java generics, how to extend from two classes?

Question

I want to have a Class object, but I want to force whatever class it represents to extend class A and also class B. I can do

Answer 1

A simple way for this problem is inheritance.

public class A { // some codes }

public class B extends A { }

<T extends A>

Answer 2

In java you cannot have a class which extends from two classes, since it doesn't support multiple inheritance. What you can have, is something like so:

public class A
...

public class B extends A
...

public class C extends B
...

In your generic signature, you can then specify that T must extend C: <T extends C>.

You could give a look at Default Methods (if you are working with Java 8), which essentially are methods declared within interfaces, and in Java, a class can implement multiple interfaces.

Answer 3

Java does not have multiple inheritance as a design decision, but you may implement multiple interfaces.

As of Java 8 these interfaces may have implementations.

To use multiple classes there are other patterns:

1. If the parent classes are purely intended for that child class, but handle entirely different aspects, and were therefore separated, place them in a single artificial hierarchy. I admit to doing this once.

2. Use delegation; duplicate the API and delegate.

3. Use a lookup/discovery mechanism for very dynamic behaviour.

   public T lookup(Class klazz);

   This would need an API change, but uncouples classes, and is dynamic.