Question

I was reading about Collections, when this question stuck me.

Following is the code I wrote to test my doubt.

public static void main(String[] args) {        
    TreeMap<Integer, String> tree = new TreeMap<Integer, String>();
    tree.put(1, "1");
    tree.put(2, "2");
    Set<Integer> set = tree.keySet();
    System.out.println(set instanceof Set);
    System.out.println(set instanceof HashSet);
}

Result :

true
false

Above code says that my set object is a instance of Set. but Set is an Interface how can it be instantiated. I’m confused. 🙁

Answer 1

Set is an interface, so no, you cannot directly instantiate it. Interfaces would be pretty useless if you couldn’t have an instance of one, though! The instance returned by tree.keySet() is some concrete implementation of the Set interface.

Let’s get super-specific, and look at the TreeMap#keySet() source code:

public Set<K> keySet() {
    return navigableKeySet();
}

Okay, that doesn’t tell us much. We need to drill down:

public NavigableSet<K> navigableKeySet() {
    KeySet<K> nks = navigableKeySet;
    return (nks != null) ? nks : (navigableKeySet = new KeySet(this));
}

So the concrete type returned is a KeySet! There’s your implementation of the Set interface. http://www.docjar.com/html/api/java/util/TreeMap.java.html#1021

Which explains this:

System.out.println(set instanceof Set); // prints true
System.out.println(set instanceof HashSet); // prints false

Set is an interface; HashSet is an implementation of that interface. foo instanceof Set will be true for every instance foo of any Set implementation. We already established that the concrete type of the object returned by TreeMap#keySet() is a KeySet, not a HashSet, so that explains why set instanceof HashSet is false – because set is a KeySet, so it cannot be a HashSet!

If that still doesn’t make sense to you, read up on instanceof:

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.