Java Polymorphism problem
I've learned all these programming terms in Swedish so please bear with me..
I'm having problems calling a method in a subclass which should override a method in the superclass.
Here is the class structure with removed code:
public interface Movable {
public void move(double delta);
}
public abstract class Unit implements Movable, Viewable{
public void move(double delta){
System.out.println("1");
}
}
public class Alien extends Unit{
public void move(long delta){
System.out.println("2");
}
}
public class Player extends Unit{
public void move(long delta){
System.out.println("3");
}
}
public void main(){
ArrayList<Unit> units = new ArrayList<Unit>();
Unit player = new Player();
Unit alien = new Alien();
units.add(player);
units.add(alien);
for (int i = 0; i < this.units.size(); i++) {
Unit u = (开发者_如何学运维Unit) this.units.get(i);
u.move();
}
}
This would output 1 and 1, but I want it to output 2 and 3.
What am I doing wrong? I thought this was how it worked.public class Alien extends Unit{
public void move(long delta){
System.out.println("2");
}
}
public class Player extends Unit{
public void move(long delta){
System.out.println("3");
}
}
Here's your problem, in the interface you have move declared as:
public void move(double delta)
But your child classes declare a:
public void move(long delta)
That creates a new function, separate from move and doesn't override it. To fix it change Alien's and Player's move functions to:
public void move(double delta)
Also, as Outlaw Programmer points out, you can have the compiler catch this sort of error by adding an @Override annotation right above the declaration of any function you intend to override a function in the parent class. Like so:
public class Alien extends Unit {
@Override
public void move(double delta) {
System.out.println("2");
}
}
Your subclasses (Alien and Player) aren't overriding the move() method in their parent class because you have declared 'delta' as a long and not a double.
You can have the compiler spot some of these errors by using the @Override annotation.
You need to add the @Override flag to the move function in your alien and player classes.
that would've helped you notice that you are not quite overriding your move since the type in alien/player are long, not doubles
Your child class does not implements the same method signature:
public void move(double delta);
is not the same as:
public void move(long delta);
To catch this kind of error at compile time, you can add @Override in the method signature. The compiler will check if the child class do in fact override a method in the parent class.
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