Java泛型限制类型

Question

In Java i have the following Class Structure:

class All
+-- class A extends All
+-- class B extends All

class ContainAll
+-- class ContainA extends ContainAll
+-- class ContainB extends ContainAll

In every "contain" class there is a linked list, and a Method

public addElement(All a) {
    list.add(a)
}

The class ContainAll should not be instanced, but the class ContainA and ContainB can.
How do i make clear, with generics, that addElement in class ContainB can only get Objects from type B, so for Example:

public abstract class ContainAll<T extends All> {}

public class ContainB<T extends B> extends ContainAll<All> {
    public addElement(T b) {
        list.add(b);
    }
}

But this example does not work, i tried it. Thanks for reading.

Answer 1

Make your ContainAll class abstract (since one of your requirements is to not be able to instantiate it) and make it implement .addElement() :

public abstract class ContainAll<T extends All>
{
    private final List<T> list = new LinkedList<T>();

    // ...

    // FINAL! Subclasses cannot override it
    public final void addElement(final T element)
    {
        list.add(element);
    }
}

Then implement ContainA as ContainAll<A> etc. The .addElement() will work "automatically".

Answer 2

You need to add

public addElement(T b) {
    list.add(b);
}

to your ContainAll class.

Answer 3

Here is a solution:

//ClassA

package limitgeneric;

public class A {

}

//ClassB

package limitgeneric;

public class B {

}

//Class ContainAll

package limitgeneric;

public abstract class ContainAll<T> {
     public abstract void addElement(T t);
}

//Class ContainA

package limitgeneric;

import java.util.ArrayList;
import java.util.List;

public class ContainA<T extends A> extends ContainAll<T> {
     List<T> locallist = new ArrayList<T>();
     public void addElement(T t) {
          locallist.add(t);
     }
}

//Class ContainB

  package limitgeneric;

  import java.util.ArrayList;
  import java.util.List;

  public class ContainB<T extends B> extends ContainAll<T> {
    List<T> locallist = new ArrayList<T>();
    public void addElement(T t) {
        locallist.add(t);
    }
  }

//Main class

  package limitgeneric;

  public class Main {
      public static void main(String[] args) {
          ContainA<A> a = new ContainA<A>();
          //a.addElement(new B());
          a.addElement(new A());
          //ContainA<B> b = new ContainA<B>();
      }
  }