Java Generics – How to Get Class of Generic Type

classgenericsjavaparameterizedtypes

My class starts with

public abstract class LastActionHero<H extends Hero>(){

Now somewhere in the code I want to write H.class but that isn't possible (like String.class or Integer.class is).

Can you tell me how I can get the Class of the generic?

Best Answer

We do it in the following way:

    private Class<T> persistentClass;

    public Class<T> getPersistentClass() {
        if (persistentClass == null) {
            this.persistentClass = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        }
        return persistentClass;
    }

Related Solutions

How to Obtain Class Instance of Generic Argument Type in Java

You can't find out T at execution time unless you add a constructor parameter to take Class<T> parameter and pass call it with Double.class in your case. You can then store that class and use it later.

This is due to type erasure... basically generics are just compiler magic to insert appropriate casts and check that things should be okay.

See the Type Erasure section of the excellent Java Generics FAQ.

You might also want to read Neal Gafter's blog posts about super type tokens and type literals.

Java Generics – Get Class of Generic Parameter

The typesafe, but boilerplatey way to do this is to pass the Class<T> token "where the compiler can see it":

public abstract class RootProcessor<T> {
    Class<T> clazz;

    protected RootProcessor<T>(Class<T> clazz) {
        this.clazz = clazz;
    }
}

public class FooProcessor extends RootProcessor<Foo> {
    public FooProcessor() {
        super(Foo.class);
    }
}

If you're doing an unchecked cast but you "know what you're doing" and want the compiler to stop complaining, the correct approach would be localising the non-type-safe-but-you-know-they-work bits and using @SuppressWarnings:

public abstract class RootProcessor<T> {
    Class<T> clazz;
    { initClazz(); }

    @SuppressWarnings("unchecked")
    private void initClazz() {
        // the usual verbiage you already have in your question
        this.clazz = this.getClass().getGenericSuperclass().yadda().blah();
    }
}

(I won't hold this against you :P)

Best Answer

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How to Obtain Class Instance of Generic Argument Type in Java

Java Generics – Get Class of Generic Parameter

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