Difference between Hashtable, HashMap, TreeMap and LinkedHashMap

Difference between Hashtable, HashMap, TreeMap and LinkedHashMap

TreeMap  

Map is one of the most important data structures and when we talk about Map all these 4 implementation classes (Hashtable, HashMap, TreeMap and LinkedHashMap) are most used. Hashtable, HashMap and LinkedHashMap are directly implements Map interface, but TreeMap implements Map interface through implementing SortedMap. Below are the few differences between these classes

• HashMap implemented as same as Hashtable except it is unsynchronized and permits null. As common both HashMap and Hashtable won't have any ordering on keys or values.
• TreeMap implemented based on red-black tree structure and it follows natural ordering on key. 
• LinkedHashMap is a subclass of HashMap and inherits all properties of HashMap class and additionally its ordering will based on insertion order.

NOTE:

• If we use any Map implementation classes and if the key is user-defined class Object then we need to override hashcode() and equals() methods. 
• Next when we use TreeMap, by default they are sorted by keys. If we use user-defined class Object for key, then to compare with each other we need to implement Comparable interface.

Next lets see simple examples for each classes. 

Hashtable:

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class MapTest {

 public static void main(String[] args) {

  Map<String, String> map = new Hashtable<String, String>();
  map.put("1", "one");
  map.put("2", "two");
  map.put("3", "three");
  map.put("4", "four");
  map.put("5", "five");

  Iterator<?> itr = map.entrySet().iterator();
  while (itr.hasNext()) {
   Map.Entry myMap = (Map.Entry) itr.next();
   System.out.println(myMap.getKey() + " : " + myMap.getValue());
  }
 }
}

OUTPUT:

5 : five
4 : four
3 : three
2 : two
1 : one

HashMap:

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class MapTest {

 public static void main(String[] args) {

  Map<String, String> map = new HashMap<String, String>();
  map.put("1", "one");
  map.put("2", "two");
  map.put("3", "three");
  map.put("4", "four");
  map.put("5", "five");

  Iterator<?> itr = map.entrySet().iterator();
  while (itr.hasNext()) {
   Map.Entry myMap = (Map.Entry) itr.next();
   System.out.println(myMap.getKey() + " : " + myMap.getValue());
  }
 }
}

OUPTUT:

3 : three
2 : two
1 : one
5 : five
4 : four

TreeMap:

import java.util.Iterator;
import java.util.Map;
import java.util.TreeMap;

public class MapTest {

 public static void main(String[] args) {

  Map<String, String> map = new TreeMap<String, String>();
  map.put("2", "two");
  map.put("3", "three");
  map.put("1", "one");
  map.put("4", "four");
  map.put("5", "five");

  Iterator<?> itr = map.entrySet().iterator();
  while (itr.hasNext()) {
   Map.Entry myMap = (Map.Entry) itr.next();
   System.out.println(myMap.getKey() + " : " + myMap.getValue());
  }
 }
}

OUTPUT:

1 : one
2 : two
3 : three
4 : four
5 : five

LinkedHashMap:

import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;

public class MapTest {

 public static void main(String[] args) {

  Map<String, String> map = new LinkedHashMap<String, String>();
  map.put("2", "two");
  map.put("3", "three");
  map.put("1", "one");
  map.put("4", "four");
  map.put("5", "five");

  Iterator<?> itr = map.entrySet().iterator();
  while (itr.hasNext()) {
   Map.Entry myMap = (Map.Entry) itr.next();
   System.out.println(myMap.getKey() + " : " + myMap.getValue());
  }
 }
}

OUTPUT:

2 : two
3 : three
1 : one
4 : four
5 : five

Difference between HashMap and HashTable in Java

Difference between HashMap and HashTable in Java

Hashtable  

Map is one of the important Interface in Java Collections and Maps contains key and value which always stores unique keys and values. Both HashMap and Hashtable implements Map interface and works based on hashing algorithm. 

Even HashMap and Hashtable implements Map interface but there are few important difference between these. Before using HashMap and Hashtable in application we need to know the functionality of both, by mistake if we select incorrect implementation class then it leads to lot of performance impact and error in our application. 

From JDK 1.4 onwards Hashtable included in Collection
Lets see few important difference between HashMap and Hashtable in Java.


Difference between HashMap and Hashtable.

• First important difference between HashMap and Hashtable is, Hashtable is thread-safe (by default synchronized) and HashMap is not thread-safe.
• HashMap allows null key and null value, but Hashmap won't allow null key and value which will give runtime NullPointerException.
• As performance wise HashMap will be faster than Hashtable since its not synchronized. 
• Iterator in HashMap class is a fail-fast iterator and enumerator in Hashtable is fail-safe and throw ConcurrentModificationException if any other Thread modifies.

Alternate for Hashtable:

If we need to use HashMap with thread-safe then we can convert HashMap to synchronized HashMap or we can for ConcurrentHashMap class. How to convert HashMap to synchronized HashMap have seen in our earlier tutorial.

Below are simple example for HashMap and Hashtable.

HashMap:

public  class MyTestClass {

 public static void main(String[] args) {
  
  Map<String, String> hm = new HashMap<String, String>();
  hm.put(null, null);
  hm.put("1", "one");
  System.out.println("Value : "+ hm.get(null));
  
  // Converting HashMap to Synchronized HashMap
  hm = Collections.synchronizedMap(hm);  
 }
}

Hashtable:

public  class MyTestClass {

 public static void main(String[] args) {
  
  Map<String, String> ht = new Hashtable<String, String>();
  
  // gives NullPointerException
  //ht.put(null, null);
  
  ht.put("1", "one");
  System.out.println("Value : "+ ht.get(null));
 }
}