Map with Class as keys and values having objects operating on key class instances

Question

I have a problem with Java Generics and I'm not sure if it's even possible what I want to get. Basically title says it all but let me give you an example. Currently I have something like this:

private static Map<Class<? extends Exception>, ExceptionTranslator<?>> MAP;

which is being used in a following way:

MAP.put(SomeException.class, new ExceptionTranslator<SomeException>());

But I would like to enforce a translator of the exception type passed as a key, something similar to what you do on methods:

<T extends Exception> Map<Class<T>, ExceptionTranslator<T>> MAP;

This of course is not a correct code and I guess my generics-foo is not strong enough for this. Is it even possible in Java?

Why your class doesn't has the <T extends Exception> generic in its definition? — Luiggi Mendoza
– Luiggi Mendoza, Commented Aug 9, 2013 at 22:27
You can't do this with just a normal Map; you have to do your own wrapping with a method that enforces the type constraint. — Louis Wasserman
– Louis Wasserman, Commented Aug 9, 2013 at 22:38
Thanks, @Louis. "You can't do it" is a valid response and I will accept your answer if you give me the chance. And well, I guess I will need to do what you guys did in Guava: code.google.com/p/guava-libraries/source/browse/guava/src/com/… — Mike Minicki
– Mike Minicki, Commented Aug 9, 2013 at 23:03

kan · Accepted Answer · 2013-08-09 23:15:45Z

I just want to explain why it is impossible.

The problem is that it is impossible to guarantee type safety. The Map relies on equals/hashMap, which is impossible for compiler to prove correctness of these methods in relation with T type.

Imagine if instead of Class you would use your own generic class which will return equals==true for all its instances. In this case you could put into the map one type, but extract with another, it will produce cast exceptions:

private<E extends Number> void mp()
{
    Map<Key<E>, E> map = new HashMap<>();
    map.put(new Key<Integer>(), 42);// Yes, there is compilation error!
    // Otherwise we could do this:
    String str = map.get(new Key<String>());// cast exception!
}

class Key<T>
{
    @Override
    public int hashCode()
    {
        return 0;
    }

    @Override
    public boolean equals(final Object obj)
    {
        return obj instanceof Key;
    }
}

Community · Accepted Answer · 2017-05-23 11:44:37Z

0

You can't do this with just a normal Map; you have to do your own wrapping with a method that enforces the type constraint. See the related question Java generics enforce same type for keys and values of map.

edited May 23, 2017 at 11:44

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answered Aug 9, 2013 at 23:07

Louis Wasserman

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