freestar freestar - 5 years ago 823
Java Question

Java: ArrayStoreException

I would like fill an external array with the elements of my ArrayQueue via

E[] toArray(E[] a)
, but somehow it throws
at the first
method. I would like to know how to solve this issue, and even more important for me to know why this exception is thrown.

Here's my code:

public E[] toArray(E[] a)
if(a.length != size)
a=(E[])Array.newInstance(a.getClass(), size);
System.arraycopy(elementData, head, a, 0, tail-head); // ArrayStoreException
System.arraycopy(elementData, head, a, 0, capacity-head);
System.arraycopy(elementData, 0, a, capacity-head, tail);
return a;

This is the external method:

String[] words = q.toArray(new String[2]);

Thanks for your time.

Answer Source

I suspect the exception doesn't actually happen on the line you've indicated, but later, in System.arraycopy.

The problem is that your call to Array.newInstance passes in the array type, when you only want to pass in the element type. In other words, you're saying "Give me a new array with an element type String[]" where you really want to say "Give me a new array with an element type String".

To do this, just use getClass().getComponentType(). Demo which is a simpler but complete example of the problem, if you remove getComponentType():

import java.lang.reflect.*;

public class Test {

    public static void main(String[] args) {
        String[] original = new String[] { "x" };
        String[] x =  Test.<String>toArray(original);

    public static <E> E[] toArray(E[] a) {
        E[] copy = (E[]) Array.newInstance(a.getClass().getComponentType(), a.length + 1);
        System.arraycopy(a, 0, copy, 0, a.length);
        return copy;
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