Java HashSet newHashSet() Method

The HashSet.newHashSet(int numElements) method in Java can be used to create a new, empty HashSet that is pre-allocated to handle the expected number of elements. 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. newHashSet Method Syntax
3. Examples
   • Creating a New HashSet with Expected Number of Elements
   • Adding Elements and Checking Capacity
4. Conclusion

Introduction

The HashSet.newHashSet(int numElements) method is used to create a new, empty HashSet that is pre-allocated to handle an expected number of elements. This is useful for optimizing performance when the approximate size of the HashSet is known in advance, as it helps reduce the number of rehash operations required as the set grows.

newHashSet Method Syntax

The syntax for the newHashSet method is as follows:

public static <T> HashSet<T> newHashSet(int numElements)

• The method takes a single parameter numElements of type int, which specifies the expected number of elements the HashSet will contain.
• The method returns a new, empty HashSet<T>.

Examples

Creating a New HashSet with Expected Number of Elements

You can create a new HashSet with a specified initial capacity using this method.

Example

import java.util.HashSet;

public class NewHashSetExample {
    public static void main(String[] args) {
        // Creating a new HashSet with an expected number of 10 elements
        HashSet<String> languages = newHashSet(10);

        // Adding elements to the HashSet
        languages.add("Java");
        languages.add("Python");
        languages.add("C");
        languages.add("C++");
        languages.add("JavaScript");
        languages.add("Ruby");
        languages.add("PHP");
        languages.add("Swift");
        languages.add("Kotlin");
        languages.add("Go");

        // Printing the HashSet
        System.out.println("HashSet: " + languages);
    }

    // Static method to create a new HashSet with the expected number of elements
    public static <T> HashSet<T> newHashSet(int numElements) {
        return new HashSet<>(numElements);
    }
}

Output:

HashSet: [Java, C++, Python, PHP, Ruby, C, Swift, JavaScript, Kotlin, Go]

Adding Elements and Checking Capacity

While the initial capacity cannot be directly checked or modified after the HashSet is created, you can add elements to the set and ensure that it handles the specified initial capacity without performance degradation.

Example

import java.util.HashSet;

public class AddElementsExample {
    public static void main(String[] args) {
        // Creating a new HashSet with an expected number of 5 elements
        HashSet<String> languages = newHashSet(5);

        // Adding elements to the HashSet
        languages.add("Java");
        languages.add("Python");
        languages.add("C");
        languages.add("C++");
        languages.add("JavaScript");

        // Printing the HashSet
        System.out.println("HashSet after adding elements: " + languages);

        // Adding more elements to check performance
        languages.add("Ruby");
        languages.add("PHP");

        // Printing the HashSet after adding more elements
        System.out.println("HashSet after adding more elements: " + languages);
    }

    // Static method to create a new HashSet with the expected number of elements
    public static <T> HashSet<T> newHashSet(int numElements) {
        return new HashSet<>(numElements);
    }
}

Output:

HashSet after adding elements: [Java, C++, Python, C, JavaScript]
HashSet after adding more elements: [Java, C++, Python, PHP, Ruby, C, JavaScript]

Conclusion

The HashSet.newHashSet(int numElements) method in Java provides a way to create a new, empty HashSet that is pre-allocated to handle an expected number of elements. By understanding how to use this method, you can optimize the performance of your Java applications when working with sets that have a known or estimated number of elements. This helps in reducing the number of rehash operations, thereby improving the efficiency of the HashSet.