HashSet removeAll(Collection) in Java

removeAll(Collection): This method is available in java.util.HashSet class of Java.

Syntax:

boolean java.util.AbstractSet.removeAll(Collection<?> c)

This method takes one argument. This method removes from this set all of its elements that are contained in the specified collection.

Parameters: One parameter is required for this method.

c: a collection containing elements to be removed from this set.

Returns: true if this set changed as a result of the call.

Throws:

1. UnsupportedOperationException - if the removeAll operation is not supported by this set.

2. ClassCastException - if the class of an element of this set is incompatible with the specified collection.

3. NullPointerException - if this set contains a null element and the specified collection does not permit null elements or if the specified collection is null.

Approach 1: When no exceptions

Java

import java.util.HashSet;

public class HashSetremoveAll {
    public static void main(String[] args) {

        HashSet<String> hashSet = new HashSet<String>();

        hashSet.add("Hello");
        hashSet.add("Java");

        HashSet<String> hashSet2 = new HashSet<String>();

        hashSet2.add("Hello");
        hashSet2.add("Java");
        hashSet2.add("C++");
        hashSet2.add("Hello");

        hashSet2.removeAll(hashSet);

        System.out.println(hashSet2);
    }
}

Output:

[C++]

Approach 2: NullPointerException 

Java

import java.util.HashSet;

public class HashSetremoveAll {
    public static void main(String[] args) {

        HashSet<String> hashSet = null;

        HashSet<String> hashSet2 = new HashSet<String>();

        hashSet2.add("Hello");
        hashSet2.add("Java");
        hashSet2.add("C++");
        hashSet2.add("Hello");

        hashSet2.removeAll(hashSet);

        System.out.println(hashSet2);
    }
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:208)
	at java.base/java.util.AbstractSet.removeAll(AbstractSet.java:167)