Question

Say I have a generic array like this:

ArrayList<Fruit>  fruits = new ArrayList<Fruit>();

Then I add lots of different fruits, which all extends the class fruit, and loop through them

for (Fruit f : fruits) {

}

If the fruit is a banana, I want to check how round it is, so..

for (Fruit f : fruits) {
    if (f instanceof Bannana)
        f.checkHowRoundBannanaIs();
}

I will have to put the checkHowRoundBannnaIs() method in the fruit class, even though the fruit may not be a banana, because I can’t use the function on f if it isn’t in the fruit class, otherwise I get a undefined method error (or something like that).

This is fine for one or two methods, but after a while it makes the class bulky and ugly. So my question is, how can I add methods to a class and use them in a for-style loop of its super class?

Answer 1

So my question is, how can I add methods to a class and use them in a for-style loop of its super class?

Short answer, you can’t really.

If you want to call the methods in the body of the for-loop, instanceof and down-casting is the only way I can think of:

for (Fruit f : fruits) {
    if (f instanceof Banana)
        ((Banana) f).checkHowRoundBananaIs();
    // ...
}

---

Both instanceof and down-casting are usually considered bad practice though. To avoid it, you could implement the visitor pattern. Here are the steps you would need to take:

1. Create a visitor interface like this:

   interface FruitVisitor {
       void visit(Banana banana);
       void visit(Apple apple);
   }
   

2. Let all fruits accept a visitor:

   abstract class Fruit {
       // ...
       public abstract void accept(FruitVisitor fv);
   }
   
   class Banana extends Fruit {
       public void accept(FruitVisitor fv) {
           fv.visit(this);
       }
   
       public void checkHowRoundBananaIs() { ... }
   }
   

3. Create a visitor that takes the appropriate action for each type of fruit, and pass it as argument to the accept method of each fruit in your list:

   FruitVisitor fv = new FruitVisitor() {
       public void visit(Banana banana) {
           banana.checkHowRoundBananaIs();
       }
   
       public void visit(Apple apple) {
           // ...
       }
   };
   
   for (Fruit f : fruits)
       f.accept(fv);