How to cast Object[] to int[] java?

Question

This leads to complie error, I think this is because int is not inherited from Object, but is there any solution without traversing every item to do casting?

Object[] A1 ={1,2,3,4};
int [] B1 = (int[]) A1;

There is no problem with casting to Integer

Object[] A1 ={1,2,3,4};
Integer [] B1 = (Integer[]) A1;

I know we can do

   int [] C = new int[B1.length];
   for(int i=0;i<B1.length;i++)
     C[i] = Integer.valueOf(B[i]);

But, is there any auto casting method?

Possible duplicate of http://stackoverflow.com/questions/1115230/casting-object-array-to-integer-array-error — toro, Jul 25 '16 at 23:46
Now that you've fixed your title, the answer is that you declared the array as `Object[]`, so that's what it is, forever. No upcast can change that. — user207421, Jul 25 '16 at 23:51
`Integer [] B1 = (Integer[]) A1;` - that might not be a compile-time error, but [it is a run-time error](http://ideone.com/4qWchz). — user2357112, Jul 25 '16 at 23:51

sprinter · Accepted Answer · 2016-07-26T00:49:25.030

4

As you have acknowledged in your question, int does not extend Object so the cast makes no sense and the compiler correctly complains.

Likely the safest and easiest way to achieve this is:

Object[] objects = {1, 2, 3, 4};
int[] ints = Arrays.stream(objects).mapToInt(o -> (int)o).toArray();

Not particularly elegant, but then again neither is storing an array of integers in an array of Object :-)

edited Jul 26 '16 at 00:49

answered Jul 26 '16 at 00:11

sprinter

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Why not `Arrays.stream(objects).mapToInt(o -> (int)o)`? – shmosel Jul 26 '16 at 00:17
If that was your concern, `o -> (Integer)o` would be equally valid. – shmosel Jul 26 '16 at 00:50

score 2 · Answer 2 · answered Jul 26 '16 at 00:23

There is no way to cast an Object[] to an int[], even if all the objects in the source array are Integer objects (like your example), because all the values need to be unboxed, not just cast.

Object[] A1 = {1,2,3,4};

That statement is actually auto-boxing the 4 integer literals for you, so the compiler is turning it into this:

Object[] A1 = new Object[4];
A1[0] = Integer.valueOf(1);
A1[1] = Integer.valueOf(2);
A1[2] = Integer.valueOf(3);
A1[3] = Integer.valueOf(4);

As can be seen from this decompilation:

public static void main(java.lang.String[]) throws java.lang.Exception;
  Code:
     0: iconst_4
     1: anewarray     #3                  // class java/lang/Object
     4: dup
     5: iconst_0
     6: iconst_1
     7: invokestatic  #19                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
    10: aastore
    11: dup
    12: iconst_1
    13: iconst_2
    14: invokestatic  #19                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
    17: aastore
    18: dup
    19: iconst_2
    20: iconst_3
    21: invokestatic  #19                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
    24: aastore
    25: dup
    26: iconst_3
    27: iconst_4
    28: invokestatic  #19                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
    31: aastore
    32: astore_1
    33: return

Any solution must do the traversing of every item to unbox the values, whether explicitly or implicitly.

Various ways to do that:

// Using cast to specific type and auto-unboxing (object cast)
int[] arr1 = new int[A1.length];
for (int i = 0; i < A1.length; i++)
    arr1[i] = (Integer)A1[i];

// Using cast to specific type and explicit unboxing
int[] arr2 = new int[A1.length];
for (int i = 0; i < A1.length; i++)
    arr2[i] = ((Integer)A1[i]).intValue();

// Using cast to more generic type and value extraction
int[] arr3 = new int[A1.length];
for (int i = 0; i < A1.length; i++)
    arr3[i] = ((Number)A1[i]).intValue();

// Using Java 8 streams with method references
int[] arr4 = Arrays.stream(A1)
                   .map(Integer.class::cast)
                   .mapToInt(Integer::intValue)
                   .toArray();

// Using Java 8 streams with lambda expression with auto-unboxing (primitive cast)
int[] arr5 = Arrays.stream(A1)
                   .mapToInt(o -> (int)o)
                   .toArray();

How to cast Object[] to int[] java?

2 Answers2