Question

I have an “Extension” class for handling general functions that’s designed to be used dynamically – that is, I want to be able to use it with many different classes and variables (kind of like how you would use something like the Math class). But I’ve encountered a little problem that I can’t seem to figure out (I’ve looked at quite a few forums and websites, but no luck).

Here’s the function from my Extension class:

public void setPlayerFriction(Object objectType, double hspeed, double vspeed){
    objectType obj = new objectType();

    if(obj.hspeed >= GameWindow.friction){
          obj.hspeed -= GameWindow.friction;
    }else if(obj.hspeed <= -GameWindow.friction){
          obj.hspeed += GameWindow.friction;
    }else{
          obj.hspeed = 0;
    }
    if(obj.vspeed >= GameWindow.friction){
          obj.vspeed -= GameWindow.friction;
    }else if(obj.vspeed <= -GameWindow.friction){
          obj.vspeed += GameWindow.friction;
    }else{
          obj.vspeed = 0;
    }
}

And the function’s use in my Player class:

public void runPhysicsEngine(){
    Extensions ext = new Extensions();
    ext.setPlayerFriction(this,hspeed,vspeed);
}

Obviously doing something like obj.var -= GameWindow.friction;, where obj and var are arguments of a method, isn’t the right way to do what I want to do – it just returns the unresolved type error. So what is the right way? How do I access a variable from a dynamic class name?

Answer 1

I think having an abstract class for your objectType with appropriate getter/setter methods might help. For example:

public abstract class ParentObject {

    private double hSpeed;
    private double vSpeed;

    /** Getter/setter methods */

    public double gethSpeed() {
        return hSpeed;
    }
    public void sethSpeed(double hSpeed) {
        this.hSpeed = hSpeed;
    }
    public double getvSpeed() {
        return vSpeed;
    }
    public void setvSpeed(double vSpeed) {
        this.vSpeed = vSpeed;
    }
}

You can then have concrete implementation of your ObjectType classes that extend this ParentObject class. And you would change your setPlayerFriction() method as follows:

public void setPlayerFriction(ParentObject objectType, double hspeed, double vspeed){

    double hSpeed = objectType.gethSpeed();
    if(hSpeed >= GameWindow.friction){
          objectType.sethSpeed(hSpeed - GameWindow.friction);
    }else if(hSpeed <= -GameWindow.friction){
          objectType.sethSpeed(hSpeed + GameWindow.friction);
    }else{
          objectType.sethSpeed(0);
    }

    double vSpeed = getvSpeed();
    if(vSpeed >= GameWindow.friction){
          objectType.setvSpeed(vSpeed - GameWindow.friction);
    }else if(obj.vspeed <= -GameWindow.friction){
          objectType.setvSpeed(vSpeed + GameWindow.friction);
    }else{
          objectType.setvSpeed(0);
    }
}

And to this method, you would basically be sending references of classes that have actually extended your ParentObject class. For example:

public class ObjImpl extends ParentObject{
    //Concrete implementation
}

public class ObjImpl2 extends ParentObject{
    //Concrete implementation
}

public class TestClass{
    public void setPlayerFriction(ParentObject objectType, double hspeed, double vspeed){   
        double hSpeed = objectType.gethSpeed();
        if(hSpeed >= GameWindow.friction){
              objectType.sethSpeed(hSpeed - GameWindow.friction);
        }else if(hSpeed <= -GameWindow.friction){
              objectType.sethSpeed(hSpeed + GameWindow.friction);
        }else{
              objectType.sethSpeed(0);
        }

        double vSpeed = getvSpeed();
        if(vSpeed >= GameWindow.friction){
              objectType.setvSpeed(vSpeed - GameWindow.friction);
        }else if(obj.vspeed <= -GameWindow.friction){
              objectType.setvSpeed(vSpeed + GameWindow.friction);
        }else{
              objectType.setvSpeed(0);
        }
    }

    public static void main(String[] args) {
        ParentObject object1    = new ObjImpl();
        ParentObject object2    = new ObjImpl2();

        setPlayerFriction(object1, 1.0d, 2.0d);
        setPlayerFriction(object2, 2.25d, 2.0d);
    }
}