Java Oddity: How an upcast can save the day

You almost never see an upcast in Java code. That is, you rarely see something like this:

…((Animal) someDog)…

Why would you need to? The fact that a Dog is an Animal should be evident to the compiler!

Here is however a program that fails to compile without an upcast:

class Player {
    private boolean isAlive = true;

    void kill(Opponent opponent) {
        // Compiles
        // Error: 'isAlive' has private access in 'Player'
.isAlive = false;

        // opponent.opponentSpecificMethod();

class Opponent extends Player {
    // ...

Note: The snippet above is written to illustrate a language oddity and should not be seen as an example of good design. In fact, the need for an upcast is a telltale of a bad design. In particular a base class should not be coupled to one of its subclasses the way Player is coupled to Opponent.

Comments (2)

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Private members cannot be accessed by child classes. You want protected access.

by holothuroid |  Reply
User avatar

No one is accessing isAlive from a child class here. The field is used only in Player, and thus anything but private would be an unncessary sacrifice of encapsulation.

by Andreas Lundblad |  Reply

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