Question

keySet() returns java.util.Set.
Why doesn’t cast to NavigableSet throw a ClassCastException?
This can be if real Object is a TreeSet with the java.util.Set reference. I can`t understand that.

import java.util.*;
class A1{}
public class Main{
     public static void main(String args[]){
         SortedMap nvs=new TreeMap();
         nvs.put(1,"one");
         nvs.put(2,"two");
         nvs.put(3,"three");
         nvs.put(4,"four");
         NavigableSet nss=(NavigableSet)nvs.keySet();
         for(Object ob: nss){
              System.out.print(nvs.get(ob)+", ");
         }
     }
}

Answer 1

The best thing to do would be to look at actual code of TreeMap (this is from java 1.7):

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

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

And here’s the KeySet class declaration:

static final class KeySet<E> extends AbstractSet<E> implements NavigableSet<E> {...}

As you can see, TreeMap#keySet method returns a type Set which is a super interface of NavigableSet. And the Set reference returned from it points to a KeySet instance, as in navigableKeySet() method.

And since KeySet class implements NavigableSet, you can always cast a reference pointing to it’s instance to a NavigableSet reference.

Just to make sure that you are independent of keySet() method implementation you could simply call navigableKeySet() method to get NavigableSet and avoid the cast