differences between TreeMap, HashMap, and LinkedHashMap in Java programming

HashMap, TreeMap, and LinkedHashMap all implement the java.util.Map interface, and the following are their features.

Hash map

• The insertion and lookup complexity of HashMap is O(1).

• HashMap allows one null key and multiple null values.

• HashMap does not maintain any order.

Tree diagram

• The insertion and lookup complexity of TreeMap is O(logN).

• TreeMap does not allow null keys, but allows multiple null values.

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

LinkedHashMap

• The insertion and lookup complexity of LinkedHashMap is O(1).

• LinkedHashMap allows one null key and multiple null values.

• LinkedHashMap maintains the order of insertion of key-value pairs.

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<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 Result

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 can see that HashMap has a random key order, LinkedHashMap retains the order of inserted keys, and TreeMap has a sorted key order.