Java generics: multiple generic parameters?

Question

I was wondering if it's possible to write a function that accepts multiple generic types as follows:

public int void myfunction(Set<T> a, Set<T> b) {     return 5; }  Set<Integer> setA = new HashSet<Integer>(); Set<String> setB = new HashSet<String>(); int result = myfunction(setA, setB); 

Will that work? Does the generic in each parameter mean that each parameter must have the same type T that's generic?

Answer 1

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!

Answer 2

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; }