How to Initialize an ArrayList in Java

In this tutorial, you will learn how to initialize an ArrayList in Java. There are several different ways to do this. Let’s discuss them with examples.

1. Basic (Normal) Initialization

One of the ways to initialize an ArrayList is to create it first and then add elements later using add() method.

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        // Creating an empty ArrayList with String type
        ArrayList<String> names = new ArrayList<>();  

        // Adding elements to the ArrayList
        names.add("Chaitanya");
        names.add("Rahul");
        names.add("Aditya");

        System.out.println(names);
    }
}

2. Initialization with Initial Capacity

If you aware of the number of elements that you are going to add to ArrayList, you can initialize it with an initial capacity. This avoid resizing overhead.

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        // Initial capacity of 10
        ArrayList<String> names = new ArrayList<>(10);  

        // Adding elements to the ArrayList
        names.add("Chaitanya");
        names.add("Rahul");
        names.add("Aditya");

        System.out.println(names);
    }
}

3. Initialization using asList() method

You can initialize an ArrayList with elements using Arrays.asList(). In this methods, all elements can be specified inside asList() method as shown below. However you can add more elements later using add() method.

import java.util.ArrayList;
import java.util.Arrays;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> names = 
new ArrayList<>(Arrays.asList("Chaitanya", "Rahul", "Aditya"));

        System.out.println(names);
    }
}

4. Initialization using anonymous inner class (Double Brace Initialization)

This method may have some overhead.

import java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<String>() {{
            add("Chaitanya");
            add("Rahul");
            add("Aditya");
        }};

        System.out.println(names);
    }
}

5. Using Java 9+ List.of() and new ArrayList

In Java 9 and later, you can use List.of() to create an immutable list and then pass it to the ArrayList constructor if you need a mutable list.

import java.util.ArrayList;
import java.util.List;

public class ArrayListExample {
    public static void main(String[] args) {
        List<String> immutableList = 
List.of("Chaitanya", "Rahul", "Aditya");
        ArrayList<String> names = new ArrayList<>(immutableList);

        System.out.println(names);
    }
}

6. Initializing using Collections.ncopies()

The Collections.ncopies() method is used when you need to initialize the ArrayList with multiple same elements. For example, if you want all 50 elements of an ArrayList as 10 then you can call the Collections.ncopies() method like this: = new ArrayList<Integer>(Collections.nCopies(50, 10))

Syntax:

ArrayList<T> obj = new ArrayList<T>(Collections.nCopies(count, element));

count is number of elements and element is the item value

Example:
In the following example, we have initialized an array with 10 elements and all the elements are 5.

import java.util.*;
public class ArrayListExample {
   public static void main(String args[]) {
     ArrayList<Integer> intlist = new ArrayList<>(Collections.nCopies(10, 5));
     System.out.println("ArrayList items: "+intlist);
   }
}

Output:

ArrayList items: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

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• Sort ArrayList in Java
• ArrayList vs LinkedList