Question

I have a method like this-

public List<Apples> returnFruits(String arg1){
List<Apples> fruits = new ArrayList<Apples>();
Apple a = new Apple();
fruits.add(a);

return fruits;
}

I would like to change it, so that I can specify the fruit type from the method call and return that fruit list. So the 2nd statement should dynamically instantiate the list of fruits that I pass. I thought of this-

public List<?> returnFruits(String arg1){
List<T> fruits = new ArrayList<T>();
T t = new T();
fruits.add(t);

return fruits;
}

But don’t know the right way to do it, as you can see.

In the second method, I just return the fruit instead of the list-

public T returnFruit(){
T t = new T();
return t;
}

The fruits that are passed are NOT in the same class hierarchy and are different types.

Thank you.

Answer 1

There are a number of approaches you could take. One is to use Class<T> as others have suggested. Another would be to create some sort of producer interface, and pass that in:

public interface FruitProducer<T> {
    T createFruit(String arg);
}

public <T> List<T> returnFruits(String arg, FruitProducer<? extends T> producer) {
    List<T> list = new ArrayList<T>();
    T fruit = producer.createFruit(arg);
    list.add(fruit);
    return list;
}

You have different producers for different fruits: AppleProducer implements FruitProducer<Apple>, OrangeProducer implements FruitProducer<Orange>, etc. This approach just kicks the can a bit — the FruitProducer still has to create the fruits somehow — but it could be a useful refactoring.

Yet another approach relies on the same polymorphism as the FruitProducer approach by making the class with returnFruits abstract:

public abstract class FruitLister<T> {
    public abstract List<T> returnFruits(String arg);
}

Now you have different listers: AppleLister implements FruitLister<Apple>, etc. Each knows how to instantiate the specific classes it needs to return in a list:

public class AppleLister implements FruitLister<Apple> {
    @Override
    public List<Apple> returnFruits(String arg) {
            List<Apple> list = new ArrayList<Apple>();
            Apple apple = new Apple(arg);
            list.add(apple);
            return list;
    }
}