Question

I have this following interface :

    public interface FruitDetails {
            public String getFruitName(); 
    }

And few Classes that implements the above interface :

    public class Banana implements FruitDetails{

        private int index;
            public Banana(int i) {
                index = i;
        }

        @Override
        public String getFruitName() {
            return "Banana Fruit " + index;
        }

    }

and

    public class Apple implements FruitDetails{

        private int index;
            public Apple(int i) {
                index = i;
        }

        @Override
        public String getFruitName() {
            return "AppleFruit " + index;
        }

    }

I have a function that prints the details of the fruits :

    static void getFruitDetails(List<FruitDetails> fruits) {
            for (FruitDetails fruitDetails : fruits) {
                    System.out.println(fruitDetails.getFruitName());
            }
    }

Now i will create a list of Bananas :

  List<Banana> bananaList = new ArrayList<Banana>();
  bananaList.add(new Banana(0));
  bananaList.add(new Banana(1));

Now i want to print Details of bananas in the above list using getFruitDetails.

Issue is if try to call

  getFruitDetails(bananaList);

I am getting compile time error :

The method getFruitDetails(List<FruitDetails>) in the type MainClass is not applicable for the arguments (List<Banana>)

How I can resolve this.

And also need to define getFruitDetails such that i should be able to print details of banana list or apple list

Answer 1

To accept all implementing classes of FruitDetails as the argument for getFruitDetails, you need to use the type List<? extends FruitDetails>:

static void getFruitDetails(List<? extends FruitDetails> fruits) {
        for (FruitDetails fruitDetails : fruits) {
                System.out.println(fruitDetails.getFruitName());
        }
}

As a side effect this makes the List fruits in getFruitDetails effectively read only for the reason that JB Nizet describes in the comments.