Java HashSet clear() Method

The HashSet.clear() method in Java is used to remove all elements from a HashSet. 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
   • Basic Example
   • Using clear in a Real-World Scenario
4. Conclusion

Introduction

The HashSet class in Java is part of the Java Collections Framework and implements the Set interface. A HashSet is used to store unique elements and provides constant-time performance for basic operations. The clear method is used to remove all elements from the set, making it empty.

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

Basic Example

In this example, we'll use the clear method to remove all elements from a HashSet.

Example

import java.util.HashSet;

public class HashSetClearExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("Java");
        set.add("Python");
        set.add("C");
        set.add("JavaScript");

        System.out.println("HashSet before clear: " + set);

        // Clearing the HashSet
        set.clear();

        System.out.println("HashSet after clear: " + set);
    }
}

Output:

HashSet before clear: [Java, JavaScript, Python, C]
HashSet after clear: []

Using clear in a Real-World Scenario

In a real-world application, you might use the clear method to reset a collection of unique items, such as clearing a set of unique user sessions at the end of the day.

Example

import java.util.HashSet;

public class UniqueSessions {
    public static void main(String[] args) {
        HashSet<String> userSessions = new HashSet<>();

        // Simulating user sessions
        userSessions.add("session1");
        userSessions.add("session2");
        userSessions.add("session3");

        System.out.println("User sessions before clearing: " + userSessions);

        // Clearing the sessions at the end of the day
        userSessions.clear();

        System.out.println("User sessions after clearing: " + userSessions);
    }
}

Output:

User sessions before clearing: [session1, session2, session3]
User sessions after clearing: []

Resetting a Set of Error Messages

In a logging system, you might want to reset the set of unique error messages after a certain period to start fresh.

Example

import java.util.HashSet;

public class UniqueErrorMessages {
    public static void main(String[] args) {
        HashSet<String> errorMessages = new HashSet<>();

        // Simulating error messages
        errorMessages.add("Error 404: Not Found");
        errorMessages.add("Error 500: Internal Server Error");
        errorMessages.add("Error 403: Forbidden");

        System.out.println("Error messages before clearing: " + errorMessages);

        // Clearing the error messages at the end of the week
        errorMessages.clear();

        System.out.println("Error messages after clearing: " + errorMessages);
    }
}

Output:

Error messages before clearing: [Error 404: Not Found, Error 500: Internal Server Error, Error 403: Forbidden]
Error messages after clearing: []

Conclusion

The HashSet.clear() method in Java provides a simple way to remove all elements from a HashSet, effectively resetting it. This method is particularly useful in scenarios where you need to reset or clear a collection of unique elements, such as user sessions, error messages, or any other set of unique items. By understanding how to use this method, you can efficiently manage collections in your Java applications.