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An Introduction to Hashtable and HashMap in Java

Joydip Kanjilal
Published: July 25, 2022

Java Developer Tutorials

In computing, a Hashtable is defined as a data structure that stores data represented as key-value pairs. Compared to a map, it is more efficient for large data sets. This Java programming tutorial discusses Hashtable and HashMap data structures, their features and benefits, and how to work with them in Java.

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What is a Hashtable in Java?

A Hashtable is a data structure used to preserve data represented as key-value pairs. Despite its similarities with a map, it is more efficient when dealing with huge data sets. This explains why a Hashtable is a great choice in applications where performance is important. 

Hashtable is used for fast lookup of data, storing data in dynamic ways, or just storing it in compact ways, which makes it more efficient compared to other solutions, such as arrays or linked lists.

A Hashtable is often used in programming languages, such as Java, to store data in a way that is easy to retrieve. A Hashtable can store large amounts of data quickly and easily, making it ideal for use in applications where speed is important.

A Hashtable works by storing data in a table, with each piece of data having a unique key. You can retrieve data from a Hashtable using its key. Once you provide the key, you can get the corresponding value.

The code snippet that follows shows how you can create an empty Hashtable instance:

Hashtable<K, V> hashTable = new Hashtable<K, V>();

How Does Hashtable Work in Java?

Hashtable is an abstract class and has two subclasses that extend this class — HashMap and LinkedHashMap. The HashMap provides the set of elements stored in it, while the LinkedHashMap allows insertion of new items at any position. 

What are the Benefits of Using Hashtable in Java?

Hashtable is one of the most efficient of all data structures as far as performance is concerned. You can take advantage of Hashtable for fast data storage and retrieval. Hashtable is also thread-safe making it an excellent choice for multithreaded applications where concurrency is essential.

When Should You Use Hashtable in Java?

A Hashtable is a data structure that stores information in key-value pairs. The key is required when retrieving items from a Hashtable. This can be advantageous if you have a lot of data and need to be able to quickly find specific items.

However, Hashtables are not well suited for storing data that needs to be sorted in any particular order. Additionally, because keys in a Hashtable must be unique, it is not possible to store duplicate keys in a Hashtable.

Overall, Hashtables are a good option for storing data when you need quick access to specific items and don’t mind if the data is unordered.

You can learn more about Hashing by reading our tutorial: Introduction to Hashing in Java.

How to Program Hashtable in Java

To create a Hashtable, programmers need to import the java.util.Hashtable package. Then, you can create a Hashtable object like this:

Hashtable hashTable = new Hashtable();

You can now add data represented as key-value pairs to the Hashtable instance. To do so, you will use the put() method, like this:

hashTable.put("key1", "value1");
hashTable.put("key2", "value2");

You can retrieve values from the Hashtable using the get() method, like so:

String str1 = hashTable.get("key1");
String str2 = hashTable.get("key2");

If you want to check if a key exists in the Hashtable, you can use the containsKey() method:

boolean containsKey = hashTable.containsKey("key1");

Finally, if you want to get a list of all the keys or values in the Hashtable, you can use the keySet() and values() methods:

Set keys = hashTable.keySet();
Collection values = hashTable.values();

How to Improve the Performance of Hashtable in Java?

Developers can use a different hashing algorithm to improve Hashtable’s performance. The default hashing algorithm used by Hashtable is known as Adler-32. However, there are other algorithms available that can be faster, such as Murmur3. To change the hashing algorithm used by Hashtable, you can use the setHashingAlgorithm() method.

Increasing the internal array size is another way to improve Hashtable’s performance. By default, Hashtable uses an array with a size of 16. The setSize() method allows programmers to increase this size. Performance will be improved because collisions will be fewer when the array is larger.

Finally, you can also consider using a different data structure altogether if performance is a major concern for you. For example, you could use a tree-based data structure, such as a red-black tree, instead of a Hashtable. Tree-based data structures tend to be much faster than Hashtables when it comes to lookup operations.

What is a HashMap in Java? How does it work?

Hashmap is a linked-list implementation of Map, where each element is assigned a unique integer hash code. An instance of HashMap contains a set of key-value pairs where the keys are instances of String and the values are instances of any Java serializable data type. The default storage mechanism used by HashMap is basically an array which is resized when the number of stored entries reaches a specified limit.

Since a hash code is used to store the key-value pair in the map using their hash codes, it means that two keys with the same hash code will end up in the same bucket and this can result in collisions. When there is a collision, HashMap uses its secondary storage to store the conflicting key-value pairs.

The code snippet that follows shows how you can create an empty HashMap instance in Java:

HashMap<K, V> hashMap = new HashMap<K, V>();

How to Program HashMap in Java

Refer to the code snippet shown below that shows how you can create an empty instance of a HashMap, insert data as key-value pairs to it and then display the data at the console window.

import java.io.*;
import java.util.*;
  public class MyHashMapHashtableExample {
    public static void main(String args[])
    {
        Map<Integer, String> hashMap = new HashMap<>();
        hashMap.put(1, "A");
        hashMap.put(2, "B");
        hashMap.put(3, "C");
        hashMap.put(4, "D");
        hashMap.put(5, "E");
        Hashtable<Integer, String> hashTable
            = new Hashtable<Integer, String>(hashMap);
        System.out.println(hashTable);
    }
}

While the put method of the HashMap class can be used to insert items, the remove method can be used to delete items from the collection.

For example, the code snippet given below can be used to remove the item having the key as 3.

hashMap.remove(3);

Final Thoughts on Hashtable and HashMap in Java

A Hashtable can store large amounts of data quickly and easily, making it ideal for use in applications where performance is important. A collision occurs when two objects pertaining to the same Hashtable have the same hash code. A Hashtable is adept at avoiding such collisions using an array of lists.

Read more Java programming tutorials and software development guides.

Source: www.developer.com