How to Initialize an ArrayList in Java

In this tutorial, we'll see 4 methods to initialize an ArrayList in Java with examples:

1) Arrays.asList

The method Arrays.asList takes a list of objects as input o1, o2...,on. Its syntax is as follows:

ArrayList arlist = new ArrayList(Arrays.asList(Object o1, Object o2,...,Object on));

Example:

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

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

 // create an array of String
 String a[] = new String[]{"a","b","c","d"}; 
 
 ArrayList arlist = new ArrayList(Arrays.asList(a)); 

 // Show objects
 System.out.println(arlist); 
 }
}

Output:

[a, b, c, d]

2) Method add()

add(E e) is the basic method for adding items to an ArrayList.

< div>Example:

import java.util.ArrayList; 

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

 ArrayList arlist = new ArrayList(Arrays.asList(a)); 

 arlist.add("a"); 
 arlist.add("b"); 
 arlist.add("c"); 
 arlist.add("d"); 
 
 System.out.println(arlist); 
 }
}

Output:

[a, b, c, d]

3) Collections.ncopies

The method Collections.ncopies() can be useful when initializing ArrayList with the same value for all items. Its syntax is as follows:

ArrayList alist=new ArrayList(Collections.nCopies(nbtimes, element));

Example;

import java.util.ArrayList; 
import java.util.Collections; 

public class Initialise {
 public static void main (String args[]) {
 
 ArrayList list = new ArrayList(Collections.nCopies(5, "a")); 
 System.out.println(list); 
 
 }
}

Output:

[a, a, a, a, a]

4) Anonymous class

An anonymous class is an expression, its syntax resembles calling a constructor, except that in its body, declaring and calling methods is allowed.

Syntax:

ArrayList arlist = new ArrayList(){{
 add(o1); 
 add(o2); 
 add(o3); 
 ... 
 ... 
}};

Example:

import java.util.ArrayList; 

public class Initialise {
 public static void main (String args[]) {
 ArrayList country = new ArrayList(){{
 add("turkey"); 
 add("Malaysia"); 
 add("france"); 
 }}; 
 System.out.println(country); 
 }
}

Output:

[Turkey, Malaysia, France]