Arraylist in Java from scratch

Article by: Manish Methani

Last Updated: October 25, 2021 at 10:04am IST
8 min 24 sec read

ArrayList is one of the type of Collection class. ArrayList implements List interface and some of the advantages ArrayList has over arrays are as follows:

â–  It can grow dynamically.

â–  It provides more powerful insertion and search mechanisms than arrays.

You cannot add an element if the array is of full length and even if you remove some elements array size will not going to shrink. Whereas, ArrayList can dynamically grow. ArrayList can add as well as remove elements from an array and it will grow and shrink as per need.

How to create empty Generic ArrayList ?

This is how you can create an Generic ArrayList.

ArrayList list1 =  new ArrayList();

Following is the list of the constructors provided by the ArrayList class.

Apart from the methods inherited from its parent classes, ArrayList defines the following methods −

Lets take a simple program to understand the concept of ArrayList

We create simple String array . We create an empty ArrayList. Add all the elements of string array to this empty ArrayList using .add(String) method . Print ArrayList using .get(index) property.

Add Elements in an ArrayList

Basic program to add elements in an ArrayList which clears the concept of adding an elements in arrayList and how to print the elements of ArrayList using for loop, while loop and for each loop.

import java.util.*;
public class JavaCollectionDemo {
  
public static void main(String[] args)
{
//String array
String[] things = {"eggs" , "hats" , "laser" , "pie"};

// Define an empty ArrayList 
ArrayList list1 =  new ArrayList();

// add array items to list1 
for(String x: things)
{
  list1.add(x);
}

//Size of ArrayList
System.out.printf("%d",list1.size());
System.out.printf("
");


//Get the items from the list1 
for (int i = 0; i//Remove element at index 2
System.out.printf("
");
list1.remove(2);

//Get the list items using for each loop 
for (String i:list1)
{
  System.out.printf("%s 	",i);
}

}
}

Output

4
eggs 	hats 	laser 	pie 	
eggs 	hats 	pie 	

Looping ArrayList using Iterator

You can also loop through ArrayList using Java Iterators also. You will learn about Java Iterators in next Article in Detail. An iterator is used to iterate through all the elements in the list. An iterator is used to go through each element of a Collection easily.

import java.util.*;
public class JavaCollectionDemo {
public static void main(String[] args)
{

//String array
String[] things = {"eggs" , "hats" , "laser" , "pie"};

// Define an empty ArrayList 
ArrayList list1 =  new ArrayList();

// add array items to list1 
for(String x: things)
{
  list1.add(x);
}

//Access elements of ArrayList using iterator
 Iterator liIterator = list1.iterator();
        while (liIterator.hasNext()) {
            String str = liIterator.next();
            System.out.printf("%s 	",str);
        }
}
}

Output :-

eggs   hats    laser   pie 	

Convert ArrayList to Array and vice-versa

To convert array to list you can use Arrays .asList method and to convert list to an array you can use List toArray.

import java.util.*;
public class JavaCollectionDemo {
public static void main(String[] args)
{
  //create an ArrayList object
   String[] things = {"eggs" , "hats" , "laser" , "pie"};
  
   //An Array to List
   ArrayList list =  new ArrayList(Arrays.asList(things));
  
  list.add("newThing");

  //Convert back list to an array
  things = list.toArray(new String[list.size()]);
  
  for(String s: things)
  {
    System.out.printf("%s ", s);  
  }
}
}

Output:

eggs hats laser pie newThing