[java] Java generics: multiple generic parameters?

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?

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;
}

You can follow one of the below approaches:

1) Basic, single type :

//One type
public static <T> void fill(List <T> list, T val) {

    for(int i=0; i<list.size(); i++){
        list.set(i, val);
    }

}

2) Multiple Types :

// multiple types as parameters
public static <T1, T2> String multipleTypeArgument(T1 val1, T2 val2) {

    return val1+" "+val2;

}

3) Below will raise compiler error as 'T3 is not in the listing of generic types that are used in function declaration part.

//Raised compilation error
public static <T1, T2> T3 returnTypeGeneric(T1 val1, T2 val2) {
    return 0;
}

Correct : Compiles fine

public static <T1, T2, T3> T3 returnTypeGeneric(T1 val1, T2 val2) {
    return 0;
}

Sample Class Code :

package generics.basics;

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

public class GenericMethods {

/*
 Declare the generic type parameter T in this method. 

 After the qualifiers public and static, you put <T> and 
 then followed it by return type, method name, and its parameters.

 Observe : type of val is 'T' and not '<T>'

 * */
//One type
public static <T> void fill(List <T> list, T val) {

    for(int i=0; i<list.size(); i++){
        list.set(i, val);
    }

}

// multiple types as parameters
public static <T1, T2> String multipleTypeArgument(T1 val1, T2 val2) {

    return val1+" "+val2;

}

/*// Q: To audience -> will this compile ? 
 * 
 * public static <T1, T2> T3 returnTypeGeneric(T1 val1, T2 val2) {

    return 0;

}*/

 public static <T1, T2, T3> T3 returnTypeGeneric(T1 val1, T2 val2) {

    return null;

}

public static void main(String[] args) {
    List<Integer> list = new ArrayList<>();
    list.add(10);
    list.add(20);
    System.out.println(list.toString());
    fill(list, 100);
    System.out.println(list.toString());

    List<String> Strlist = new ArrayList<>();
    Strlist.add("Chirag");
    Strlist.add("Nayak");
    System.out.println(Strlist.toString());
    fill(Strlist, "GOOD BOY");
    System.out.println(Strlist.toString());


    System.out.println(multipleTypeArgument("Chirag", 100));
    System.out.println(multipleTypeArgument(100,"Nayak"));

}

}

// class definition ends

Sample Output:

[10, 20]
[100, 100]
[Chirag, Nayak]
[GOOD BOY, GOOD BOY]
Chirag 100
100 Nayak

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);
    }

In your function definition you're constraining sets a and b to the same type. You can also write

public <X,Y> void myFunction(Set<X> s1, Set<Y> s2){...}

a and b must both be sets of the same type. But nothing prevents you from writing

myfunction(Set<X> a, Set<Y> b)