java generic class cast

Question

Hello i have got hash map with value:

    private static final LinkedHashMap<Class<? extends Annotation> , Class<? extends AGenerator<?>>> annotationGeneratorMap = new LinkedHashMap<Class<? extends Annotation>, Class<? extends AGenerator<?>>>();

and my numeric Generator looks like :

public abstract class NumericGenerator<T extends Number> extends AGenerator<T>{


public NumericGenerator(Field field) {
    super(field);
    // TODO Auto-generated constructor stub
}

public T random() {
    // TODO Auto-generated method stub
    return null;
}

  }

And i have got problem when i need to put this class into hashmap :

annotationGeneratorMap.put(GenerateNumeric.class, NumericGenerator.class);

And in eclipse i have got error the method is not applicable for the argument ???

but :

        annotationGeneratorMap.put(GenerateNumeric.class, (Class<? extends AGenerator<?>>) NumericGenerator.class);

and @SuppressWarnings("unchecked") is good ..:/

Can i do this without casting ?? (Class<? extends AGenerator<?>>) NumericGenerator.class

irreputable · Accepted Answer · 2012-09-17 21:48:31Z

2

use Class<? extends AGenerator> instead

LinkedHashMap<
    Class<? extends Annotation>, 
    Class<? extends AGenerator> > annotationGeneratorMap = new LinkedHashMap<>()

In Java, class can represent a raw type, but cannot represent a generic type.

There is a class for List, but there is no class for List<String>.

So you can declare Class<? extends List>, which is compatible with ArrayList.class etc. because raw type ArrayList is a subtype of List.

But Class<? extends List<?>> doesn't make much sense, because there is no class that is a subtype of List<?>.

And all thanks to erasure.

edited Sep 17, 2012 at 21:48

answered Sep 17, 2012 at 21:40

irreputable

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Comments

nullpotent · Accepted Answer · 2012-09-17 21:57:22Z

1

Compile time check won't allow you to do that unless you cast it of course.

Information about generic types is lost at runtime, not compile time.

answered Sep 17, 2012 at 21:57

nullpotent

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Miserable Variable · Accepted Answer · 2012-09-17 22:11:21Z

1

Following compiles without error on version 1.7.0_02:

import java.util.*;
import java.lang.annotation.*;

interface AGenerator<T> {}

interface A extends Annotation {}
class X implements AGenerator<X> {}

class E7 {
    private static final LinkedHashMap<Class<? extends Annotation>, 
        Class<? extends AGenerator<?>>> annotationGeneratorMap 
        = new LinkedHashMap<Class<? extends Annotation>, 
                            Class<? extends AGenerator<?>>>();


    void foo() {
        annotationGeneratorMap.put(A.class, X.class);
    }

}

answered Sep 17, 2012 at 22:11

Miserable Variable

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2 Comments

irreputable Over a year ago

yes, X is a subtype of AGenerator<?>. but X is not generic, unlike OP's example.

Łukasz Woźniczka Over a year ago

@Miserable Variable but consider scenario like that that your X class user generics: class X<T extends Numver> implements AGenerator<T> this is not allowed :/

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