Why is System.out.println(super) not permitted?

Question

Why is System.out.println(super) not permitted?

System.out.println(this);

This is OK and this.toString() is called and printed automatically. Of course, instance variable is OK instead of this.

However, this and super can be used in same way as I know.

System.out.println(super);

So why does this fail? I think it's supposed to call super.toString() implicitly. I have read Java specification document, but I haven't found the reason.

Answer 1

Check the grammar at http://java.sun.com/docs/books/jls/second_edition/html/syntax.doc.html

The super keywords must always be followed by SuperSuffix, which cannot be empty.

So super can never stand alone as an expression.

Answer 2

Implementing a standalone variant of super that breaks virtual method dispatch would be an extremely bad idea.

Let's think about it for a while.

abstract class Base {
    abstract String Description();
    String toString() { return "Base"; }
}
class Derived extends Base {
    String Description() { return "Derived description"; }
    String toString() { return "Derived"; }

    static void use(Base instance) {
        System.out.println(instance.toString());
        System.out.println(instance.Description());
    }
}

Now, let us take your suggestion and suppose that super is valid and does what you suggest; then we may write in Derived:

class Derived extends Base {
    // Previous declarations omitted.
    void useSuper() { Derived.use(super); }
    void useThis() { Derived.use(this); }

    static void main() {
        Derived instance = new Derived();
        instance.useThis();
        instance.useSuper();
    }
}

Now, if I understood you, you suggest that the main function should print in order:

• the implementation of toString() from Derived: "Derived".
• the implementation of Description() from Derived: "Derived description"
• the implementation of toString() from Base: "Base".
• the implementation of Description() from Base: It does not exist. And the two solutions I can think of leads to bigger problems:
  • Raise an exception: congratulations, you can now break any program which relies on abstract methods actually being implemented without even thinking about it. (How would you know that a function will call the abstract method?)
  • Return the implementation from Derived: breaks consistency.

In short, such a use of the word super conceptually breaks object-oriented programming.