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Java ArrayList addAll()

The Java ArrayList addAll() method adds all the element of a collection to the arraylist.

The syntax of the addAll() method is:

arraylist.addAll(int index, Collection c)

Here, arraylist is an object of the ArrayList class.


addAll() Parameters

The ArrayList addAll() method can take two parameters:

  • index (optional) - index at which all elements of a collection is inserted
  • collection - collection that contains elements to be inserted

If the index parameter is not passed the collection is appended at the end of the arraylist.


addAll() Return Value

  • returns true if the collection is successfully inserted into the arraylist
  • raises NullPointerException if the specified collection is null
  • raises IndexOutOfBoundsException if the index is out of range

Example 1: Inserting Elements using ArrayList addAll()

import java.util.ArrayList;

class Main {
    public static void main(String[] args){
        
        // create an arraylist
        ArrayList<Integer> primeNumbers = new ArrayList<>();
        // add elements to arraylist
        primeNumbers.add(3);
        primeNumbers.add(5);
        System.out.println("Prime Numbers: " + primeNumbers);

        // create another arraylist
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);

        // Add all elements from primeNumbers to numbers
        numbers.addAll(primeNumbers);
        System.out.println("Numbers: " + numbers);
    }
}

Output

Prime Numbers: [3, 5]
Numbers: [1, 2, 3, 5]

In the above example, we have created two arraylists named primeNumbers and numbers. Notice the line,

numbers.addAll(primeNumbers);

Here, the addAll() method does not contain the optional index parameter. Hence, all elements from the arraylist primeNumbers are added at the end of the arraylist numbers.

Note: We have used the add() method to add single elements to arraylist. To learn more, visit Java ArrayList add().


Example 2: Inserting Elements to the Specified Position

import java.util.ArrayList;

class Main {
    public static void main(String[] args){
        ArrayList<String> languages1 = new ArrayList<>();
        languages1.add("Java");
        languages1.add("Python");
        System.out.println("ArrayList 1: " + languages1);

        // create another arraylist
        ArrayList<String> languages2 = new ArrayList<>();
        languages2.add("JavaScript");
        languages2.add("C");
        System.out.println("ArrayList 2: " + languages2);

        // Add elements from languages1 to languages2 at index 1
        languages2.addAll(1, languages1);
        System.out.println("Updated ArrayList 2: " + languages2);
    }
}

Output

ArrayList 1: [Java, Python]
ArrayList 2: [JavaScript, C]
Updated ArrayList 2: [JavaScript, Java, Python, C]

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

languages2.addAll(1, languages1);

Here, the addAll() contains the optional index parameter. Hence, all elements from the arraylist languages1 are added to languages at index 0.


Example 3: Inserting Elements from Set to ArrayList

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

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

        // create a hashset of String type
        HashSet<String> set = new HashSet<>();

        // add elements to the hashset
        set.add("Java");
        set.add("Python");
        set.add("JavaScript");
        System.out.println("HashSet: " + set);

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

        // add element to arraylist
        list.add("English");
        System.out.println("Initial ArrayList: " + list);

        // Add all elements from hashset to arraylist
        list.addAll(set);
        System.out.println("Updated ArrayList: " + list);
    }
}

Output

Set: [Java, JavaScript, Python]
Initial ArrayList: [English]
Updated ArrayList: [English, Java, JavaScript, Python]

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

list.addAll(set);

Here, we have used the addAll() method to add all the elements of hashset to the arraylist. The optional index parameter is not present in the method. Hence, all elements are added at the end of the arraylist.