CODEWITHSUNDEEP
javaclassobject
Daan
Daan

Reputation: 65

How to check if child object is from specific class (and not parent) (Java)

I have to write a small text-based game for school in which I have a Player class and a Computer class. In the Game class I need to differentiate between these two classes so I can print instructions ONLY when the player is not a computer.

public class Player {
    private String name;

    public Player(String name) {
        this.name = name;
    }
}

public class Computer extends Player {
    public Computer(String name) {
        super(name);
    }
}

I've tried the following:

player1 = new Player;

if(player1.getClass() != Computer.class) {
    System.out.println("Your turn, player 1.");
}

This should print "Your turn, player 1." because a player is not a computer, right? Well it doesn't and I have no idea how to do it otherwise.

I have tried instanceof, but that always returns true because Computer is an instance of Player.

Help would be very much appreciated!

~ Daan

Upvotes: 3

Views: 18011

Answers (4)

vefthym
vefthym

Reputation: 7462

Computer is always an instance of Player, but a Player is not always a Computer. Try this instead:

if (!player.instanceof(Computer)) {...}

But keep in mind that if you initialize player1 as a new Player, as you do, it will never be an instance of Computer. So, if you want to initialize a Player as a Computer, you will need to use the Computer's constructor. The following code will return "true":

Player player1 = new Computer();
System.out.println(player1.instanceof(Computer));

Upvotes: 0

mvieghofer
mvieghofer

Reputation: 2896

You could create a method that returns the instructions for a player in the Player class. Every subclass can then override the getInstructions method in order to provide more specific instructions. For a computer this would be an empty string. You could do it like this:

public class Player {
    private String name;

    public Player(String name) {
        this.name = name;
    }

    public String getInstructions() {
        return "Instructions";
    }
}

public class Computer extends Player {
    public Computer(String name) {
        super(name);
    }

    @Override
    public String getInstructions() {
         return "";
    }
}

Then you can just print the instructions if they are not empty.

Player p = new Player("asdf");
Player c = new Computer("qwer");

String instruction = p.getInstruction();
if (!instruction.empty()) {
    System.out.printLine(instruction);
}

instruction = c.getInstruction();
if (!instruction.empty()) {
    System.out.printLine(instruction);
}

Of course you can pass an Output Stream to the getInstruction method an write it directly to the output stream. Then you wouldn't need the ifs.

Upvotes: 0

Joe Rinehart
Joe Rinehart

Reputation: 763

You can do this with instanceof:

if(!(player1 instanceof Computer))

...but that's a bad design. Since a computer is-a player (inheritance), they should both have the same abilities (API). A really simple idea would be to have them both implement an IPlayer interface with a printGreeting() method:

public interface IPlayer {
    void printGreeting();
}

public class Player implements IPlayer {
    private String name;

    public Player(String name) {
        this.name = name;
    }

    public void printGreeting() {
        system.out.println('Your turn! (or whatever)');
    }
}

public class Computer extends Player {
    public Computer(String name) {
        super(name);
    }

    @Override
    public void printGeeting() {
        // NO-OP
    }

}

Upvotes: 4

Kevin Workman
Kevin Workman

Reputation: 42174

This smells like a symptom of a bad design. You shouldn't usually have to check what kind of class a value is. In this case you could have a boolean in the Player class specifying whether the player is human.

If you really want to go with this design though, what you're looking for is the "instanceof" operator. Do a google search of "java instanceof" for more info.

Edit: I see now that you tried instanceof already. Why are you comparing instanceof to the Player class? Don't you want to compare instanceof to the Computer class?

Upvotes: 0

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