Collections addAll() method in Java with Examples

The addAll() method of the java.util.Collections class is used to add multiple elements to a collection at once. It is faster than repeatedly calling add() and is a convenient way to insert elements individually or from an array. It throws NullPointerException if the collection or elements are null, or if the collection does not allow null values.

Example:

import java.util.*;
public class GFG {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));
        Collections.addAll(list, "D", "E", "F");
        System.out.println("Updated List: " + list);
    }
}

Updated List: [A, B, C, D, E, F]

Explanation:

• Collections.addAll() adds multiple elements to a collection in one call.
• Works with any collection like ArrayList or HashSet.
• Saves time compared to adding elements one by one.

Syntax

public static <T> boolean addAll(Collection<T> c, T... elements)

Parameters:

• c : The collection to which elements are to be added.
• elements : Elements to add to the collection.

Return Value: Returns true if the collection changed as a result of the call.

Example 1: This code shows how to use Collections.addAll() to efficiently add multiple elements to a Java List at once.

import java.util.*;
public class GFG {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C", "Tajmahal"));
        System.out.println("list before operation: " + list);
        boolean result = Collections.addAll(list, "1", "2", "3");
        System.out.println("Result of addAll: " + result);
        System.out.println("list after operation: " + list);
    }
} 

list before operation: [A, B, C, Tajmahal]
Result of addAll: true
list after operation: [A, B, C, Tajmahal, 1, 2, 3]

Explanation:

• An ArrayList is created with initial elements "A", "B", "C", "Tajmahal".
• Collections.addAll() adds "1", "2", "3" to the list in a single call.
• The method returns true if the list is modified.

Example 2: This code demonstrates that Collections.addAll() throws a NullPointerException if the target collection is null, showing proper exception handling.

import java.util.*;
public class GFG {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C", "Tajmahal"));
        System.out.println("list before operation: " + list);
        try {
            Collections.addAll(null, list.toArray(new String[0]));
        } catch (NullPointerException e) {
            System.out.println("Exception thrown: " + e);
        }
    }
}

list before operation: [A, B, C, Tajmahal]
Exception thrown: java.lang.NullPointerException: Cannot invoke "java.util.Collection.add(Object)" because "c" is null

Explanation :

• The program attempts to add these elements to a null collection using Collections.addAll().
• Since the target collection is null, a NullPointerException is thrown.
• The exception is caught in the catch block and printed

Related Topics:

• Arrays.asList() method
• HashSet in Java
• ArrayList in Java