Java generics: multiple generic parameters?

Yes - it's possible (though not with your method signature) and yes, with your signature the types must be the same.

With the signature you have given, T must be associated to a single type (e.g. String or Integer) at the call-site. You can, however, declare method signatures which take multiple type parameters

public <S, T> void func(Set<S> s, Set<T> t)

Note in the above signature that I have declared the types S and T in the signature itself. These are therefore different to and independent of any generic types associated with the class or interface which contains the function.

public class MyClass<S, T> {   public        void foo(Set<S> s, Set<T> t); //same type params as on class   public <U, V> void bar(Set<U> s, Set<V> t); //type params independent of class}

You might like to take a look at some of the method signatures of the collection classes in the java.util package. Generics is really rather a complicated subject, especially when wildcards (? extends and ? super) are considered. For example, it's often the case that a method which might take a Set<Number> as a parameter should also accept a Set<Integer>. In which case you'd see a signature like this:

public void baz(Set<? extends T> s);

There are plenty of questions already on SO for you to look at on the subject!

• Java Generics: List, List<Object>, List<?>
• Java Generics (Wildcards)
• What are the differences between Generics in C# and Java... and Templates in C++?

Not sure what the point of returning an int from the function is, although you could do that if you want!

You can declare multiple type variables on a type or method. For example, using type parameters on the method:

<P, Q> int f(Set<P>, Set<Q>) {  return 0;}

Even more, you can inherit generics :)

@SuppressWarnings("unchecked")public <T extends Something<E>, E extends Enum<E> & SomethingAware> T getSomething(Class<T> clazz) {        return (T) somethingHolderMap.get(clazz);    }