Java how to implement and design an abstract class

Question

I've run into a design problem in my java code. My application uses missiles, and there are different types of missiles that all work identical except they have 3 unique attributes. The constructor of a missile must know these attributes. I decided to make missile an abstract class, but I can't assign values to protected variables in a subclass outside of a method/constructor. Also I can't declare the variables in the constructor, because I must make the call to the super-constructor first thing. How can I be smart about this problem?

public abstract class Missile {

private int x, y;
private Image image;
boolean visible;

private final int BOARD_WIDTH = 390;

protected final int MISSILE_SPEED;
protected final int MISSILE_HEIGHT;
protected String file;

public Missile(int x, int y) {
    ImageIcon ii =
        new ImageIcon(this.getClass().getResource(file));
    image = ii.getImage();
    visible = true;
    this.x = x;
    this.y = y - Math.floor(MISSILE_HEIGHT/2);
}


public Image getImage() {
    return image;
}

public int getX() {
    return x;
}

public int getY() {
    return y;
}

public boolean isVisible() {
    return visible;
}

public void move() {
    x += MISSILE_SPEED;
    if (x > BOARD_WIDTH)
        visible = false;
}
}

And there is an ideal implementation of a subclass, except it doesn't work. (it can't recognize the protected variables). What do I do?

public class Laser extends Missile {

    MISSILE_SPEED = 2;
    MISSILE_HEIGHT = 5;
    file = "laser.jpg";

public Laser(int x, int y) {
    super(x, y);
}

}

Answer 1

Change the base class fields and constructors to

protected final int speed;
protected final int height;

public Missile(int x, int y, int speed, int height, String file) {
    ImageIcon ii =
        new ImageIcon(this.getClass().getResource(file));
    image = ii.getImage();
    visible = true;
    this.speed = speed;
    this.height = height;
    this.x = x;
    this.y = y - Math.floor(height/2);
}

And the subclass to:

public class Laser extends Missile {
    public Laser(int x, int y) {
        super(x, y, 2, 5, "laser.jpg");
    }

    ...
}

The attributes are already in the base class, so they must not be redefined in the subclass. All-uppercase naming is reserved to constants in Java.

Answer 2

I think the best way to do what you want it to do is make abstract methods in Missile that the subclasses have to implement. For example, add these to Missile:

public abstract int getMissileSpeed();
public abstract int getMissileHeight();
public abstract int getFileName();

Then your subclass has to implement it, and you can make it constant like so:

public class Laser extends Missile {

    public Laser(int x, int y) {
        super(x, y);
    }

    public int getMissileSpeed() {
        return 2;
    }

    public int getMissileHeight() {
        return 5;
    }

    public String getFileName() {
        return "laser.jpg";
    }

}

edit: And then of course anywhere that you want to retrieve the constant value you just call those methods.

Answer 3

I'm not sure if missile needs to be an abstract class, but I think something like this might be what you're going for:

public abstract class Missile {

    private int x, y;
    private Image image;
    boolean visible;

    private final int BOARD_WIDTH = 390;

    protected final int MISSILE_SPEED;
    protected final int MISSILE_HEIGHT;

    public Missile(int x, int y, int speed, int height, String file) {
        MISSILE_SPEED = speed;
        MISSILE_HEIGHT = height;

        ImageIcon ii = new ImageIcon(this.getClass().getResource(file));
        image = ii.getImage();
        visible = true;
        this.x = x;
        this.y = y - Math.floor(MISSILE_HEIGHT/2);
    }

}

public class Laser extends Missile {

    public Laser(int x, int y) {
        super(x, y, 2, 5, "laser.jpg");
    }

}

Answer 4

Create an interface and put all your final fields in it. Now implement this interface within Missile and Laser both. At least that would solve the issue of access.