Java quiz. What will be the output of the following code, ass

What will be the output of the following code, assuming all imports are in place and the code is inside a main method?


NavigableMap<String, String> nm = new TreeMap<String, String>();
nm.put("1", "one");
nm.put("3", "three");
nm.put("2", "two");
nm.put("4", "four");

NavigableSet<String> keys = nm.keySet();
NavigableSet<String> subKeys = keys.subSet("1", true, "3", false);

for(String s: subKeys) {
    System.out.print(s + " ");
}

1 2
1 2 3
Runtime exception
Compilation error
1 3
1 3 2

Explanation

This code will fail to compile. The keySet() method will return a Set. Set is a superclass of NavigableSet, hence it is-not-a NavigableSet (we've got exactly the opposite: a NavigableSet is-a Set). An explicit cast would fix the code through: (NavigableSet<String>) nm.keySet();