Cast object array to generic array

Question

Currently, I am viewing the source code of java.util.ArrayList. Now I find the function public void ensureCapacity(int minCapacity) casts an object array to a generic array, just like code below:

 E[] newData = (E[]) new Object[Math.max(current * 2, minCapacity)];

However, when I declare the array to a specific type, IDE will show an error.

Object[] arr = new Object[10];
    int[] arr1 = (int[]) new Object[arr.length];

Any one is able to tell me the differences between them? Thanks a lot.

Konstantin Yovkov · Accepted Answer · 2015-07-03 08:41:43Z

4

It's because E (in the source code of ArrayList) stands for some reference type, but not for some primitive type.

And that's why you get a compile-time error when trying to cast an array of Object instances to an array of primitives.

If you do (for example)

Object[] arr = new Object[10];
Integer[] arr1 = (Integer[]) new Object[arr.length];

the error will be gone.

answered Jul 3, 2015 at 8:41

Konstantin Yovkov

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Erwin Bolwidt · Accepted Answer · 2015-07-03 08:41:14Z

3

You can never cast a reference type (anything that extends from Object) to a primitive type (int, long, boolean, char, etc.).

You can also not cast an array of a reference type like Object[] to an array of a primitive type like int[].

And primitives cannot stand in for a generic parameter.

answered Jul 3, 2015 at 8:41

Erwin Bolwidt

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3 Comments

Mattias Buelens Over a year ago

One exception though: auto-boxing, e.g. boxing a primitive int into an Integer object. But you're correct about array casts: even with auto-boxing, you cannot cast an int[] into an Integer[].

Vince Over a year ago

@MattiasBeulens But that's not casting. That's simply wrapping the value

Mattias Buelens Over a year ago

@VinceEmigh Technically correct, but it looks like a cast so I thought I'd at least mention it.

TheBakker · Accepted Answer · 2015-07-03 08:42:52Z

2

int is not Object, but it's primitive.

Use Integer and it will work.

Object[] arr = new Object[10];
    Integer[] arr1 = (Integer[]) new Object[arr.length];

edited Jul 3, 2015 at 8:42

answered Jul 3, 2015 at 8:41

TheBakker

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