Question

I am trying to write a method that adapts to different types of interfaces using generics.

In this code I would have expected that the method f an g would print out “BB” instead of “interface A”. Why did this happen?

interface A {
    String n="interface A";
}

interface B extends A {
    String n = "BB";
}

public class Main {
    <T extends A> void f() {
        T a = null;
        System.out.println(a.n);
    }

    <T extends A> void g() {
        System.out.println(T.n);    
    }

    public static void main(String[] args) {
        Main m = new Main();
        m.<B>f();
        m.<B>g();
    }

Answer 1

In your code, any classes or interfaces that extend/implement A will inherit the constant variable n.

The declaration of interface A actualy become like this:

interface A {
    public static final String n = "interface A"; //constant static varible
}

and you cannot override final variables in java.

If you write

System.out.println(B.n);

it will print “BB”.

You can find an answered similar question here