How to Initialize an ArrayList

In this tutorial, you will learn multiple ways to initialize an ArrayList.

1. ArrayList Initialization using Arrays.asList() method

The asList() method of Arrays class converts an array to ArrayList. This is a perfect way to initialize an ArrayList because you can specify all the elements inside asList() method.

Syntax:

ArrayList<Type> obj = new ArrayList<Type>(
        Arrays.asList(Object o1, Object o2, Object o3, ....so on));

Example:
Here, we have initialized an ArrayList of string type using Arrays.asList() method. We have specified three string elements inside asList(), these elements are added as arraylist elements. As you can see, when we print the arraylist elements, it prints the three strings that we passed inside asList().

import java.util.*;
public class InitializationExample1 {
   public static void main(String args[]) {
     ArrayList<String> obj = new ArrayList<String>(
		Arrays.asList("Pratap", "Peter", "Harsh"));
     System.out.println("Elements are:"+obj);
   }
}

Output:

Elements are:[Pratap, Peter, Harsh]

Another example: Using Arrays.asList() to initialize an ArrayList of integer type.

import java.util.*;
public class JavaExample {
  public static void main(String args[]) {
    ArrayList<Integer> numbers = new ArrayList<Integer>(
            Arrays.asList(10, 20, 30, 40, 50));
    System.out.println("ArrayList elements are: "+numbers);
  }
}

Output:

ArrayList elements are: [10, 20, 30, 40, 50]

2. Anonymous inner class method to initialize an ArrayList

Here, we initialize an ArrayList using anonymous inner class. Note the double curly braces. The elements are specified using add().
Syntax:

ArrayList<T> obj = new ArrayList<T>(){{
		   add(Object o1);
		   add(Object o2);
		   add(Object o3);
                   ...
                   ...
		   }};

Example:

import java.util.*;
public class JavaExample {
  public static void main(String args[]) {
    ArrayList<String> cities = new ArrayList<>(){{
      add("Delhi");
      add("Agra");
      add("Chennai");
      add("Pune");
      add("Noida");
    }};
    System.out.println("Content of Array list cities:"+cities);
  }
}

Output:

Content of Array list cities:[Delhi, Agra, Chennai, Pune, Noida]

3. Normal way of ArrayList initialization

This is the most common and frequently used method of initializing an array. Here we are using add() method of ArrayList class to add elements to an ArrayList.

Syntax:

ArrayList<T> obj = new ArrayList<T>();
	   obj.add("Object o1");
	   obj.add("Object o2");
	   obj.add("Object o3");
                        ...
                        ...

Example:
In the following example, we have created an ArrayList with the name books. We can add as many elements as we want by calling add() method. Please keep in mind the type of the ArrayList. In this example, we have created an arraylist of string type so we are specifying strings in add() method. If the arraylist is of integer type, you need to specify the numbers in add() method else the program will throw compilation error.

import java.util.*;
public class Details {
   public static void main(String args[]) {
     ArrayList<String> books = new ArrayList<String>();
     books.add("Java Book1");
     books.add("Java Book2");
     books.add("Java Book3");
     System.out.println("Books stored in array list are: "+books);
   }
}

Output:

Books stored in array list are: [Java Book1, Java Book2, Java Book3]

4. Initializing an ArrayList with multiple same elements using Collections.ncopies()

Collections.ncopies() method is used, when we need to initialize the ArrayList with multiple same elements. For example, if you want an ArrayList with 50 elements and all elements as 10 then you can call the 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 Details {
   public static void main(String args[]) {
     ArrayList<Integer> intlist = new ArrayList<Integer>(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