Why doesn't polymorphism work in a generics method when using an interface?

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();
    }

*"Why this happens?"* — *What* happens? You only told us what you expected to happen. — stakx - no longer contributing, Jun 17 '12 at 08:34
`interface`s in Java cannot have instance variables. They can only have constant values (i.e `final` instance variable) but it's considered a bad practice to put constant into an `interface`. — Genzer, Jun 17 '12 at 08:34
@Cicada: I think the code snippet misses 1 bracket. But the question is *"I expected that function f an g print "BB" instead of "interface A. Why this happens?"* so I think he did run the code. — Genzer, Jun 17 '12 at 08:52
@SubinS *Any* variable declared in an interface is (a) constant (b) static and (c) final. — user207421, Jun 17 '12 at 11:55

score 4 · Accepted Answer · edited May 23 '17 at 12:12

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

score 0 · Answer 2 · answered Jun 17 '12 at 08:51

There are too many problems in your code snippet.

You tried to define field into interfaces. This is forbidden. Only methods or static fields are allowed in interface.
Your method f is not terminated.
It is not exactly clear what are you trying to do.

I'd suggest you to read about metamorphism first. Then try to implement some exercise without generics. Then read about generics, think what are you going to do and implement the second exercise with generics. Then if you still have questions post them here and we will be happy to help you.

Good luck.

"This is forbidden." No, it is not forbidden. It is just implicitly public, static, and final. — newacct, Jun 17 '12 at 21:47

Why doesn't polymorphism work in a generics method when using an interface?

2 Answers2