扩展类和静态变量

Question

I've DynamicObject , Player , Enemy classes

public class DynamicObject {

    protected static float speed;
}



public class Player extends DynamicObject {
       /*some code*/
       Player.speed = 100;
}


public class Enemy extends DynamicObject {
       /*some code*/
       Enemy.speed = 50;
}

And always speed value is overridden. Of course I can create new speed variable in Player and Enemy , but then the existing DynamicObject class is pointless. I want to have different speed values on each class. All objects of current class will have the same speed . How I should make it in correct way?

Answer 1

The speed variable should not be static . Otherwise it won't be bound to any of the instances of the DynamicObject class, nor any of it's subclasses instances.

If you want to have a different speed value for each of the subclasses, you can do:

public class DynamicObject {    
    protected float speed;

    public DynamicObject(float speed) {
        this.speed = speed;
    }

    public float getSpeed() {
       return this.speed;
    }
}

public class Player extends DynamicObject {
   /*some code*/
   public Player(float speed) {
       super(speed);
   }

}


public class Enemy extends DynamicObject {
       /*some code*/
   public Enemy(float speed) {
       super(speed);
   }
}

Answer 2

If every DynamicObject has a speed, and the speed is the same for every instance of, for example, Player, then you should have

public abstract int getSpeed();

in DynamicObject, and

@Override
public int getSpeed() {
    return 100;
}

in Player.

If you need to have access to the constant speed returned for evry instance of Player without instantiateing a Player, just use

public static final int CONSTANT_PLAYER_SPEED = 100;

@Override
public int getSpeed() {
    return 100;
}

and use Player.CONSTANT_PLAYER_SPEED to access the speed of all players.

Answer 3

Remove the static from that variable, if a variable is static, it's belong to the class, not to instances.

And making a static variable protected doesn't make any scene, since, inheritance and static are totally different.

protected float speed;

Answer 4

You could also have an abstract method, which would mean that you would always have to override it in your sub classes.

public abstract class Dynamic {

protected float speed; 

abstract void setSpeed();
}
public class Player extends Dynamic{
@Override
void setSpeed() {
    this.speed=50;
}

}