Why Does Java Prohibit Calling a Superclass's Superclass Method (super.super.method())?

Calling Super's Super Method in Java: Why is it Prohibited?

Despite the ease with which one might circumvent certain scenarios by accessing the parent's parent's method directly using super.super.method(), Java forbids such invocation. This article explores the reasons behind this restriction and discusses whether analogous mechanisms exist in other languages.

Encapsulation Violation

The primary reason Java disallows super.super.method() is to uphold encapsulation. This principle prevents the bypass of a parent class's behavior by accessing its superclass directly. While bypassing one's own class's behavior may occasionally be necessary, such as from within the same method, it should not be possible for the subclasses to bypass the behavior of their parents.

Example: Maintaining Class Invariants

Consider a hierarchy of classes: Items, RedItems, and BigRedItems. Items represents a collection of items, RedItems a collection of red items, and BigRedItems a collection of big red items. To maintain the integrity of each class:

public class Items {
    public void add(Item item) { ... }
}

public class RedItems extends Items {
    @Override
    public void add(Item item) {
        if (!item.isRed()) {
            throw new NotRedItemException();
        }
        super.add(item);
    }
}

public class BigRedItems extends RedItems {
    @Override
    public void add(Item item) {
        if (!item.isBig()) {
            throw new NotBigItemException();
        }
        super.add(item);
    }
}

This ensures that RedItems contains only red items and BigRedItems contains only big red items.

Breaking the Encapsulation Contract

If super.super.add() were permitted:

public class NaughtyItems extends RedItems {
    @Override
    public void add(Item item) {
        // Add any item to NaughtyItems, ignoring the rule in RedItems
        super.super.add(item);
    }
}

NaughtyItems could bypass the invariant in RedItems, adding items indiscriminately, rendering the encapsulation of RedItems meaningless.

Conclusion

The prohibition of super.super.method() in Java is an important safeguard that prevents encapsulation violation, maintaining the integrity and consistency of class hierarchies. While it may prevent certain convenient scenarios, it ensures the robustness and reliability of Java codebases.

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