Generics parameter extends another parameter on java method

Question

Sorry, maybe it is a silly question, but I can't find answer.

Two generics parameters on a method, could one extends another?

public class A {
}

public class B extends A {
}

public class C {
}

public class Foo {

    public static <R extends A> void f1 (A t, R r){
    }

    // T and R are generics parameter, R bounds on T
    public static <T, R extends T > void f2(T t, R r) {
    }

    public static void main(String[] args) {

        A a = new A();
        B b = new B();
        C c = new C();

        Foo.f1(a, b); // no error
        Foo.f1(a, c); // compile error, it's ok

        Foo.f2(a, b); // no error
        Foo.f2(a, c); // no error !  why? 
   }
}

The last f2 method call has no compile error, but I think C is not subclass of A, compile error should be arised. Any help?

Why do you think T is A?

user2357112
– user2357112

2016-10-11 18:55:52 +00:00
Commented Oct 11, 2016 at 18:55 — user2357112
– user2357112, Commented Oct 11, 2016 at 18:55

Filipp Voronov · Accepted Answer · 2016-10-11 18:53:56Z

3

Because type parameters in your code where you call the method are implicit and for example if java compiler infers T and R to Object it's fine, isn't it? But if you declare them explicit it raises an error:

 Foo.<A, C>f2(a, c); //error as you wished
 Foo.<Object, Object>f2(a, c); //no errors and it's ok, isn't it?

answered Oct 11, 2016 at 18:53

Filipp Voronov

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