Java TreeMap Example

• 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.

Hierarchy of TreeMap class:

Example of TreeMap class:

package com.mnb;

import java.util.Map;

import java.util.NavigableMap;

import java.util.Set;

import java.util.SortedMap;

import java.util.TreeMap;

public class TreeMapDemo {

public static void main(String[] args) {

TreeMap<Integer, String> tm = new TreeMap<Integer, String>();

tm.put(1, "Power");

tm.put(2, "Star");

tm.put(4, "Kalyan");

tm.put(3, "Pawan");

System.out.println("Automatically sorted by natural order of Keys from TreeMap:");

for (Map.Entry<Integer,String> me : tm.entrySet()) {

System.out.println(me.getKey() + " : " + me.getValue());

}

System.out.println("To get a reverse order view of Mapping from TreeMap:");

NavigableMap<Integer,String> reverseTreeMap = tm.descendingMap();

Set<Integer> keySet = reverseTreeMap.keySet();

for (Integer key : keySet) {

System.out.println(key + " : " + reverseTreeMap.get(key));

}

// To get first and last entry from TreeMap in Java

System.out.println("First Entry: " + tm.firstEntry());

System.out.println("Last Entry: " + tm.lastEntry());

// To get first and last key from TreeMap in Java

System.out.println("First Key: " + tm.firstKey());

System.out.println("Last Key: " + tm.lastKey());

// Creating subMap from TreeMap in Java

SortedMap<Integer,String> subMap = tm.subMap(3, 5);

System.out.println("SubMap: " + subMap);

// Creating headMap and tailMap from TreeMap in Java

SortedMap<Integer,String> headTreeMap = tm.headMap(3);

SortedMap<Integer,String> tailTreeMap = tm.tailMap(3);

System.out.println("Head Map: " + headTreeMap);

System.out.println("Tail Map: " + tailTreeMap);

}

}

Output:

Automatically sorted by natural order of Keys from TreeMap:

1 : Power

2 : Star

3 : Pawan

4 : Kalyan

To get a reverse order view of Mapping from TreeMap:

4 : Kalyan

3 : Pawan

2 : Star

1 : Power

First Entry: 1=Power

Last Entry: 4=Kalyan

First Key: 1

Last Key: 4

SubMap: {3=Pawan, 4=Kalyan}

Head Map: {1=Power, 2=Star}

Tail Map: {3=Pawan, 4=Kalyan}