如何从列表访问子类对象的方法

Question

I have a class Sprites and a class Soldier , Tank and so on. Soldier and Tank extends Sprites . Now the subclasses have a special update() method which is different in each class (like different movement and so on).

I have a List<Sprites> ListUnits1Player1 where all objects of eg Soldiers are stored and also a List<Sprites> ListUnits2Player1 where all objects of Tanks are stored. These lists are of the type <Sprites> . Now please see the follwing code:

        for(Iterator<Sprites> itr_unit1_player1 =     ListUnits1Player1.iterator();itr_unit1_player1.hasNext();) {
        Sprites unit1_player1 = itr_unit1_player1.next();
        unit1_player1.update();
    }

I get an error can't resolve method update() .

The debugger tells me however, that unit1_player1 is of type Soldier , which is correct since I stored the objects of Soldier in the List ListUnits1Player1 .

But then why can't I call the method update() of the class Soldier?

I guess I could explicitly cast unit1_player1 to Soldier with ((Soldier)unit1_player1).update(); but since it is variable which unit (Soldier, Tank,...) is stored in ListUnits1Player1 I don't want to cast it to a certain type.

I have tried casting with:

unitClass = unit1_player1.getClass();
((unitClass)unit1_player1).update(); 

to always get the correct class, but this just gives me the error Unknown class 'unitClass'

Any suggestions?

Answer 1

You could move the method update in the parent class Sprites as an abstract method.

The subclasses then will have to override the method and unit1_player1.update(); would work without any cast.

Answer 2

If you can not call unpdate on Sprites , means you did not declare update method in the super class or declare it as private.

It's always a good habit to use override annotation:

class Sprites {
    void update() {

    }
}

class Tank extends Sprites {

    @Override
    void update() {
        // ...
    }
}

Answer 3

Put your update method as abstract in Sprites class.

//superclass
public abstract void update ();

Then override it in each subclass

//subclass
public void update (){
    //code
}

Answer 4

You declared an iterator over Sprite s, so from the compiler's point of view, you could be dealing with any subtype of that, but there's no way to know which

If you will only ever have objects in your collection that support update , you should declare an interface for that method, make both Soldier and Tank implement it, and then debate your collection as containing objects of that interface type.