Difference between TreeMap, HashMap and LinkedHashMap in Java programming

HashMap, TreeMap and LinkedHashMap all implements java.util.Map interface and following are their characteristics.

HashMap

• HashMap has complexity of O(1) for insertion and lookup.

• HashMap allows one null key and multiple null values.

• HashMap does not maintain any order.

TreeMap

• TreeMap has complexity of O(logN) for insertion and lookup.

• TreeMap does not allow null key but allow multiple null values.

• TreeMap maintains order. It stores keys in sorted and ascending order.

LinkedHashMap

• LinkedHashMap has complexity of O(1) for insertion and lookup.

• LinkedHashMap allows one null key and multiple null values.

• LinkedHashMap maintains order in which key-value pairs are inserted.

Example

 Live Demo

import java.util.HashMap; import java.util.Hashtable; import java.util.LinkedHashMap; import java.util.Map; import java.util.TreeMap; public class Tester {    public static void main(String args[]) {       Map<String, String> map = new HashMap<String, String>();       map.put("One", "1");       map.put("Five", "5");       map.put("Four", "4");       map.put("Two", "2");       map.put("Three", "3");       System.out.println("HashMap: \n" + map);       Map<String, String> map1 = new LinkedHashMap<String, String>();       map1.put("One", "1");       map1.put("Five", "5");       map1.put("Four", "4");       map1.put("Two", "2");       map1.put("Three", "3");       System.out.println("LinkedHashMap: \n" + map1);       Map<String, String> map2 = new TreeMap<String, String>();       map2.put("One", "1");       map2.put("Five", "5");       map2.put("Four", "4");       map2.put("Two", "2");       map2.put("Three", "3");       System.out.println("TreeMap: \n" + map2);    } }

Output

HashMap: {Five=5, One=1, Four=4, Two=2, Three=3} LinkedHashMap: {One=1, Five=5, Four=4, Two=2, Three=3} TreeMap: {Five=5, Four=4, One=1, Three=3, Two=2}

Here you see, HashMap has random order of Keys, LinkedHashMap has preserved the order in which keys are inserted and TreeMap has sorted order of keys.