Puzzle 70: Package Deal

This program involves the interaction of two classes in different packages. The main method is in class hack.TypeIt. What does the program print?

package hack;

import click.CodeTalk;



public class TypeIt {

    private static class ClickIt extends CodeTalk {

        void printMessage() {

            System.out.println("Hack");

        }

    }



    public static void main(String[] args) {

        ClickIt clickit = new ClickIt();

        clickit.doIt();

    }

}



package click;

public class CodeTalk {

    public void doIt() {

        printMessage();

    }



    void printMessage() {

        System.out.println("Click");

    }

}

Solution 70: Package Deal

This puzzle appears straightforward. The main method in hack.TypeIt instantiates the class TypeIt.ClickIt and invokes its doIt method, which is inherited from CodeTalk. This method, in turn, calls printMessage, which is declared in TypeIt.ClickIt to print Hack. And yet, if you run the program, it prints Click. How can this be?

This analysis incorrectly assumes that hack.TypeIt.ClickIt.printMessage overrides click.CodeTalk.printMessage. A package-private method cannot be directly overridden by a method in a different package [JLS 8.4.8.1]. The two printMessage methods in this program are unrelated; they merely have the same name. When the program calls printMessage from within the package hack, the package-private method hack.TypeIt.ClickIt.printMessage is run. This method prints Click, which explains the observed behavior.

If you want the printMessage method in hack.TypeIt.ClickIt to override the method in click.CodeTalk, you must add the protected or public modifier to the method declaration in click.CodeTalk. To make the program compile, you must also add a modifier to the overriding declaration in hack.TypeIt.ClickIt. This modifier must be no more restrictive than the one you placed on the declaration for printMessage in hack.TypeIt.ClickIt [JLS 8.4.8.3]. In other words, both printMessage methods may be declared public, both may be declared protected, or the superclass method may be declared protected and the subclass method public. If any of these three changes is made, the program will print Hack, indicating that overriding is taking place.

In summary, package-private methods cannot be directly overridden outside the package in which they're declared. Although the combination of package-private access and overriding can lead to some confusion, Java's current behavior enables packages to support encapsulation of abstractions larger than a single class. Package-private methods are implementation details of their package, and reuse of their names outside the package should have no effect inside the package.