Java HashSet containsAll() Method

The HashSet.containsAll() method in Java is used to check if the HashSet contains all elements from a specified collection. 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. containsAll Method Syntax
3. Examples
   • Checking if HashSet Contains All Elements from Another Collection
   • Checking if HashSet Contains All Elements from a List
4. Conclusion

Introduction

The HashSet.containsAll() method is a member of the HashSet class in Java. It allows you to check if the HashSet contains all elements from a specified collection. This is useful for verifying the presence of multiple elements in the set at once.

containsAll Method Syntax

The syntax for the containsAll method is as follows:

public boolean containsAll(Collection<?> c)

• The method takes a single parameter c of type Collection<?>, which specifies the collection whose elements are to be checked for containment in the HashSet.
• The method returns a boolean value:
  • true if the HashSet contains all elements of the specified collection.
  • false if the HashSet does not contain all elements of the specified collection.

Examples

Checking if HashSet Contains All Elements from Another Collection

The containsAll method can be used to check if a HashSet contains all elements from another HashSet.

Example

import java.util.HashSet;

public class ContainsAllExample {
    public static void main(String[] args) {
        // Creating two HashSets of Strings
        HashSet<String> languages1 = new HashSet<>();
        HashSet<String> languages2 = new HashSet<>();

        // Adding elements to the first HashSet
        languages1.add("Java");
        languages1.add("Python");
        languages1.add("C");

        // Adding elements to the second HashSet
        languages2.add("Java");
        languages2.add("Python");

        // Checking if languages1 contains all elements of languages2
        boolean containsAll = languages1.containsAll(languages2);

        // Printing the result
        System.out.println("Does languages1 contain all elements of languages2? " + containsAll);
    }
}

Output:

Does languages1 contain all elements of languages2? true

Checking if HashSet Contains All Elements from a List

The containsAll method can also be used to check if a HashSet contains all elements from a List.

Example

import java.util.HashSet;
import java.util.ArrayList;
import java.util.List;

public class ContainsAllFromListExample {
    public static void main(String[] args) {
        // Creating a HashSet of Strings
        HashSet<String> languages = new HashSet<>();

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

        // Creating a List of Strings
        List<String> moreLanguages = new ArrayList<>();
        moreLanguages.add("Java");
        moreLanguages.add("Python");

        // Checking if the HashSet contains all elements of the List
        boolean containsAll = languages.containsAll(moreLanguages);

        // Printing the result
        System.out.println("Does the HashSet contain all elements of the List? " + containsAll);
    }
}

Output:

Does the HashSet contain all elements of the List? true

Example with Missing Elements

This example shows what happens if the HashSet does not contain all elements of the specified collection.

Example

import java.util.HashSet;
import java.util.List;
import java.util.ArrayList;

public class ContainsAllMissingExample {
    public static void main(String[] args) {
        // Creating a HashSet of Strings
        HashSet<String> languages = new HashSet<>();

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

        // Creating a List of Strings with an element not present in the HashSet
        List<String> moreLanguages = new ArrayList<>();
        moreLanguages.add("Java");
        moreLanguages.add("Ruby"); // Not present in the HashSet

        // Checking if the HashSet contains all elements of the List
        boolean containsAll = languages.containsAll(moreLanguages);

        // Printing the result
        System.out.println("Does the HashSet contain all elements of the List? " + containsAll);
    }
}

Output:

Does the HashSet contain all elements of the List? false

Conclusion

The HashSet.containsAll() method in Java provides a way to check if a HashSet contains all elements from a specified collection. By understanding how to use this method, you can efficiently verify the presence of multiple elements in your collections. This method is particularly useful for ensuring that a set contains all necessary elements from another collection, such as another HashSet or a List.