How to convert int into List Integer in Java

Question

How do I convert int   into List lt Integer gt  in Java   Of course  I m interested in any other answer than doing it in a loop  item by item  But if there s no other answer  I ll pick that one as the best to show the fact that this functionality is not part of Java

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It s also worth checking out this bug report  which was closed with reason  Not a defect  and the following text    Autoboxing of entire arrays is not specified behavior  for good reason  It can be prohibitively expensive for large arrays

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In Java 8 with stream   int   ints    1  2  3   List lt Integer gt  list   new ArrayList lt Integer gt     Collections addAll list  Arrays stream ints  boxed   toArray Integer    new      or with Collectors  List lt Integer gt  list    Arrays stream ints  boxed   collect Collectors toList

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The smallest piece of code would be   public List lt Integer gt  myWork int   array        return Arrays asList ArrayUtils toObject array        where ArrayUtils comes from commons-lang

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What about this   int   a    1 2 3   Integer   b   ArrayUtils toObject a   List lt Integer gt  c   Arrays asList b

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I ll add another answer with a different method  no loop but an anonymous class that will utilize the autoboxing features   public List lt Integer gt  asList final int   is        return new AbstractList lt Integer gt                  public Integer get int i    return is i                 public int size     return is length

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Arrays stream ints  forEach list  add

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In Java 8     int   arr    1 2 3   IntStream of arr  boxed   collect Collectors toList

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Arrays asList will not work as some of the other answers expect   This code will not create a list of 10 integers  It will print 1  not 10   int arr       1  2  3  4  5  6  7  8  9  10    List lst   Arrays asList arr   System out println lst size       This will create a list of integers   List lt Integer gt  lst   Arrays asList 1  2  3  4  5  6  7  8  9  10     If you already have the array of ints  there is not quick way to convert  you re better off with the loop   On the other hand  if your array has Objects  not primitives in it  Arrays asList will work   String str        Homer    Marge    Bart    Lisa    Maggie     List lt String gt  lst   Arrays asList str

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Integer   to List lt Integer gt                Integer   intArr     10  20  30  40  50  60  70  80  90  100            List lt Integer gt  arrList   new ArrayList lt  gt             arrList addAll Arrays asList intArr            System out println arrList        Integer   to Collection lt Integer gt           Integer   intArr     10  20  30  40  50  60  70  80  90  100        Collection lt Integer gt  c   Arrays asList intArr

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give a try to this class   class PrimitiveWrapper lt T gt  extends AbstractList lt T gt         private final T   data       private PrimitiveWrapper T   data            this data   data     you can clone this array for preventing aliasing            public static  lt T gt  List lt T gt  ofIntegers int    data            return new PrimitiveWrapper toBoxedArray Integer class  data               public static  lt T gt  List lt T gt  ofCharacters char    data            return new PrimitiveWrapper toBoxedArray Character class  data               public static  lt T gt  List lt T gt  ofDoubles double    data            return new PrimitiveWrapper toBoxedArray Double class  data                    ditto for byte  float  boolean  long      private static  lt T gt  T   toBoxedArray Class lt T gt  boxClass  Object components            final int length   Array getLength components           Object res   Array newInstance boxClass  length            for  int i   0  i  lt  length  i                  Array set res  i  Array get components  i                       return  T    res              Override     public T get int index            return data index               Override     public int size             return data length            testcase   List lt Integer gt  ints   PrimitiveWrapper ofIntegers 10  20   List lt Double gt  doubles   PrimitiveWrapper ofDoubles 10  20      etc

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Here is another possibility  again with Java 8 Streams   void intArrayToListOfIntegers int   arr  List lt Integer gt  list        IntStream range 0  arr length  forEach i - gt  list add arr i

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If you are using java 8  we can use the stream API to convert it into a list    List lt Integer gt  list   Arrays stream arr         IntStream                                   boxed             Stream lt Integer gt                                   collect Collectors toList       You can also use the IntStream to convert as well    List lt Integer gt  list   IntStream of arr     return Intstream                                      boxed             Stream lt Integer gt                                       collect Collectors toList       There are other external library like guava and apache commons   also available convert it    cheers

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int   arr     1  2  3  4  5     List lt Integer gt  list   Arrays stream arr         IntStream                              boxed             Stream lt Integer gt                               collect Collectors toList       see this

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Here is a solution   int   array     1  2  3  4  5  6  7  8  9  10     Integer   iArray   Arrays stream array  boxed   toArray Integer    new   System out println Arrays toString iArray     List lt Integer gt  list   new ArrayList lt  gt     Collections addAll list  iArray   System out println list     Output    1  2  3  4  5  6  7  8  9  10   1  2  3  4  5  6  7  8  9  10

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If you re open to using a third party library  this will work in Eclipse Collections   int   a    1  2  3   List lt Integer gt  integers   IntLists mutable with a  collect i - gt  i   Assert assertEquals Lists mutable with 1  2  3   integers     Note  I am a committer for Eclipse Collections

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Streams  In Java 8 you can do this  int   ints    1 2 3   List lt Integer gt  list   Arrays stream ints  boxed   collect Collectors toList

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Also from guava libraries    com google common primitives Ints   List lt Integer gt  Ints asList int

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Here is a generic way to convert array to ArrayList   lt T gt  ArrayList lt T gt  toArrayList Object o  Class lt T gt  type       ArrayList lt T gt  objects   new ArrayList lt  gt         for  int i   0  i  lt  Array getLength o   i                noinspection unchecked         objects add  T  Array get o  i              return objects      Usage  ArrayList lt Integer gt  list   toArrayList new int   1 2 3   Integer class

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There is no shortcut for converting from int   to List lt Integer gt  as Arrays asList does not deal with boxing and will just create a List lt int   gt  which is not what you want  You have to make a utility method   int   ints    1  2  3   List lt Integer gt  intList   new ArrayList lt Integer gt  ints length   for  int i   ints        intList add i

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The best shot         Integer modifiable fix length list of an int array or many int s         author Daniel De Leon      public class IntegerListWrap extends AbstractList lt Integer gt         int   data       public IntegerListWrap int    data            this data   data              Override     public Integer get int index            return data index               Override     public Integer set int index  Integer element            int r   data index           data index    element          return r              Override     public int size             return data length             Support get and set  No memory data duplication  No wasting time in loops    Examples   int   intArray   new int   1  2  3   List lt Integer gt  integerListWrap   new IntegerListWrap intArray   List lt Integer gt  integerListWrap1   new IntegerListWrap 1  2  3

[java] How to convert int[] into List<Integer> in Java?

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