Why Does Java’s toArray Sometimes Return a Non‑Object[] and Cause ArrayStoreException?

Background

While studying JDK source code, the author noticed bug comments in the constructors of CopyOnWriteArrayList and ArrayList that reference bug IDs 6260652 and 6515694, which actually describe the same problem that remained unfixed in JDK 8.

Case 1

This example shows that assigning a Child[] to a Father[] is allowed, but storing a Father instance into the array triggers java.lang.ArrayStoreException because the runtime type of the array remains Child[].

package com.javaedge;

public class Test {
    public static void main(String[] args) {
        Child[] childArray = {new Child(), new Child()};
        System.out.println(childArray.getClass());

        Father[] fatherArray = childArray;
        System.out.println(fatherArray.getClass());

        // ArrayStoreException
        fatherArray[0] = new Father();
    }
}

Case 2

This case demonstrates that Arrays.asList("JavaEdge") returns an internal class java.util.Arrays$ArrayList, and calling list.toArray() on it returns a String[], not a generic Object[]. Consequently, attempting to store an arbitrary object into that array results in an exception.

List<String> list = Arrays.asList("JavaEdge"); // returns java.util.Arrays$ArrayList
Object[] objects = list.toArray(); // returns String[] array
// cannot store an arbitrary Object into objects

Case 3

This example shows that ArrayList.toArray() returns an Object[], allowing any object type to be stored, which differs from the behavior observed in case 2.

Conclusion

The examples prove that List<String> stringList may produce an array whose actual runtime type is not Object[]. Therefore the JDK comment “c.toArray might (incorrectly) not return Object[] (see 6260652)” is accurate. To avoid ArrayStoreException, one should check the array type before storing or use Arrays.copyOf(elementData, size, Object[].class) to guarantee an Object[] result.