ArrayList c.toArray might (incorrectly) not return Object[] (see 6260652)

    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

这段代码让我疑惑的是当 elementData.getClass() != Object[].class 时为什么需要转换,然后接着找了点资料,很多都只是通过列子来说明问题,没有把造成这个问题的原因说明白

        List<String> list = new ArrayList<>(Arrays.asList("list"));
        System.out.println(list.getClass());//class java.util.ArrayList
        Object[] listArray = list.toArray();
        System.out.println(listArray.getClass());//class [Ljava.lang.Object;
        listArray[0] = new Object();


        List<String> asList = Arrays.asList("asList");
        System.out.println(asList.getClass());//class java.util.Arrays$ArrayList
        Object[] asListArray = asList.toArray();
        System.out.println(asListArray.getClass());//class [Ljava.lang.String;
        asListArray[0] = new Object();//! java.lang.ArrayStoreException


        String[] strings = {new String()};
        Object[] objects = strings;
        objects[0] = new Object();//! java.lang.ArrayStoreException

最主要的原因其实是list.toArray()实现方式不一样，导致返回的数组真实类型不一样

    //java.util.Arrays$ArrayList
    @Override
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        int size = size();
        if (a.length < size)
            return Arrays.copyOf(this.a, size,
                    (Class<? extends T[]>) a.getClass());
        System.arraycopy(this.a, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    //java.util.ArrayList
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }