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!
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);
}
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