How to convert an ArrayList containing Integers to primitive int array

Question

I m trying to convert an ArrayList containing Integer objects to primitive int   with the following piece of code  but it is throwing compile time error  Is it possible to convert in Java   List lt Integer gt  x    new ArrayList lt Integer gt     int   n    int   x toArray int x size

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Apache Commons has a ArrayUtils class  which has a method toPrimitive   that does exactly this   import org apache commons lang ArrayUtils          List lt Integer gt  list   new ArrayList lt Integer gt         list add new Integer 1        list add new Integer 2        int   intArray   ArrayUtils toPrimitive list toArray new Integer 0       However  as Jon showed  it is pretty easy to do this by yourself instead of using external libraries

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List lt Integer gt  list   new ArrayList lt Integer gt          list add 1       list add 2        int   result   null      StringBuffer strBuffer   new StringBuffer        for  Object o   list            strBuffer append o           result   new int     Integer parseInt strBuffer toString               for  Integer i   result                System out println i                     strBuffer delete 0  strBuffer length

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You can simply copy it to an array   int   arr   new int list size     for int i   0  i  lt  list size    i          arr i    list get i       Not too fancy  but  hey  it works

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If you re using Eclipse Collections  you can use the collectInt   method to switch from an object container to a primitive int container   List lt Integer gt  integers   new ArrayList lt  gt  Arrays asList 1  2  3  4  5    MutableIntList intList     ListAdapter adapt integers  collectInt i - gt  i   Assert assertArrayEquals new int   1  2  3  4  5   intList toArray       If you can convert your ArrayList to a FastList  you can get rid of the adapter   Assert assertArrayEquals    new int   1  2  3  4  5     Lists mutable with 1  2  3  4  5       collectInt i - gt  i  toArray       Note  I am a committer for Eclipse collections

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You can convert  but I don t think there s anything built in to do it automatically   public static int   convertIntegers List lt Integer gt  integers        int   ret   new int integers size         for  int i 0  i  lt  ret length  i                  ret i    integers get i  intValue              return ret       Note that this will throw a NullPointerException if either integers or any element within it is null    EDIT  As per comments  you may want to use the list iterator to avoid nasty costs with lists such as LinkedList   public static int   convertIntegers List lt Integer gt  integers        int   ret   new int integers size         Iterator lt Integer gt  iterator   integers iterator        for  int i   0  i  lt  ret length  i                  ret i    iterator next   intValue              return ret

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Integer   arr    Integer    x toArray new Integer x size        access arr like normal int

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This works nice for me     Found at https   www techiedelight com convert-list-integer-array-int   import java util Arrays  import java util List   class ListUtil          Program to convert list of integer to array of int in Java     public static void main String args                  List lt Integer gt  list   Arrays asList 1  2  3  4  5            int   primitive   list stream                                mapToInt Integer  intValue                               toArray             System out println Arrays toString primitive

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Java 8   int   intArr   Arrays stream integerList  mapToInt i- gt i  toArray

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Arrays setAll   will work for most scenarios   Integer List to primitive int array  public static int   convert final List lt Integer gt  list       final int   out   new int list size        Arrays setAll out  list  get      return out      Integer List  made of Strings  to primitive int array  public static int   convert final List lt String gt  list       final int   out   new int list size        Arrays setAll out  i - gt  Integer parseInt list get i        return out      Integer array to primitive int array  public static int   convert final Integer   array       final int   out   new int array length      Arrays setAll out  i - gt  array i       return out      Primitive int array to Integer array  public static Integer   convert final int   array       final Integer   out   new Integer array length      Arrays setAll out  i - gt  array i       return out

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Arrays setAll        List lt Integer gt  x   new ArrayList lt  gt  Arrays asList 7  9  13        int   n   new int x size         Arrays setAll n  x  get        System out println  Array of primitive ints      Arrays toString n      Output      Array of primitive ints   7  9  13    The same works for an array of long or double  but not for arrays of boolean  char  byte  short or float  If you   ve got a really huge list  there   s even a parallelSetAll method that you may use instead   To me this is good and elgant enough that I wouldn   t want to get an external library nor use streams for it   Documentation link  Arrays setAll int    IntUnaryOperator

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A very simple one-line solution is   Integer   i   arrlist stream   toArray Integer    new

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It bewilders me that we encourage one-off custom methods whenever a perfectly good  well used library like Apache Commons has solved the problem already   Though the solution is trivial if not absurd  it is irresponsible to encourage such a behavior due to long term maintenance and accessibility   Just go with Apache Commons

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This code segment is working for me  try this   Integer   arr   x toArray new Integer x size        Worth to mention ArrayList should be declared like this   ArrayList lt Integer gt  list   new ArrayList lt  gt

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using Dollar should be quite simple   List lt Integer gt  list     5  toList       the list 0  1  2  3  4   int   array       list  toArray    toIntArray      I m planning to improve the DSL in order to remove the intermediate toArray   call

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Next lines you can find convertion from int   - gt  List - gt  int      private static int   convert int   arr            List lt Integer gt  myList new ArrayList lt Integer gt             for int number arr                  myList add number                                   int   myArray new int myList size             for int i 0 i lt myList size   i                myArray i  myList get i                     return myArray

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I believe iterating using the List s iterator is a better idea  as list get i  can have poor performance depending on the List implementation   private int   buildIntArray List lt Integer gt  integers        int   ints   new int integers size         int i   0      for  Integer n   integers            ints i      n            return ints

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If you are using java-8 there s also another way to do this   int   arr   list stream   mapToInt i - gt  i  toArray      What it does is    getting a Stream lt Integer gt  from the list obtaining an IntStream by mapping each element to itself  identity function   unboxing the int value hold by each Integer object  done automatically since Java 5  getting the array of int by calling toArray   You could also explicitly call intValue via a method reference  i e   int   arr   list stream   mapToInt Integer  intValue  toArray      It s also worth mentioning that you could get a NullPointerException if you have any null reference in the list  This could be easily avoided by adding a filtering condition to the stream pipeline like this                             filter Objects  nonNull  also works int   arr   list stream   filter i - gt  i    null  mapToInt i - gt  i  toArray      Example   List lt Integer gt  list   Arrays asList 1  2  3  4   int   arr   list stream   mapToInt i - gt  i  toArray       1  2  3  4   list set 1  null      1  null  3  4  arr   list stream   filter i - gt  i    null  mapToInt i - gt  i  toArray       1  3  4

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Google Guava  Google Guava provides a neat way to do this by calling Ints toArray   List lt Integer gt  list        int   values   Ints toArray list

[java] How to convert an ArrayList containing Integers to primitive int array?

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