Using a generic type of a subclass within it's abstract superclass?

问题

Within my code a have the following abstract superclass

public abstract class AbstractClass<Type extends A> {...}

and some child classes like

public class ChildClassA extends AbstractClass<GenericTypeA> {...}

public class ChildClassB extends AbstractClass<GenericTypeB> {...}

I'm searching for an elegant way how I can use the generic type of the child classes (GenericTypeA, GenericTypeB, ...) inside the abstract class in a generic way.

To solve this problem I currently defined the method

protected abstract Class<Type> getGenericTypeClass();

in my abstract class and implemented the method

@Override
protected Class<GenericType> getGenericTypeClass() {
    return GenericType.class;
}

in every child class.

Is it possible to get the generic type of the child classes in my abstract class without implementing this helper method?

BR,

Markus

回答1:

I think its possible. I saw this was being used in the DAO patterns along with generics. e.g. Consider classes:

public class A {}
public class B extends A {}

And your generic class:

  import java.lang.reflect.ParameterizedType;
  public abstract class Test<T extends A> {

     private Class<T> theType;

     public Test()  {
        theType = (Class<T>) (
               (ParameterizedType) getClass().getGenericSuperclass())
              .getActualTypeArguments()[0];
     }

     // this method will always return the type that extends class "A"
     public Class<T> getTheType()   {
        return theType;
     }

     public void printType() {
        Class<T> clazz = getTheType();
        System.out.println(clazz);
     }
  }

You can have a class Test1 that extends Test with class B (it extends A)

  public class Test1 extends Test<B>  {

     public static void main(String[] args) {
        Test1 t = new Test1();

        Class<B> clazz = t.getTheType();

        System.out.println(clazz); // will print 'class B'
        System.out.println(printType()); // will print 'class B'
     }
  }

回答2:

I'm not sure I fully understand your question - <Type> is the generic type of the subclass, even when it's being expressed in the abstract class. For example, if your abstract superclass defines a method:

public void augment(Type entity) {
   ...
}

and you instantiate a ChildClassA, you'll only be able to call augment with an instance of GenericTypeA.

Now if you want a class literal, then you'll need to provide the method as you indicated. But if you just want the generic parameter, you don't need to do anything special.

来源：https://stackoverflow.com/questions/3139018/using-a-generic-type-of-a-subclass-within-its-abstract-superclass

标签

java

generics

class

abstract-class