Differences Between HashMap and HashTable in Java

If you’re learning about Java and its applications, you must have come across HashMap and HashTable. Both of them are among the most important classes in the Java Collection framework. You’ll be using them extensively while developing with Java, which is why it’s vital to understand their differences. 

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In the following points, we’ll cover the HashTable vs HashMap topic in detail and explain the distinctions between the two: 

What is HashMap?

HashMap has been in Java’s collection since the introduction of Java 1.2. It allows you to perform basic implementations of the Map interface of Java. HashMap stores data in (Key, Value) pairs, and to access them, you’ll have to use an index of another type, such as an integer. 

Here, you use one object as a key (index) for another object (value), hence the use of (key, value) pair. If you add a duplicate key, it will replace the element of the corresponding key. 

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Features of HashMap

To understand the distinctions between HashTable vs HashMap, first, you must understand their features. It would make you familiar with the basics of both of them. HashMap has the following features:

• It is part of java.util package.
• It extends an abstract class AbstractMap that provides an incomplete implementation of the Map interface.
• Along with implementing Map interface, it also implements Serializable and Cloneable interfaces. 
• It allows duplicate values but doesn’t allow duplicate keys. This means one key can’t have more than one value, but multiple keys can have a single value. 
• You can use the null key only once in a HashMap, but you can use multiple null values. 
• It makes no guarantees about the order of the map, particularly whether or not the order would remain constant over time. HashMap is nearly similar to HashTable but is unsynchronised. 
• HashSet uses HashMap internally.

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What is HashTable?

You use the HashTable class to implement a hash table that maps keys to values. Here, you can use non-null objects as a key or as a value. Keep in mind that to store and retrieve objects from a hashtable successfully, the objects you used as keys must implement the hashCode method and the equals method. 

A hashtable stores data in an array format, and every data value has a unique index value. This allows you to access particular data quite quickly if you know the required index. 

Features of HashTable

HashTable has its peculiar features, just like HashMap. However, these features make it unique and different from HashMap in many respects: 

• HashTable is quite similar to HashMap but is synchronised.
• It stores key-value pairs in a hash table.
• Here, you specify an object used as a key and the value you want to associate to the same. Then you hash the key and use the generated hash code as the index where you store the value within the table. 
• The default capacity of the Hashtable class is 11, and the load factor is 0.75.
• HashMap doesn’t offer Enumeration, whereas Hashtable offers not fail-fast Enumeration. 

Similarities in HashMap and HashTable

We may better understand the difference between HashMap and HashTable after we are familiar with fundamental ideas and the similarities between HashMaps and Hash Tables.

• In Java, neither HashMap nor HashTable maintains insertion order.
• The Java.util.Map interface is implemented by both HashMap and HashTable.
• The complexity offered by methods of both HashTable and HashMap is the same in Java:

Differences Between HashMap and HashTable

Now that you know the particular highlights of HashMap and HashTable, we’ll now compare them and see the prominent differences between the two:

Coding Example

Following is an example of HashMap and HashTable at work so you can understand the distinctions of HashMap vs HashTable. 

Input: 
import java.util.*;

import java.lang.*;

import java.io.*;

public class JavaTester{

   public static void main(String args[]){

      Hashtable ht=new Hashtable();

      ht.put(1,”Uday”);

      ht.put(1,”Ujjwal”);

      ht.put(2,”Sumit”);

      ht.put(3,”Vijay”);

      System.out.println(“————-Hash table————–“);

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

      for (Integer key:keySet) {

         System.out.println(key + ” “+ht.get(key));

      }

      HashMap hm=new HashMap();

      hm.put(0,”Uday”);

      hm.put(4,”Uday”); // you can have duplicate values in a hashmap

      hm.put(1,”Sumit”);

      hm.put(2,”Vijay”);

      System.out.println(“———–Hash map———–“);

      Set<Integer> keySet1 = ht.keySet();

      for (Integer key:keySet) {

         System.out.println(key + ” “+hm.get(key));

      }

   }

}

Output: 

Hash table: 

3 Vijay

2 Sumit

1 Ujjwal

Hash map:

0 Uday

1 Sumit

2 Vijay

4 Uday

When to use HashMap vs HashTable?

The primary factor that determines whether you’ll use HashMap or HashTable is synchronisation. If you need a thread-safe task, then you should use HashTable because all its methods are synchronised. However, it is a legacy class, and you should avoid them.

If you have a multithreaded environment, you should use ConcurrentHashMap as it’s pretty similar to HashTable. It allows you to make the HashMap synchronised properly.

Synchronised operations cause poor performance, so you should avoid them in most cases. Moreover, HashMap is suitable for a non-threaded environment, so you can use it easily. 

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Concurrent HashMap vs HashTable

ConcurrentHashMap uses HashTable as its basic data structure. But there still exist a few differences to be noted:

Hashmap vs Hashtable: Limitations

When discussing the differences, you must also be familiar with their limitations which can help you choose the correct data structure in Java.

Limitations of HashMap

• The HashMap performs poorly in case of a chance of collision when two distinct keys produce the same hashCode() value.
• Sometimes HashMap needs to be resized since the buckets’ original size is full. The elements from the old HashMap are transferred to a new, larger HashMap, which takes O(n) time.

Limitations of Hash table

• The data to be read is distributed in-memory seemingly at random, which is a problem with HashTables in general.
• Databases may deteriorate if there are a lot of collisions, which is one of the drawbacks of HashTables. The more data, the greater the chances of collision. Many hash algorithms lack the ability to switch from one data set to another or to the previous data set.

Conclusion

HashMap and HashTable are popular Java codes with similar functions. However, as you can see there are several prominent differences between the two. HashMap is a Java class, whereas HashTable is a data structure. 

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