Get class of generic

问题:

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?

回答1:

You can provide the type dynamically, however the compiler doesn't do this for you automagically.

public abstract class LastActionHero<H extends Hero>(){
    protected final Class<H> hClass;
    protected LastActionHero(Class<H> hClass) {
        this.hClass = hClass;
    }
    // use hClass how you like.
}

BTW: It not impossible to get this dynamically, but it depends on how it is used. e.g

public class Arnie extends LastActionHero<MuscleHero> { }

It is possible to determine that Arnie.class has a super class with a Generic parameter of MuscleHero.

public class Arnie<H extend Hero> extends LastActionHero<H> { }

The generic parameter of the super class will be just H in this case.

回答2:

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;
    }

回答3:

One way is to keep reference to your parameterized type like having an attribute of

private Class<H> clazz;

And create a setter or a constructor that takes in a Class<H>.

Parameterized Types are erased at runtime, hence why you can't do what you ask.

回答4:

You can do it without passing in the class:

public abstract class LastActionHero<H extends Hero>() {
  Class<H> clazz = (Class<H>) DAOUtil.getTypeArguments(LastActionHero.class, this.getClass()).get(0);
}

You need two functions from this file: http://code.google.com/p/hibernate-generic-dao/source/browse/trunk/dao/src/main/java/com/googlecode/genericdao/dao/DAOUtil.java

For more explanation: http://www.artima.com/weblogs/viewpost.jsp?thread=208860

回答5:

You can't- the type is erased at run-time and exists only at compile-time.