Collections list new sun misc CompoundEnumeration new Enumeration         new Vector list1  elements        new Vector list2  elements

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Use a Helper class   I suggest   public static  lt E gt  Collection lt E gt  addAll Collection lt E gt  dest  Collection lt   extends E gt     src        for Collection lt   extends E gt  c   src            dest addAll c              return dest     public static void main String   args        System out println addAll new ArrayList lt Object gt     Arrays asList 1 2 3   Arrays asList  a    b    c              does not compile        System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList  a    b    c           System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList 4  5  6

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Almost of answers suggest to use an ArrayList   List lt String gt  newList   new LinkedList lt  gt  listOne   newList addAll listTwo     Prefer to use a LinkedList for efficient add operations   ArrayList add is  O 1  amortized  but O n  worst-case since the array must be resized and copied  While LinkedList add is always constant O 1    more infos https   stackoverflow com a 322742 311420

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I m not claiming that it s simple  but you mentioned bonus for one-liners  -   Collection mergedList   Collections list new sun misc CompoundEnumeration new Enumeration         new Vector list1  elements        new Vector list2  elements

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Almost of answers suggest to use an ArrayList   List lt String gt  newList   new LinkedList lt  gt  listOne   newList addAll listTwo     Prefer to use a LinkedList for efficient add operations   ArrayList add is  O 1  amortized  but O n  worst-case since the array must be resized and copied  While LinkedList add is always constant O 1    more infos https   stackoverflow com a 322742 311420

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public static  lt T gt  List lt T gt  merge List lt T gt     args        final List lt T gt  result   new ArrayList lt  gt          for  List lt T gt  list   args            result addAll list              return result

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In Java 8  the other way    List lt   gt  newList    Stream of list1  list2  flatMap List  stream  collect Collectors toList

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You could do it with a static import and a helper class  nb the generification of this class could probably be improved  public class Lists       private Lists          can t be instantiated     public static List lt T gt  join List lt T gt     lists          List lt T gt  result   new ArrayList lt T gt           for List lt T gt  list   lists             result addAll list                 return results            Then you can do things like  import static Lists join  List lt T gt  result   join list1  list2  list3  list4

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Probably not simpler  but intriguing and ugly   List lt String gt  newList   new ArrayList lt String gt        addAll listOne   addAll listTwo          Don t use it in production code

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No way near one-liner  but I think this is the simplest   List lt String gt  newList   new ArrayList lt String gt  l1   newList addAll l2    for String w newList          System out printf   s    w

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Use a Helper class   I suggest   public static  lt E gt  Collection lt E gt  addAll Collection lt E gt  dest  Collection lt   extends E gt     src        for Collection lt   extends E gt  c   src            dest addAll c              return dest     public static void main String   args        System out println addAll new ArrayList lt Object gt     Arrays asList 1 2 3   Arrays asList  a    b    c              does not compile        System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList  a    b    c           System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList 4  5  6

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Probably not simpler  but intriguing and ugly   List lt String gt  newList   new ArrayList lt String gt        addAll listOne   addAll listTwo          Don t use it in production code

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I can t improve on the two-liner in the general case without introducing your own utility method  but if you do have lists of Strings and you re willing to assume those Strings don t contain commas  you can pull this long one-liner   List lt String gt  newList   new ArrayList lt String gt  Arrays asList  listOne toString   subString 1  listOne length   - 1           listTwo toString   subString 1  listTwo length   - 1   split            If you drop the generics  this should be JDK 1 4 compliant  though I haven t tested that   Also not recommended for production code  -

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Off the top of my head  I can shorten it by one line   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Use a Helper class   I suggest   public static  lt E gt  Collection lt E gt  addAll Collection lt E gt  dest  Collection lt   extends E gt     src        for Collection lt   extends E gt  c   src            dest addAll c              return dest     public static void main String   args        System out println addAll new ArrayList lt Object gt     Arrays asList 1 2 3   Arrays asList  a    b    c              does not compile        System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList  a    b    c           System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList 4  5  6

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Java 8 version with support for joining by object key   public List lt SomeClass gt  mergeLists final List lt SomeClass gt  left  final List lt SomeClass gt  right  String primaryKey        final Map lt Object  SomeClass gt  mergedList   new LinkedHashMap lt  gt          Stream concat left stream    right stream             map someObject - gt  new Pair lt Object  SomeClass gt  someObject getSomeKey    someObject            forEach pair- gt  mergedList put pair getKey    pair getValue           return new ArrayList lt  gt  mergedList values

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Use a Helper class   I suggest   public static  lt E gt  Collection lt E gt  addAll Collection lt E gt  dest  Collection lt   extends E gt     src        for Collection lt   extends E gt  c   src            dest addAll c              return dest     public static void main String   args        System out println addAll new ArrayList lt Object gt     Arrays asList 1 2 3   Arrays asList  a    b    c              does not compile        System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList  a    b    c           System out println addAll new ArrayList lt Integer gt     Arrays asList 1 2 3   Arrays asList 4  5  6

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You could do it with a static import and a helper class  nb the generification of this class could probably be improved  public class Lists       private Lists          can t be instantiated     public static List lt T gt  join List lt T gt     lists          List lt T gt  result   new ArrayList lt T gt           for List lt T gt  list   lists             result addAll list                 return results            Then you can do things like  import static Lists join  List lt T gt  result   join list1  list2  list3  list4

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You could do it with a static import and a helper class  nb the generification of this class could probably be improved  public class Lists       private Lists          can t be instantiated     public static List lt T gt  join List lt T gt     lists          List lt T gt  result   new ArrayList lt T gt           for List lt T gt  list   lists             result addAll list                 return results            Then you can do things like  import static Lists join  List lt T gt  result   join list1  list2  list3  list4

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In Java 8   List lt String gt  newList   Stream concat listOne stream    listTwo stream                                  collect Collectors toList

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another one liner solution using Java8 stream  since flatMap solution is already posted  here is a solution without flatMap  List lt E gt  li   lol stream   collect ArrayList  new  List  addAll  List  addAll     or  List lt E gt  ints   Stream of list1  list2  collect ArrayList  new  List  addAll  List  addAll     code      List lt List lt Integer gt  gt  lol   Arrays asList Arrays asList 1  2  3   Arrays asList 4  5  6        List lt Integer gt  li   lol stream   collect ArrayList  new  List  addAll  List  addAll       System out println lol       System out println li     output    1  2  3    4  5  6    1  2  3  4  5  6

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public class TestApp            param args     public static void main String   args        System out println  Hi        Set lt List lt String gt  gt  bcOwnersList   new HashSet lt List lt String gt  gt         List lt String gt  bclist   new ArrayList lt String gt         List lt String gt  bclist1   new ArrayList lt String gt         List lt String gt  object   new ArrayList lt String gt         object add  BC11        object add  C2        bclist add  BC1        bclist add  BC2        bclist add  BC3        bclist add  BC4        bclist add  BC5        bcOwnersList add bclist       bcOwnersList add object        bclist1 add  BC11        bclist1 add  BC21        bclist1 add  BC31        bclist1 add  BC4        bclist1 add  BC5         List lt String gt  listList  new ArrayList lt String gt         for List lt String gt  ll   bcOwnersList           listList    List lt String gt   CollectionUtils union listList CollectionUtils intersection ll  bclist1                for List lt String gt  lists   listList           test    List lt String gt   CollectionUtils union test  listList               for Object l   listList           System out println l toString               System out println bclist contains  BC

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You can do a oneliner if the target list is predeclared    newList   new ArrayList lt String gt  list1   addAll list2

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Found this question looking to concatenate arbitrary amount of lists  not minding external libraries  So  perhaps it will help someone else   com google common collect Iterables concat     Useful if you want to apply the same logic to a number of different collections in one for

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You could use the Apache commons-collections library   List lt String gt  newList   ListUtils union list1  list2

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Slightly simpler   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Off the top of my head  I can shorten it by one line   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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public static  lt T gt  List lt T gt  merge List lt T gt     args        final List lt T gt  result   new ArrayList lt  gt          for  List lt T gt  list   args            result addAll list              return result

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One of your requirements is to preserve the original lists   If you create a new list and use addAll    you are effectively doubling the number of references to the objects in your lists   This could lead to memory problems if your lists are very large   If you don t need to modify the concatenated result  you can avoid this using a custom list implementation   The custom implementation class is more than one line  obviously   but using it is short and sweet   CompositeUnmodifiableList java   public class CompositeUnmodifiableList lt E gt  extends AbstractList lt E gt         private final List lt E gt  list1      private final List lt E gt  list2       public CompositeUnmodifiableList List lt E gt  list1  List lt E gt  list2            this list1   list1          this list2   list2              Override     public E get int index            if  index  lt  list1 size                  return list1 get index                     return list2 get index-list1 size                 Override     public int size             return list1 size     list2 size              Usage   List lt String gt  newList   new CompositeUnmodifiableList lt String gt  listOne listTwo

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You could do it with a static import and a helper class  nb the generification of this class could probably be improved  public class Lists       private Lists          can t be instantiated     public static List lt T gt  join List lt T gt     lists          List lt T gt  result   new ArrayList lt T gt           for List lt T gt  list   lists             result addAll list                 return results            Then you can do things like  import static Lists join  List lt T gt  result   join list1  list2  list3  list4

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another one liner solution using Java8 stream  since flatMap solution is already posted  here is a solution without flatMap  List lt E gt  li   lol stream   collect ArrayList  new  List  addAll  List  addAll     or  List lt E gt  ints   Stream of list1  list2  collect ArrayList  new  List  addAll  List  addAll     code      List lt List lt Integer gt  gt  lol   Arrays asList Arrays asList 1  2  3   Arrays asList 4  5  6        List lt Integer gt  li   lol stream   collect ArrayList  new  List  addAll  List  addAll       System out println lol       System out println li     output    1  2  3    4  5  6    1  2  3  4  5  6

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A little shorter would be   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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A little shorter would be   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Java 8  Stream of and Stream concat    The proposed solution is for three lists though it can be applied for two lists as well  In Java 8 we can make use of Stream of or Stream concat as   List lt String gt  result1   Stream concat Stream concat list1 stream   list2 stream    list3 stream    collect Collectors toList     List lt String gt  result2   Stream of list1 list2 list3  flatMap Collection  stream  collect Collectors toList       Stream concat takes two streams as input and creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream  As we have three lists we have used this method  Stream concat  two times   We can also write a utility class with a method that takes any number of lists  using varargs  and returns a concatenated list as   public static  lt T gt  List lt T gt  concatenateLists List lt T gt     collections            return Arrays stream collections  flatMap Collection  stream  collect Collectors toList          Then we can make use of this method as   List lt String gt  result3   Utils concatenateLists list1 list2 list3

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I m not claiming that it s simple  but you mentioned bonus for one-liners  -   Collection mergedList   Collections list new sun misc CompoundEnumeration new Enumeration         new Vector list1  elements        new Vector list2  elements

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import java util AbstractList  import java util List           The   code ConcatList  is a lightweight view of two   code List s      lt p gt     This implementation is  lt em gt not lt  em gt  thread-safe even though the underlying lists can be          param  lt E gt                the type of elements in this list     public class ConcatList lt E gt  extends AbstractList lt E gt             The first underlying list         private final List lt E gt  list1          The second underlying list         private final List lt E gt  list2                  Constructs a new   code ConcatList  from the given two lists                  param list1                   the first list         param list2                   the second list             public ConcatList final List lt E gt  list1  final List lt E gt  list2            this list1   list1          this list2   list2              Override     public E get final int index            return getList index  get getListIndex index                Override     public E set final int index  final E element            return getList index  set getListIndex index   element               Override     public void add final int index  final E element            getList index  add getListIndex index   element               Override     public E remove final int index            return getList index  remove getListIndex index                Override     public int size             return list1 size     list2 size                Override     public boolean contains final Object o            return list1 contains o     list2 contains o               Override     public void clear             list1 clear            list2 clear                          Returns the index within the corresponding list related to the given index                  param index                   the index in this list                 return the index of the underlying list             private int getListIndex final int index            final int size1   list1 size            return index  gt   size1   index - size1   index                        Returns the list that corresponds to the given index                  param index                   the index in this list                 return the underlying list that corresponds to that index             private List lt E gt  getList final int index            return index  gt   list1 size     list2   list1

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public static  lt T gt  List lt T gt  merge  Nonnull final List lt T gt     list           calculate length first     int mergedLength   0      for  List lt T gt  ts   list          mergedLength    ts size               final List lt T gt  mergedList   new ArrayList lt  gt  mergedLength        for  List lt T gt  ts   list          mergedList addAll ts              return mergedList

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This is simple and just one line  but will add the contents of listTwo to listOne  Do you really need to put the contents in a third list   Collections addAll listOne  listTwo toArray

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Slightly simpler   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Here s an approach using streams and java 8 if your lists have different types and you want to combine them to a list of another type   public static void main String   args        List lt String gt  list2   new ArrayList lt  gt         List lt Pair lt Integer  String gt  gt  list1   new ArrayList lt  gt          list2 add  asd        list2 add  asdaf        list1 add new Pair lt  gt  1   werwe         list1 add new Pair lt  gt  2   tyutyu          Stream stream   Stream concat list1 stream    list2 stream          List lt Pair lt Integer  String gt  gt  res    List lt Pair lt Integer  String gt  gt   stream              map item - gt                    if  item instanceof String                        return new Pair lt  gt  0  item                                     else                       return new Pair lt  gt    Pair lt Integer  String gt  item  getKey      Pair lt Integer  String gt  item  getValue                                                   collect Collectors toList

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Slightly simpler   List lt String gt  newList   new ArrayList lt String gt  listOne   newList addAll listTwo

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Java 8  Stream of and Stream concat    The proposed solution is for three lists though it can be applied for two lists as well  In Java 8 we can make use of Stream of or Stream concat as   List lt String gt  result1   Stream concat Stream concat list1 stream   list2 stream    list3 stream    collect Collectors toList     List lt String gt  result2   Stream of list1 list2 list3  flatMap Collection  stream  collect Collectors toList       Stream concat takes two streams as input and creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream  As we have three lists we have used this method  Stream concat  two times   We can also write a utility class with a method that takes any number of lists  using varargs  and returns a concatenated list as   public static  lt T gt  List lt T gt  concatenateLists List lt T gt     collections            return Arrays stream collections  flatMap Collection  stream  collect Collectors toList          Then we can make use of this method as   List lt String gt  result3   Utils concatenateLists list1 list2 list3

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Not simpler  but without resizing overhead   List lt String gt  newList   new ArrayList lt  gt  listOne size     listTwo size     newList addAll listOne   newList addAll listTwo

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I m not claiming that it s simple  but you mentioned bonus for one-liners  -   Collection mergedList   Collections list new sun misc CompoundEnumeration new Enumeration         new Vector list1  elements        new Vector list2  elements

[java] How do I join two lists in Java?

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