Are unsound type systems wrong?

Re:Are unsound type systems wrong? by brabel on Saturday April 07, 2018 @12:42PM (#16781482)

> If you have a Map<S, T> Java will silently take an object that has NOTHING to do with S and return null because that key is obviously not in the map

        Map map = new HashMap();
        map.put("one", 1);
        
        class Whatever {}

        Map<Whatever, Integer> whateverToIntMap = (Map<Whatever, Integer>) map;

        Integer i = whateverToIntMap.get("one");
        System.out.println("i = " + i);

Prints:

    i = 1

That's a consequence of generics type-erasure for backwards compatibility with non-generic code, so there _is_ a reason for `get` accepting any Object.

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Re:Are unsound type systems wrong? by _asummers on Saturday April 07, 2018 @05:44PM (#16783173)

Like I said, I know the history and why it exists. I learned Java before 1.5 came out. That API existed long before generics did (I think 1.0 or earlier). If they had designed the language with generics in mind, they would have probably done it differently; hindsight is 20/20. Doesn't make it not a suboptimal API. I don't think there's a good way to fix it this late in the game without breaking old code, but such are the warts of backwards compatibility.

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