Difference between TreeMap, HashMap, and LinkedHashMap in Java

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

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: 
" + 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: 
" + 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: 
" + 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.