Check if a Java HashSet Collection contains another Collection

HashSet is a class in Java that implements the Set interface. It is used for storing unique elements. Our task is to check if a HashSet contains another collection in Java. Let's look at some scenarios to understand the problem better:

Scenario 1

Input: {1, 2, 3, 4, 5}
Collection to check: {2, 3}
Output: true
Explanation: The HashSet contains all the elements of the collection {2, 3}.

Scenario 2

Input: {1, 2, 3, 4, 5}
Collection to check: {6, 7}
Output: false
Explanation: The HashSet does not contain all the elements of the collection {6, 7}.

Checking if HashSet Contains another Collection

The following are the ways we can check if a HashSet contains another collection in Java:

• Using containsAll() Method
• Using Streams API

Using containsAll() Method

The containsAll() method is a method of the Collection interface, which is implemented by the HashSet class. This method accepts a collection object as a parameter and verifies whether the given object exists in the current HashSet. If it does, this method returns TRUE; otherwise, it returns FALSE.

Syntax

Following is the syntax of the containsAll() method:

containsAll(collection);

Example

In the below example, we will create a HashSet and an ArrayList, and then we will check if the HashSet contains all the elements of an ArrayList using the containsAll() method:

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;   
public class CheckIfContainsCollection {
   public static void main(String[] args){
      Set<Integer> hashSet = new HashSet<>();
      hashSet.add(1);
      hashSet.add(2);
      hashSet.add(3);
      hashSet.add(4);
      hashSet.add(5);

      List<Integer> collectionToCheck = new ArrayList<>();
      collectionToCheck.add(2);
      collectionToCheck.add(3);

      if(hashSet.containsAll(collectionToCheck)) {
         System.out.println("HashSet contains all elements of the collection.");
      } else {
         System.out.println("HashSet does not contain all elements of the collection.");
      }
   }
}

When you run the above code, it will produce the following output:

HashSet contains all elements of the collection.

Using Streams API

Java Stream API was introduced in Java8. It provides methods that help us to process collections. We can use the allMatch() method of the Stream interface to check if a HashSet contains all the elements of another collection.

Example

In the below example, we will create a HashSet, next we will convert the HashSet to a Stream using the stream() method, and then we will use the allMatch() method to check if all elements of the collection are present in the HashSet:

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Stream;
public class CheckIfContainsCollection {
   public static void main(String[] args){
      Set<Integer> hashSet = new HashSet<>();
      hashSet.add(1);
      hashSet.add(2);
      hashSet.add(3);
      hashSet.add(4);
      hashSet.add(5);

      List<Integer> collectionToCheck = new ArrayList<>();
      collectionToCheck.add(2);
      collectionToCheck.add(3);

      boolean exists = collectionToCheck.stream().allMatch(hashSet::contains);

      if(exists) {
         System.out.println("HashSet contains all elements of the collection.");
      } else {
         System.out.println("HashSet does not contain all elements of the collection.");
      }
   }
}

When you run the above code, it will produce the following output:

HashSet contains all elements of the collection.

In this article, we have learned how to check if a HashSet contains another collection in Java using the containsAll() method and the Streams API.