Java extending class types

Question

I have the following code snippet which returns a type mismatch error:

@SuppressWarnings("rawtypes")
public Map<String, Class<? extends SomeClass.SuperClass>> subClass() {
    return Collections.singletonMap("hello", SomeClass.SubClass.class);
}

The class declaration looks like the following:

public class SomeClass {

    public static abstract class SuperClass { }

    public static class SubClass extends SuperClass{ }
}

This returns a type mismatch error, stating:

Type mismatch: cannot convert from Map<String,Class<SubClass>> to Map<String,Class<? extends SuperClass>>

Why is this occurring? Regardless of whether I use "? extends SuperClass" or just "SuperClass" in the parameterized return type of subClass(), I still see this same error. I've also tried just extending SomeClass instead but that hasn't worked either.

Tkachuk_Evgen · Accepted Answer · 2014-12-02 19:42:00Z

2

Your SubClass is subtype of SuperClass, but Map<String, Class<SubClass>> isn't subtype of Map<String, Class<SuperClass>>. For more information you can see Generics, Inheritance, and Subtypes

For Java SE 7 you can use:

return Collections.<String, Class<? extends SomeClass.SuperClass>>singletonMap("hello", SomeClass.SubClass.class);

edited Dec 2, 2014 at 19:42

answered Dec 2, 2014 at 19:15

Tkachuk_Evgen

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2 Comments

b4da Over a year ago

Explicit type parameter is a good point, can Java 8 be calling that automatically?

Tkachuk_Evgen Over a year ago

This are Type inference and Target Typing stuff.

Jatin · Accepted Answer · 2014-12-02 19:25:20Z

2

I think there is a typo. It should be:

@SuppressWarnings("rawtypes")
    public Map<String, Class<? extends SomeClass.SuperClass>> subClass() {
        return Collections.singletonMap("hello", SomeClass.SubClass.class);
    }

You missed .class :)

In Java, generics are invariant. This means if:

Dog extends Animal then List<Dog> does not extend List<Animal>

List<Animal> ls = new ArrayList<Dog>() //illegal

Hence this is invalid code and doesn't work:

Map<String, Class<SomeClass.SuperClass>> hello2 = Collections.singletonMap("hello", SomeClass.SubClass.class);

And this is perfectly alright:

Map<String, Class<? extends SomeClass.SuperClass>> hello = Collections.singletonMap("hello", SomeClass.SubClass.class);
return hello;

edited Dec 2, 2014 at 19:25

answered Dec 2, 2014 at 19:05

Jatin

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6 Comments

b4da Over a year ago

That still gives the Type mismatch error in Netbeans.

Jatin Over a year ago

I am using Java8. And it compiles fine on mine.

b4da Over a year ago

I have Java 7 (1.7.0_67)

user1553248 Over a year ago

So is this only a Java 8 feature? I'm looking into it

b4da Over a year ago

I read the Java 8 whats new page but did not see anything new that might cause this. I thought it might be Netbeans so compiled with javac and Ideone, both did not compile.

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