Java HashSet size() Method

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The HashSet.size() method in Java is used to determine the number of elements in the HashSet. 

Table of Contents

1. Introduction
2. size Method Syntax
3. Examples
   • Basic Example
   • Real-World Use Case: Checking Number of Active Users
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 like add, remove, contains, and size. The size method is used to determine the number of elements currently in the HashSet.

size() Method Syntax

The syntax for the size method is as follows:

public int size()

• The method does not take any parameters.
• The method returns the number of elements in the HashSet.

Examples

Basic Example

In this example, we'll use the size method to determine the number of elements in a HashSet.

Example

import java.util.HashSet;

public class HashSetSizeExample {
    public static void main(String[] args) {
        // Creating a HashSet of Strings
        HashSet<String> set = new HashSet<>();
        set.add("Java");
        set.add("Python");
        set.add("C");
        set.add("JavaScript");

        // Getting the size of the HashSet
        int size = set.size();

        // Printing the size of the HashSet
        System.out.println("Number of elements in the HashSet: " + size);
    }
}

Output:

Number of elements in the HashSet: 4

Real-World Use Case: Checking Number of Active Users

In a web application, you might want to check the number of active users currently logged in.

Example

import java.util.HashSet;

public class ActiveUsersSizeExample {
    public static void main(String[] args) {
        // Creating a HashSet to store active users
        HashSet<String> activeUsers = new HashSet<>();
        activeUsers.add("john_doe");
        activeUsers.add("jane_smith");
        activeUsers.add("alice_jones");

        // Getting the number of active users
        int numberOfActiveUsers = activeUsers.size();

        // Printing the number of active users
        System.out.println("Number of active users: " + numberOfActiveUsers);
    }
}

Output:

Number of active users: 3

Example: Tracking Inventory Items

In an inventory management system, you might want to keep track of the number of unique items in stock.

Example

import java.util.HashSet;

public class InventorySizeExample {
    public static void main(String[] args) {
        // Creating a HashSet to store inventory items
        HashSet<String> inventoryItems = new HashSet<>();
        inventoryItems.add("Laptop");
        inventoryItems.add("Monitor");
        inventoryItems.add("Keyboard");
        inventoryItems.add("Mouse");

        // Getting the number of inventory items
        int numberOfItems = inventoryItems.size();

        // Printing the number of inventory items
        System.out.println("Number of unique inventory items: " + numberOfItems);
    }
}

Output:

Number of unique inventory items: 4

Example: Checking Number of Unique Error Codes

In a logging system, you might want to keep track of the number of unique error codes encountered.

Example

import java.util.HashSet;

public class ErrorCodesSizeExample {
    public static void main(String[] args) {
        // Creating a HashSet to store unique error codes
        HashSet<Integer> errorCodes = new HashSet<>();
        errorCodes.add(404);
        errorCodes.add(500);
        errorCodes.add(403);

        // Getting the number of unique error codes
        int numberOfErrorCodes = errorCodes.size();

        // Printing the number of unique error codes
        System.out.println("Number of unique error codes: " + numberOfErrorCodes);
    }
}

Output:

Number of unique error codes: 3

Conclusion

The HashSet.size() method in Java provides a way to determine the number of elements in a HashSet. This method is useful in various scenarios, such as managing active users in a web application, tracking inventory items, or counting unique error codes. By understanding how to use this method, you can efficiently manage and manipulate sets in your Java applications.