Method argument extends class implements interface

Question

I have the following class and interface:

public class BasicObject{...} public interface CodeObject{...} 

I want to create a method in which the argument needs to be of type BasicObject and implements CodeObject. I tried the following code but it doesn't guarantee clazz to be a class that implements CodeObject.

myMethod(Class<? extends BasicObject> clazz){...} 

I want to do something like this but it doesn't compile:

myMethod(Class<? extends BasicObject implements CodeObject> clazz){...} 

Answer 1

Your pattern class has to extend BasicObject and extend/implement CodeObject (which is actually an interface). You can do it with multiple classes declared in the wildcard definition of the method signature, like this:

public <T extends BasicObject & CodeObject> void myMethod(Class<T> clazz) 

Note that it won't work if you do it any of these ways:

• public <T extends BasicObject, CodeObject> void myMethod(Class<T> clazz)

  This is technically valid syntax, but CodeObject is unused; the method will accept any classes that extends BasicObject, no matter whether they extend/implement CodeObject.

• public void myMethod(Class<? extends BasicObject & CodeObject> clazz)
public void myMethod(Class<? extends BasicObject, CodeObject> clazz)

  These are just wrong syntax according to Java.

Answer 2

Here is an approach which is a bit verbose, but avoids generics headaches. Create another class which does the extending/implementing:

public abstract class BasicCodeObject      extends BasicObject      implements CodeObject {...} 

Then your method can be:

public <T extends BasicCodeObject> void myMethod(Class<T> clazz) {...}