Question

Let’s say Java has these hierarchical classes:

class A 
{
}
class B extends A
{
    public void m()
    {
        System.out.println("B\n");
    }
}
class C extends B
{
    public void m()
    {
        System.out.println("C\n");
    }
}
class D extends C
{
    public static void main(String[] args)
    {
        A a = new D();
        // a.m(); // doesn't work
        B b = new D();
        b.m();
        C c = new D();
        c.m();
        D d = new D();
        d.m();
    }
}

This is the (blind) duplication of the same code in C#:

using System;
class A 
{   
}
class B : A
{
    public void M()
    {
        Console.WriteLine("B");
    }
}
class C : B
{
    public void M() // I need to use public new void M() to avoid the warning
    {
        Console.WriteLine("C");
    }
}
class D : C
{
    public static void Main(String[] args)
    {
        A a = new D();
        // a.M(); // doesn't work       
        B b = new D();
        b.M();
        C c = new D();
        c.M();
        D d = new D();
        d.M();
    }
}

When I execute the Java code, I got C-C-C whereas C# returns B-C-C.

To me C#’s result makes more sense, as reference B invokes its own method.

• What’s the logic behind the Java designer’s decision to print C-C-C instead of B-C-C? I mean, why reference B uses the overriding method in C? What’s the advantage of this approach?
• How can I change Java code to print out B-C-C just like C# does? I mean, how can I teach java to invoke the method of the exact reference it uses?
• How can I change C# code to print out C-C-C? I mean, how can I teach C# to invoke the overriding method?

Answer 1

It’s for the virtual function definition:

a virtual function or virtual method is a function or method whose behavior can be overridden within an inheriting class by a function with the same signature. This concept is a very important part of the polymorphism portion of object-oriented programming (OOP).

In C#,you should declare the method as virtual in order to be overriden, as shown in MSDN:

• C# Lang Spec. Virtual keyword
• C# Virtual

Since the M method is not virtual, it will execute b.M() even if b variable is actually a D instance.

In Java, every non-static method is virtual by default, so you when you override a method (even without the @Override annotation) the behavior of the b.M() will be the d.M() that inherits the c.M() method behavior.

How can I change Java code to print out B-C-C just like C# does? I mean, how can I teach java to invoke the method of the exact reference it uses?

You simply can’t do this in Java. The M method in C class would override the M method in B. Adding the final modifier to B#M will just make that C or other B children can’t override the M() method.

How can I change C# code to print out C-C-C? I mean, how can I teach C# to invoke the overriding method?

Change the M method in B class to virtual and override it in C class:

class B : A
{
    public virtual void M()
    {
        Console.WriteLine("B");
    }
}
class C : B
{
    public override void M() // I need to use public new void M() to avoid the warning
    {
        Console.WriteLine("C");
    }
}