Java HashMap generics with Class key

Question

How do I stop the compiler from complaining at map.get()?

"Type mismatch: cannot convert from ClassInfo<capture#1-of ?> to ClassInfo<T>"

class Context
{
    final private Map<Class<?>,ClassInfo<?>> map = new HashMap<>();

    <T> ClassInfo<T> getClassInfo(Class<T> c)
    {
        ClassInfo<T> ci = map.get(c);
        if (ci == null) {
            ci = new ClassInfo<T>(c);
            map.put(c, ci);
        }
        return(ci);
    }
}

Some more information:

ClassInfo contains data about Class, gathered via reflection.

The compiler error does NOT occur with the JDK javac, only when using the Eclipse compiler.

what do you think this line is supposed to do? <T> ClassInfo<T> getClassInfo(Class<T> c) — user177800
– user177800, Commented Nov 28, 2013 at 17:51

Flame_Phoenix · Accepted Answer · 2013-11-28 17:55:36Z

1

I fixed the problem adding a ()cast to the code:

import java.util.HashMap; import java.util.Map;

class Context
{
    private final Map<Class<?>,ClassInfo<?>> map = new HashMap<Class<?>,ClassInfo<?>>();

    <T> ClassInfo<T> getClassInfo(Class<T> c)
    {
        @SuppressWarnings("unchecked")
        ClassInfo<T> ci = (ClassInfo<T>) map.get(c);
        if (ci == null) {
            ci = new ClassInfo<T>(c);
            map.put(c, ci);
        }
        return(ci);
    }
}

Be aware though, casts are usually dangerous, and you should only use them if you are really sure that object is in fact what you are expecting.

The downside of this solution is that it will create a warning, which I had to suppress. Hope it helps!

answered Nov 28, 2013 at 17:55

Flame_Phoenix

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2 Comments

caprona Over a year ago

Doesn't really work. Now there is a compiler error and a compiler warning. The error reads: Type safety: Unchecked cast from ... to .... And the warning reads: Unnecessary @SuppressWarnings("unchecked"). It could be that my eclipse compiler settings are a little bit too restricted.

caprona Over a year ago

I enabled the following compiler option of Eclipse: "Suppress optional errors with '@SuppressWarnings'". Now the code is without errors or warnings. But when executing this method, I get an Error exception "Unresolved compilation problem".

Eldhose M Babu · Accepted Answer · 2013-11-30 04:25:03Z

1

Just cast it and use :

@SuppressWarnings("unchecked")
ClassInfo<T> ci = (ClassInfo<T>) map.get(c);

Make sure that you are putting Type T into the hash map other wise casting exception can happen.

That is if T represents String, then map.get(c) should always return either null or ClassInfo

edited Nov 30, 2013 at 4:25

answered Nov 28, 2013 at 17:57

Eldhose M Babu

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2 Comments

caprona Over a year ago

The compiler error changed from "Type mismatch" to "Type safety"... (see first answer)

Eldhose M Babu Over a year ago

that is just a warning.. to clear that warning, check my updated answer

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