Java HashMap clear() Method

The HashMap.clear() method in Java is used to remove all mappings from a HashMap. This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. clear Method Syntax
3. Examples
   • Clearing All Entries from a HashMap
   • Real-World Use Case: Resetting a User Session Store
4. Conclusion

Introduction

The clear() method is a member of the HashMap class in Java. It allows you to remove all key-value mappings from the HashMap, effectively resetting it to an empty state. This can be useful when you need to reuse the same HashMap instance without retaining any of the previous data.

clear() Method Syntax

The syntax for the clear method is as follows:

public void clear()

• The method does not take any parameters.
• The method does not return any value.

Examples

Clearing All Entries from a HashMap

The clear method can be used to remove all entries from a HashMap.

Example

import java.util.HashMap;

public class ClearExample {
    public static void main(String[] args) {
        // Creating a HashMap with String keys and Integer values
        HashMap<String, Integer> people = new HashMap<>();

        // Adding entries to the HashMap
        people.put("Ravi", 25);
        people.put("Priya", 30);
        people.put("Vijay", 35);

        // Printing the HashMap before clearing
        System.out.println("HashMap before clear: " + people);

        // Clearing all entries from the HashMap
        people.clear();

        // Printing the HashMap after clearing
        System.out.println("HashMap after clear: " + people);
    }
}

Output:

HashMap before clear: {Ravi=25, Priya=30, Vijay=35}
HashMap after clear: {}

Real-World Use Case: Resetting a User Session Store

In a real-world scenario, you might use the clear method to reset a user session store, removing all active sessions.

Example

import java.util.HashMap;

public class SessionStore {
    public static void main(String[] args) {
        // Creating a HashMap with String keys (session IDs) and String values (user names)
        HashMap<String, String> sessions = new HashMap<>();

        // Adding active sessions
        sessions.put("SID001", "Ravi Kumar");
        sessions.put("SID002", "Priya Sharma");
        sessions.put("SID003", "Vijay Singh");

        // Printing the active sessions before clearing
        System.out.println("Active sessions before clear: " + sessions);

        // Clearing all active sessions
        sessions.clear();

        // Printing the active sessions after clearing
        System.out.println("Active sessions after clear: " + sessions);
    }
}

Output:

Active sessions before clear: {SID001=Ravi Kumar, SID002=Priya Sharma, SID003=Vijay Singh}
Active sessions after clear: {}

Conclusion

The HashMap.clear() method in Java provides a way to remove all key-value mappings from a HashMap, effectively resetting it to an empty state. By understanding how to use this method, you can efficiently manage the lifecycle of your HashMap instances in your Java applications. This method is useful when you need to clear the contents of a HashMap without creating a new instance, such as resetting user sessions, clearing caches, or reinitializing data structures.