Java ArrayList removeAll()

The syntax of the removeAll() method is:

arraylist.removeAll(Collection c);

Here, arraylist is an object of the ArrayList class.

removeAll() Parameters

The removeAll() method takes a single parameter.

• collection -all elements present in collection are deleted from the arraylist.

removeAll() Return Value

• returns true if elements are deleted from the arraylist
• throws ClassCastException if the class of elements present in arraylist is incompatible with the class of elements in specified collection
• throws NullPointerException if the arraylist contains null element and the specified collection does not allow null elements

Example 1: Remove all elements from an ArrayList

import java.util.ArrayList;

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

        // create an arraylist
        ArrayList<String> languages = new ArrayList<>();

        // add elements to arraylist
        languages.add("Java");
        languages.add("JavaScript");
        languages.add("Python");
        System.out.println("Programming Languages: " + languages);

        // remove all elements from arraylist
        languages.removeAll(languages);
        System.out.println("ArrayList after removeAll(): " + languages);
    }
}

Output

Programming Languages: [Java, JavaScript, Python]
ArrayList after removeAll(): []

In the above example, we have created an arraylist named languages. The arraylist stores the name of programming languages. Notice the line,

languages.removeAll(languages);

Here, we are passing the ArrayList languages as an argument of the removeAll() method. Hence, the method removes all the elements from the arraylist.

Note: The clear() method is preferred to remove all elements from the arraylist. To know more, visit Java ArrayList clear().

Example 2: Remove all Elements from an ArrayList Present in Another ArrayList

import java.util.ArrayList;

class Main {
    public static void main(String[] args) {
        // create an ArrayList
        ArrayList<String> languages1 = new ArrayList<>();

        // insert element at the end of arraylist
        languages1.add("Java");
        languages1.add("English");
        languages1.add("C");
        languages1.add("Spanish");
        System.out.println("Languages1: " + languages1);

        // create another arraylist
        ArrayList<String> languages2 = new ArrayList<>();

        // add elements to the arraylist
        languages2.add("English");
        languages2.add("Spanish");
        System.out.println("Languages2: " + languages2);

        // remove all elements of ArrayList2 from ArrayList1
        languages1.removeAll(languages2);
        System.out.println("Languages1 after removeAll(): " + languages1);
    }
}

Output

Languages1: [Java, English, C, Spanish]
Languages2: [English, Spanish]
Languages1 after removeAll(): [Java, C]

In the above example, we have created two arraylists named languages1 and languages2. Notice the line,

languages1.removeAll(languages2);

Here, the removeAll() method is used to remove all those elements from languages1 that are also present in languages2. Hence, English and Spanish are removed from languages1.

Example 3: Remove all Elements from an ArrayList Present in a HashSet

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

class Main {
    public static void main(String[] args) {
        // create an ArrayList
        ArrayList<Integer> numbers = new ArrayList<>();

        // add element to ArrayList
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println("ArrayList: " + numbers);

        // create a HashSet
        HashSet<Integer> primeNumbers = new HashSet<>();

        // add elements to the HashSet
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("HashSet: " + primeNumbers);

        // remove all elements of HashSet from ArrayList
        numbers.removeAll(primeNumbers);
        System.out.println("ArrayList after removeAll(): " + numbers);
    }
}

Output

ArrayList: [1, 2, 3, 4]
HashSet: [2, 3]
ArrayList after removeAll(): [1, 4]

In the above example, we have created an arraylist named numbers and a hashset named primeNumbers. Notice the line,

numbers.removeAll(primeNumbers);

Here, the removeAll() method removes all those elements from numbers that are also present in primeNumbers. Hence, 2 and 3 are removed from the arraylist numbers.