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Introduction to Hashing in Java

Joydip Kanjilal
Published: June 28, 2022

Java Developer Tutorials

Hash functions are a fundamental part of computing, and Java provides excellent support for working with them. In Java, hashing is a common way to store data in collections such as a HashMap and HashSet. This programming tutorial talks about hashing, its benefits and downsides, and how you can work with it in Java.

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What is Hashing?

Hashing is defined as the process of transforming one value into another based on a particular key. A hash is a function that converts an input value into an output value that is usually shorter, and is designed to be unique for each input value. Although collisions are unavoidable, your hash function should attempt to reduce collisions, which implies that different input values should not generate the same hash code.

Hashes are used in many different applications, such as storing passwords, creating unique identifiers, and verifying data. A hash function produces what is known as a hash value, a hash code, or a hash. A hash table is a data structure that stores key-value pairs, where each key is used to calculate an index in the table that corresponds to the location of the value.

Hash functions are used in computer programming for various purposes, such as storing data in a database or verifying data integrity. Hashing is used to secure credentials; for example, passwords before they are stored in the data store. When a user enters their password, a hash function creates a hash code from the password. To verify the password entered by the user, this generated hash code is compared with the stored hash code.

Although there are several types of hash functions, they all accept a fixed-sized input and produce a fixed-sized output. The output size is usually smaller than the input size, which makes hashing a space-efficient way to store data.

Hash functions are designed to be one-way functions, meaning that it should be very difficult to compute the original input from the output (hash code). Nonetheless, collisions can occur if two different inputs result in the same output.

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Types of Hashing Algorithms in Java

There are several hashing algorithms – the most common ones are: MD5, SHA-1, and SHA-256. These algorithms are used to generate a hash of a given piece of data, which can then be used to verify the integrity of that data.

For example, you can leverage a hash algorithm to generate a hash of the file. If the file is modified and a hash is generated again, the new hash value will differ from the earlier has value. This can help you to verify whether or not a file has been tampered with.

What are the Advantages and Disadvantages of Hashing

The main advantage of hashing is that it can be used to store data of any size in a relatively small amount of space. The data is stored in a “hash table”, which is a collection of data values that are each assigned a unique key. When you want to retrieve the data, you simply provide the key and the hash table looks up the associated value.

The main disadvantage of hashing is that it can be hard to retrieve data if you do not know the exact key that was used to store the data. This can be a problem if you are trying to recover lost data or if you want to find all the data that matches a certain criterion. Also, if two pieces of data have the same key, only one will be stored in the hash table resulting in data loss.

Hashing will not be efficient if collisions occur, meaning two or more items are assigned the same key. Additionally, hash functions can be complex, and the data in a hash table must be carefully organized so that the keys can be quickly found.

How to Choose a Java Hashing Algorithm

You should consider a few points before you select a hashing algorithm for your application. The first point is the security, you should to choose an algorithm that is difficult to break. The second is the speed of the algorithm – you should select an algorithm that is high performant. The third is the size of the input: you should select an algorithm that can handle the size of the data you need to hash.

The most popular hashing algorithms are SHA-1, SHA-256, and SHA-512. All of these algorithms are secure and fast and can handle large amounts of data.

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HashMap and HashSet in Java

Java provides multiple ways to implement hashing. Some of the most popular ways are using the HashMap and HashSet classes. Both the HashMap and HashSet classes use hashing algorithms to store and retrieve data.


The HashMap class is a part of the Java Collections Framework. It stores data represented as key-value pairs where the keys are non-null and unique; for example, duplicate keys are not allowed.


The HashSet class is also a part of the Java Collections Framework. It stores data in a set, which means that similar to HashMap, it would not allow duplicate values. However, unlike the HashMap class, the HashSet class does not store data in key-value pairs.

How to Program Hashing in Java

There are many ways to hash in Java. Some of the most common methods are using the built-in hashCode method. To hash a String using the built-in hashCode method, you can use the following code:

String str = "Hello, world!"; 
int hash = str.hashCode();

To hash a String using the SHA-256 hashing algorithm, you can use the following code:

String str = "Hello, world!"; 
String algorithm = "SHA-256"; 
byte[] bytes = Hashing.digest(algorithm, str.getBytes()).asBytes();

The following code listing shows how you can generate hash code for variables in Java. Note that the hash code for str1 and str2 will differ but the hash code for str3 and str4 will be identical:

public class Test {
    public static void main(String args[]) {
      String str1 = "Hello";
      String str2 = "World!";
      System.out.println("The hash code of str1 is: " + str1.hashCode());
      System.out.println("\nThe hash code of str2 is: " + str2.hashCode());
      String str3 = "Same value";
      String str4 = "Same value";
       System.out.println("The hash code of str3 is: " + str3.hashCode());
      System.out.println("\nThe hash code of str4 is: " + str4.hashCode());

Final Thoughts on Hashing in Java

In this programming tutorial, we examined hashing, its types, benefits, and how to work with hashing in Java. We also looked at how to use a salt to improve the security of your hashes. By understanding how hashing works, you can make more informed choices about which algorithm is best for your needs.

Read more Java programming tutorials and software development guides.