Core Java

Alter Keys in HashMap

A Java HashMap is a data structure that stores key-value pairs, allowing for efficient retrieval and storage of values based on unique keys. It is part of the Java Collections Framework and implements the Map interface. HashMap uses a hash table to internally organize data, providing constant-time (O(1)) average complexity for basic operations like put and get. Keys are unique and unordered, making them suitable for tasks like caching, indexing, or fast data retrieval. However, it does not guarantee any specific order of elements. HashMap is widely used in Java for tasks requiring efficient key-based data storage and retrieval in various applications. Let’s dive into how to alter keys in HashMap.

1. Introduction

In the realm of Java programming, the HashMap is a versatile and indispensable data structure. It belongs to the Java Collections Framework and serves as a dynamic, high-performance container for storing key-value pairs. A HashMap in Java is a data structure that enables efficient storage and retrieval of values based on unique keys. It is designed to provide lightning-fast access to values by leveraging a hash table internally. HashMaps are part of Java’s rich collection of data structures and implement the Map interface, ensuring the uniqueness of keys within the collection.

1.1 Benefits

  • Constant-Time Access: HashMaps offer constant-time complexity (O(1)) for key-based operations such as inserting (putting) and retrieving (getting) values. This makes them exceptionally fast for tasks that require rapid data access.
  • Dynamic Sizing: HashMaps automatically resize themselves as needed to accommodate more data. This dynamic sizing ensures efficient memory usage, and developers don’t need to worry about managing the underlying data structure.
  • Key-Based Retrieval: HashMaps are ideal for scenarios where you need to associate data with specific keys. Whether you’re building a cache, an index, or managing configurations, HashMaps excels at mapping keys to values.
  • Null Values: Unlike some other data structures, HashMaps allows the storage of null values for both keys and values, making them versatile and flexible.

1.2 Use Cases

  • Caching: HashMaps are frequently employed in caching systems. They can store frequently used data in memory, allowing for rapid retrieval without the need to regenerate or re-fetch the data.
  • Indexing: In database management systems and search engines, HashMaps are used to build indexes that accelerate data retrieval. By mapping keys to data records, they facilitate efficient querying.
  • Configuration Management: HashMaps can be used to store and manage configuration settings in applications. This allows for quick lookups of configuration values based on their names or keys.
  • Counting and Aggregating: When processing large datasets, HashMaps can be used to tally and aggregate data efficiently. For example, you can count the occurrences of specific elements in a collection.
  • Implementing Custom Data Structures: Developers often use HashMaps as a foundational data structure to implement more complex data structures like graphs, sets, and multimaps.

2. Modifying a Java HashMap

Java HashMaps provide two essential methods for modifying their content:

  • remove(Key key): The remove() method allows you to remove a key-value pair from the HashMap based on the specified key. It returns the value associated with the key that was removed, or null if the key was not found.
  • put(Key key, Value value): The put() method is used to insert or update a key-value pair in the HashMap. If the key already exists, the associated value will be updated. If the key is not present, a new key-value pair will be added to the HashMap.

These methods are essential for dynamically managing the contents of a HashMap during runtime, allowing you to add, update, or remove data as needed.

2.1 Working Example

Here’s a practical example in Java demonstrating how to use the remove() and put() methods in a HashMap. In this example, we’ll create a simple phone book application:

package com.jcg.example;

import java.util.HashMap;

public class PhoneBookExample {

    public static void main(String[] args) {
        // Create a HashMap to represent a phone book
        HashMap<String, String> phoneBook = new HashMap<>();

        // Adding entries to the phone book
        phoneBook.put("John Smith", "+1234567890");
        phoneBook.put("Alice Johnson", "+9876543210");
        phoneBook.put("Bob Brown", "+5555555555");

        // Display the phone book
        System.out.println("Phone Book:");

        // Suppose John Smith changed his phone number
        // We can update the entry using put()
        phoneBook.put("John Smith", "+9999999999");

        // Display the updated phone book
        System.out.println("\nUpdated Phone Book:");

        // Now, let's remove Alice Johnson from the phone book
        // We can use remove() for this purpose
        phoneBook.remove("Alice Johnson");

        // Display the phone book after removal
        System.out.println("\nPhone Book after Removing Alice Johnson:");

    // Helper method to display the phone book
    public static void displayPhoneBook(HashMap<String, String> phoneBook) {
        for (String name : phoneBook.keySet()) {
            String phoneNumber = phoneBook.get(name);
            System.out.println(name + ": " + phoneNumber);

In this example:

  • We create a HashMap named phoneBook to store phone book entries, where names are keys, and phone numbers are values.
  • We add entries to the phone book using the put() method.
  • We display the phone book entries before and after updating John Smith’s phone number and removing Alice Johnson.
  • The remove("Alice Johnson") method is used to remove the entry for Alice Johnson from the phone book.
  • The put("John Smith", "+9999999999") method is used to update John Smith’s phone number.

When you run this Java program, you’ll see the phone book entries before and after modifications, demonstrating the use of the remove() and put() methods in a HashMap.

2.1.1 Ide Output

Here’s the sample output of the Java code provided earlier:

Console Output

Phone Book:
John Smith: +1234567890
Alice Johnson: +9876543210
Bob Brown: +5555555555

Updated Phone Book:
John Smith: +9999999999
Alice Johnson: +9876543210
Bob Brown: +5555555555

Phone Book after Removing Alice Johnson:
John Smith: +9999999999
Bob Brown: +5555555555

This output demonstrates the behavior of the Java program using a HashMap for a phone book. It first displays the phone book entries, then updates John Smith’s phone number, and finally removes Alice Johnson’s entry, showing the modified phone book at each step.

3. Never Modify Keys in a HashMap

You are correct. Modifying keys in a HashMap is not recommended and can lead to unexpected behavior or data corruption. In Java, keys in a HashMap (or any object used as keys in a hash-based collection) should be treated as immutable, meaning they should not be changed after they are used as keys in the collection.

The reason for this recommendation is that the hash code of an object is used to determine its storage location within the HashMap. If you modify an object used as a key, its hash code may change, and the HashMap won’t be able to locate the associated value correctly. This can result in lost data or erroneous behavior.

If you need to update a key-value pair in a HashMap, the typical approach is to remove the old entry using the original key and then add a new entry with the updated key and value. Here’s an example:

// Create a HashMap
HashMap<String, Integer> scores = new HashMap<>();

// Add an entry
scores.put("Alice", 100);

// Update the entry
scores.remove("Alice"); // Remove the old entry
scores.put("Alice", 120); // Add the updated entry

In this example, we remove the old entry with the key “Alice” and then put a new entry with the same key and the updated value. This ensures that the HashMap remains consistent and behaves as expected.

4. Conclusion

In conclusion, when utilizing the Java HashMap data structure, it’s imperative to adhere to a set of best practices to ensure code reliability and data integrity. One fundamental guideline is treating keys within a HashMap as immutable entities; modifying them after they’ve been used as keys can lead to unpredictable outcomes due to potential changes in hash codes. To update key-value pairs, the recommended approach involves removing the old entry using the remove() method and subsequently adding the updated entry via put(). Furthermore, when custom objects serve as keys, ensuring their immutability and implementing proper equals() and hashCode() methods is vital. By meticulously following these practices, developers can harness the full potential of HashMap while avoiding common pitfalls associated with key modification. These principles not only maintain code reliability but also safeguard data consistency, contributing to more robust and error-free Java applications.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
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