Using type casting and generics in java

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

Maybe it's my OCD but it would be great if the interface was called Fruit instead of FruitDetails :-) — Rahul
– Rahul, Commented Jan 20, 2012 at 7:49

x4u · Accepted Answer · 2012-01-20 08:04:47Z

5

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.

edited Jan 20, 2012 at 8:04

answered Jan 20, 2012 at 7:49

x4u

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4 Comments

JB Nizet Over a year ago

Just to explain (once again): If a List<Banana> was a List<Fruit>, you could add an Apple to the list, and it would thus break the type-safety of the list.

Pavan Over a year ago

But I have FruitDetails as an interface.

Pavan Over a year ago

this is correct. Sorry i was under the impression that as FruitDetails was a interface, expression "? extends FruitDetails" should be "? implements FruitDetails" which was giving error. Also can i know how this works.

newacct Over a year ago

@Pavan: in generics, type bounds are expressed using extends and super regardless of whether the type happens to be a class or interface (or, even a type variable, in which case we don't know whether it is a class or interface)

Rahul · Accepted Answer · 2012-01-20 07:50:51Z

1

This is because you can not assign List<Banana> to List<FruitDetails>. Try changing the code to

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

you should read about generics subtyping to understand this.

answered Jan 20, 2012 at 7:50

Rahul

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1 Comment

Pavan Over a year ago

Again FruitDetails is not a Baseclass. Its an interface

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