从超类的对象数组访问子类的功能

Question

I have class Carte and Croyant , DeuxEx are subclass of Carte . In Croyant and DeuxEx , there are attributes valueCroyant or valueDeuxEx which just exists in each class. I create an array of Object of class Carte . How can I access to method getValueCroyant() or getValueDeuxEx() from object of this array.

Class Carte {
    private int id;
    public Carte(int id){
        this.id=id;
    }
}
Class Croyant extends Carte{
    private int valueCroyant;
    public Croyant(int id){
        super(id);
    }
    public int getValueCroyant(){
        return this.valueCroyant;
    }
}
Class DeuxEx extends Carte{
    private String valueDeuxEx;
    public DeuxEx(int id){
      super(id); 
    }
    public String getValueDeuxEx(){
       return this.ValueDeuxEx;
    }
}
public static void main(String[] agrs){
    ArrayList<Carte> array_carte = new ArrayList();
    Croyant cr1 = new Croyant(1);
    Croyant cr2 = new Croyant(2);
    DeuxEx de1= new DeuxEx(3);
    DeuxEx de2 = new DeuxEx(4);
    array_carte.add(cr1);
    array_carte.add(cr2);
    array_carte.add(de1);
    array_carte.add(de2);
    for(Carte c:array_carte){
       if(c instanceof Croyant){
           System.out.println(c.getValueCroyant()); 
        }else{
           System.out.println(c.getValueDeuxEx()); 
        }
    }
}

I want to do a for loop like this, but it doesn't work. Can somebody help me please!

Answer 1

c.getValueCroyant() won't compile because its type is Carte which is the superclass. Even if it is not elegant, to call this method, using instanceOf is fine but you have also to cast to Croyant

Answer 2

What you're trying to do hints strongly at a problem with your design. The suggestion in another answer of testing with instanceof and casting would work, but it's very anti-OO. Basically, that kind of solution circumvents polymorphism.

I see that valueCroyant is an int while valueDeuxEx is a String . The best solution would be to rethink your design so these values are either the same type or implement some common interface. Then move the value and the getter to the common superclass Carte . If it's impossible to represent both values using the same type, then give Carte an abstract method that is meant to do something with the value, and implement it differently in each subclass.

Answer 3

I think you should cast. The code snippet shown by you won't compile.

Something like this:

for(Carte c:array_carte){
       if(c instanceof Croyant){
           System.out.println(((Croyant)c).getValueCroyant()); 
        }else if(c instanceof DeuxEx){
           System.out.println(((DeuxEx)c).getValueDeuxEx()); 
        }
    }

Check my answer here .