why base class method is invoked?

Question

In the following code:

b.show("str");
//prints: from base
d.show("");
//prints: from str

Could one please explain why it's behaving differently? I am wondering why Base b = new Sub(), that b.show() from base class would be invoked. I am merely using DifferentClass, as an reference showing b.show(String) is called under an non-inheritance occasion.

public class TestClass {

    public static void main(String[] args) {
        Base b = new Sub();
        b.show("str");
        DifferentClass d = new DifferentClass ();
        d.show("");
    }
}

class Base {
    public void show(Object obj) {
        System.out.println("from base");
    }
}

class Sub extends Base {
    public void show(String str) {
        System.out.println("from sub");
    }
}

class DifferentClass {
    public void show(String str) {
        System.out.println("from str");
    }

    public void show(Object obj) {
        System.out.println("from obj");
    }
}

score 3 · Accepted Answer · edited May 23 '17 at 12:14

Because of the reference type.

    Base b = new Sub();
    b.show("str");
    Sub2 s2 = new Sub2();
    s2.show("");

In that code, though b is an instance of Sub, the reference type of the variable is Base. Method overloading is evaluated at compile-time, not run-time. That matters because at compile time, once the new Sub() constructor runs and we assign the variable, the compiler is no longer aware of the concrete class. Only that it's a valid Base.

Why does this matter?

Because when the compiler tries to resolve b.show(String), there is no method on Base (the only type it knows of for sure for b) which takes a String, so it passes the string to the show(Object) method.

Sub does not over-ride the show(String) method (though Sub2 does).

By the way, the @Override annotation will help you with this: When do you use Java's @Override annotation and why?

+1. Exactly. The key is that sub's method does not 'override' the base class method, but overload it. I agree to use @Override as often as possible. — BrianT., Nov 15 '14 at 14:37

score 0 · Answer 2 · answered Nov 15 '14 at 14:26

0

You're invoking with a string so the method show() overloaded with string is called.

Maybe you did not realize that sub2 does not inherit??...

answered Nov 15 '14 at 14:26

eduyayo

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score 0 · Answer 3 · answered Nov 15 '14 at 14:31

You declare your variable b to be of type Base, then you invoke show() on it. Since it has a show method which matches the signature, that is the method invoked.

Then you declare your variable s2 to be of type Sub2, and invoke show(String) on it. The runtime invokes that method since it matches.

why base class method is invoked?

3 Answers3