TreeMap class in collection framework

• A TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
• It contains only unique elements.
• It cannot have null key but can have multiple null values.
• It is same as HashMap instead maintains ascending order.

The TreeMap class implements the Map interface by using a tree. A TreeMap provides an efficient means of storing key/value pairs in sorted order, and allows rapid retrieval.

You should note that, unlike a hash map, a tree map guarantees that its elements will be sorted in ascending key order.

The TreeMap class supports four constructors. The first form constructs an empty tree map that will be sorted by using the natural order of its keys:

TreeMap( )

The second form constructs an empty tree-based map that will be sorted by using the Comparator comp:

TreeMap(Comparator comp)

The third form initializes a tree map with the entries from m, which will be sorted by using the natural order of the keys:

TreeMap(Map m)

The fourth form initializes a tree map with the entries from sm, which will be sorted in the same order as sm:

TreeMap(SortedMap sm)

Apart from the methods inherited from its parent classes, TreeMap defines following methods:

Example:

import java.util.*;
class TreeMapDemo
{
 public static void main(String args[])
 {
  TreeMap< String,Integer> tm = new TreeMap< String,Integer>();
  tm.put("a",new Integer(100));
  tm.put("b",new Integer(200));
  tm.put("c",new Integer(300));
  tm.put("d",new Integer(400));

  Set< Map.Entry< String,Integer> > st = tm.entrySet();
  for(Map.Entry me:st)  
  {
   System.out.print(me.getKey()+":");
   System.out.println(me.getValue());
  }
 }
}