Why is superclass method called while that method is overriden? (from OCA practice test)

Question

From question three in these OCA practice questions (pdf):

abstract class Writer {
   public static void write() {
      System.out.println("Writing...");
   }
}

class Author extends Writer {
   public static void write() {
      System.out.println("Writing book");
   }
}

public class Programmer extends Writer {
   public static void write() {
      System.out.println("Writing code");
   }
   public static void main(String[] args) {
      Writer w = new Programmer();
      w.write();
   }
}

The output is Writing....

I don't understand why. As Programmer overrides Writer's write method, I thought it should call the method in Programmer and not in Writer.

Why?

static methods can not be overriden, only hidden. You declare your object as type of superclass, so that's the one that will be taken — Stultuske, Jan 30 '18 at 09:19

score 4 · Accepted Answer · edited Jan 30 '18 at 21:55

4

You have to understand two points here.

There is no overriding concept in case of static members. They are simply static and never change based on instance.

And static members bind to class rather than instance. So no matter what is the instance, they look the type they got called and execute.

edited Jan 30 '18 at 21:55

Ivar

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answered Jan 30 '18 at 09:23

Suresh Atta

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I would have upvoted this if you hadn't misused code formatting for text that isn't code. – user207421 Jan 30 '18 at 09:34
Ahh .. sorry. My bad. Will keep in mind. I kept using `ctrl+k` unnecessarily . – Suresh Atta Jan 30 '18 at 09:35

Andrew Tobilko · Answer 2 · 2018-01-30T09:40:24.697

The type of the reference is Writer. You have called a static method for which overriding isn't applied - the method from the Writer is going to be invoked.

The mechanism is called method hiding.

Check out these cases:

new Programmer().write();             // code  [Programmer]
((Writer)new Author()).write();       // ...   [Writer]

new Author().write();                 // book  [Author]
((Writer)new Programmer()).write();   // ...   [Writer]

new Writer() {{}}.write();            // ...   [Writer]

score 1 · Answer 3 · edited Jan 30 '18 at 10:04

As we know, static methods cannot be overridden. If we try to do so, it turns out to be method hiding instead. In the above case, both class- Writer and Programmer contain write() method.

When Programmer extends the Writer class and provides its own implementation of the write() method, it just hides the Writer implementation of it.

Now, on runtime, the compiler just checks the Reference type (since it is a static method, compiler is not concerned about the object created to call the method. Remember, static methods are class methods). Hence, compiler checks and finds that reference w is of type Writer, it calls the Writer version of the write method instead. If the methods would not have been static, what you expect would have been the output instead.

Rick · Answer 4 · 2018-01-30T09:39:06.103

Found the answer myself.

There is no such thing as overriding a static method in Java. This is why when you call a static method from the superclass reference, the superclass static method will be called.

So

public class SuperClass {
    public static void write() {
        System.out.println("Writing Super");
    }
    public void writeMore() {
        System.out.println("super something");
    }
}
public class SubClass extends SuperClass {
    public static void write() {
        System.out.println("Writing Sub");
    }
    public void writeMore() {
        System.out.println("sub something");
    }
}
public class Test {
    public static void main(String[] args) {
        SuperClass super = new SubClass();
        super.write();
        super.writeMore();
    }
}

Will output

Writing super
sub something

If you want to call the static write() method from the subclass. You have to reference it from a subclass. E.g.:

SubClass sub = new Subclass();
sub.write();

Some sources where I learned about this: https://www.geeksforgeeks.org/can-we-overload-or-override-static-methods-in-java/ Why doesn't Java allow overriding of static methods?

Found the answer yourself after you had already been told six times. I don't know what the scare-quotes around "'allow'" are supposed to mean. The fact is that there is *no such thing* as overriding static methods, contrary to the assertion in your question. Yon don't need any reference other than the JLS. — user207421, Jan 30 '18 at 09:32
You are right, I did not word that sentence in a way that represents what is actually happening. I have edited it to say that there isn't overriding static methods in Java. — Rick, Jan 30 '18 at 09:41

Why is superclass method called while that method is overriden? (from OCA practice test)

4 Answers4