Why doesn't ArrayList.contains(Object.class) work for finding instances types?

Question

Say I have an ArrayList which is populated with objects of different types...

ArrayList<Fruit> shelf = new ArrayList<Fruit>();
Apple apple = new Apple();
Orange orange = new Orange();
Pear pear = new Pear();

shelf.add(apple);
shelf.add(orange);
shelf.add(pear);

I want to find out if shelf contains an Orange object. I've tried

shelf.contains(Orange.class)

but this doesn't return true. My understanding is that contains makes use of the equals method for object comparison, so I'm not sure why this is the case.

I realise I can simply iterate through the ArrayList and check the type of the objects individually, but I'm curious as to why contains doesn't behave the way I expect it to.

Answer 1

You are correct, contains uses equals. However, an instance of a class is not equal to an object of the class, i.e. Orange.class.equals(new Orange()) is false.

You will need a custom method to check for a list containing an instance of a class.

public static <E> boolean containsInstance(List<E> list, Class<? extends E> clazz) {
    for (E e : list) {
        if (clazz.isInstance(e)) {
            return true;
        }
    }
    return false;
}

And here's a Java 8 version making use of the Stream API and lambdas:

public static <E> boolean containsInstance(List<E> list, Class<? extends E> clazz) {
    return list.stream().anyMatch(e -> clazz.isInstance(e));
}