array cast Java 8 vs Java 9

Question

Does anyone have insights on why this code works on java 8 but not on java 9

String[] strings = (String[]) Arrays.asList(\"foo\", \"bar\").toArray();
for (St         


        

Answer 1

The implementation of Arrays.ArrayList.toArray seems to have been changed. The old implementation was to just clone the backing array:

private final E[] a;

ArrayList(E[] array) {
    a = Objects.requireNonNull(array);
}

@Override
public Object[] toArray() {
    return a.clone();
}

The new implementation forces the returned array to be an Object[]:

@Override
public Object[] toArray() {
    return Arrays.copyOf(a, a.length, Object[].class);
}

To be clear, though, in Java 8 the cast only worked because the backing array was originally a String[], created by the asList varargs. Implicitly all that was happening was new String[] {"foo", "bar"}.clone(), but the array was passed through the asList List implementation.

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As for fixing the broken dependency, I don't think there's a way besides either using a Java 8 run-time environment or rewriting what was introduced in that commit. Filing a bug report seems like the right thing to do.