Question

I understand what an ArrayStoreException is. My question is: why isn’t this caught by the compiler?

This might be an odd example, but say you do this:

    HashMap[] h = new LinkedHashMap[4];
    h[0] = new PrinterStateReasons();

Why can’t the compiler recognize that this isn’t valid?

Answer 1

Because the information you’ve given the compiler allows what you’re doing. It’s only the runtime state that’s invalid. Your h variable is declared as being a HashMap[], which means that as far as h is concerned, anything implementing HashMap is a valid element. PrinterStateReasons implements HashMap, and so h[0] = new PrinterStateReasons(); is a perfectly valid statement. Similarly, since LinkedHashMap implements HashMap, the statement HashMap[] h = new LinkedHashMap[4]; is a perfectly valid statement. It’s only at runtime that you try to store a PrinterStateReasons object as an element in a LinkedHashMap array, which you can’t do as it isn’t assignment-compatible.

The two statements you’ve given are contiguous, but of course the generalized reality is far more complex. Consider:

HashMap[] h = foo.getHashMapArray();
h[0] = new PrinterStateReasons();

// ... elsewhere, in some `Foo` class -- perhaps compiled
// completely separately from the code above, perhaps
// even by a completely different team and even a different
// compiler --  and only combined with the code above at runtime...

public HashMap[] getHashMapArray() {
    return new LinkedHashMap[4];
}