Java - The SortedMap Interface


The SortedMap interface extends Map. It ensures that the entries are maintained in ascending key order

Several methods throw a NoSuchElementException when no items are in the invoking map. A ClassCastException is thrown when an object is incompatible with the elements in a map. A NullPointerException is thrown if an attempt is made to use a null object when null is not allowed in the map.

The methods declared by SortedMap are summarized in the following table:

SN Methods with Description
1 Comparator comparator( )

Returns the invoking sorted map's comparator. If the natural ordering is used for the invoking map, null is returned.

2 Object firstKey( )

Returns the first key in the invoking map.

3 SortedMap headMap(Object end)

Returns a sorted map for those map entries with keys that are less than end.

4 Object lastKey( )

Returns the last key in the invoking map.

5 SortedMap subMap(Object start, Object end)

Returns a map containing those entries with keys that are greater than or equal to start and less than end

6 SortedMap tailMap(Object start)

Returns a map containing those entries with keys that are greater than or equal to start.


SortedMap have its implementation in various classes like TreeMap, Following is the example to explain SortedMap functionlaity:

import java.util.*;

public class TreeMapDemo {

   public static void main(String args[]) {
      // Create a hash map
      TreeMap tm = new TreeMap();
      // Put elements to the map
      tm.put("Zara", new Double(3434.34));
      tm.put("Mahnaz", new Double(123.22));
      tm.put("Ayan", new Double(1378.00));
      tm.put("Daisy", new Double(99.22));
      tm.put("Qadir", new Double(-19.08));
	  // Get a set of the entries
      Set set = tm.entrySet();
      // Get an iterator
      Iterator i = set.iterator();
      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry);
         System.out.print(me.getKey() + ": ");
      // Deposit 1000 into Zara's account
      double balance = ((Double)tm.get("Zara")).doubleValue();
      tm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " +

This would produce the following result:

Ayan: 1378.0
Daisy 99.22
ahnaz: 123.22
Qadir: -19.08
Zara: 3434.34
Zara.s current balance: 4434.34