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AbstractCollection addAll() Method in Java

Last Updated : 11 Jul, 2025

The addAll() method of Java AbstractCollection is used to append all elements from a given collection to the current collection. If the collection being appended is a TreeSet, the elements are stored in sorted order, as TreeSet maintains a natural ordering. It is important to note that AbstractCollection cannot be instantiated directly, so we use Collection as the type instead. 

Example 1: Adding Elements from One Collection to Another

In this example, we append all the elements from one collection to another using the addAll() method.

import java.util.*;

public class Geeks {
    public static void main(String args[]) {
        
        // Creating an empty collection 
        // (TreeSet maintains elements in sorted order)
        Collection<String> c1 = new TreeSet<>();

        // Adding elements to the collection
        c1.add("Welcome");
        c1.add("To");
        c1.add("Geeks");
        c1.add("4");
        c1.add("Geeks");  // Duplicate, will be removed
        c1.add("TreeSet");

        System.out.println("Collection 1: " + c1);

        // Creating another empty Collection
        Collection<String> c2 = new TreeSet<>();

        // Displaying the empty Collection
        System.out.println("Collection 2 (before addAll): " + c2);

        // Using addAll() method to append elements
        c2.addAll(c1);

        // Displaying the Collection after adding elements
        System.out.println("Collection 2 (after addAll): " + c2);
    }
}

Collection 1: [4, Geeks, To, TreeSet, Welcome]
Collection 2 (before addAll): []
Collection 2 (after addAll): [4, Geeks, To, TreeSet, Welcome]

boolean addAll(Collection c);

• Parameters: The parameter "c" is a collection of any type that is to be added to the collection. 
• Return Value: The method returns "true" if it successfully appends the elements of collection c to the existing collection otherwise, it returns "false".
• Exceptions:
  • NullPointerException: If the specified collection is null.
  • UnsupportedOperationException: If the collection does not support the add() operation. 

Example 2: Using addAll() method with integer value

import java.util.*;

public class Geeks {
    public static void main(String args[]) {
        
        // Creating an empty collection
        Collection<Integer> c1 = new TreeSet<>();

        // Adding elements into the Collection
        c1.add(10);
        c1.add(20);
        c1.add(30);
        c1.add(40);
        c1.add(50);

        // Displaying the Collection
        System.out.println("Collection 1: " + c1);

        // Creating another empty Collection
        Collection<Integer> c2 = new TreeSet<>();

        // Displaying the empty Collection
        System.out.println("Collection 2 (before addAll): " + c2);

        // Using addAll() method to append elements
        c2.addAll(c1);

        // Displaying the Collection after adding elements
        System.out.println("Collection 2 (after addAll): " + c2);
    }
}

Collection 1: [10, 20, 30, 40, 50]
Collection 2 (before addAll): []
Collection 2 (after addAll): [10, 20, 30, 40, 50]

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