A Guide to ArrayList in Java

An ArrayList in Java represent a resizable list of objects. We can add, remove, find, sort and replace elements in this list. ArrayList is part of Java’s collection framework and implements Java’s List interface.

Hierarchy of ArrayList class

Java ArrayList class extends AbstractList class which implements List interface. The List interface extends Collection and Iterable interfaces in hierarchical order.

1. ArrayList Features

ArrayList has following features –

Ordered – Elements in arraylist preserve their ordering which is by default the order in which they were added to the list.
Index based – Elements can be randomly accessed using index positions. Index start with '0'.
Dynamic resizing – ArrayList grows dynamically when more elements needs to be added than it’s current size.
Non synchronized – ArrayList is not synchronized, by default. Programmer needs to use synchronized keyword appropiately or simply use Vector class.
Duplicates allowed – We can add duplicate elements in arraylist. It is not possible in sets.

2. Internal Working of ArrayList

ArrayList class is implemented arounda backing array. The elements added or removed from arraylist are actually modified in this backing array. All arraylist methods access this array and get/set elements in the array.

ArrayList basically cab be seen as resizable-array implementation in Java.

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, 
        		   Cloneable, java.io.Serializable
{
	transient Object[] elementData;		//backing array
	private int size;					//array or list size

	//more code
}

3. ArrayList Examples

3.1. Create ArrayList

Usually we will create either an empty list and add elements to it. Or we will create an arraylist with another exising collection.

//Empty arraylist
List<String> names = new ArrayList<>(); 

//Arraylist initialized with another collection
List<Integer> numbers = new ArrayList<>(Arrays.asList(1,2,3,4,5));

3.2. Add and remove elements

Use add(), set() and remove() methods to add or update the elements in the list.

//Create arraylist
List<String> names = new ArrayList<>(); 

names.add("lokesh");    //[lokesh]
names.add("alex");      //[lokesh, alex]

names.set(1, "brian");  //[lokesh, brian]

names.remove(1);        //[lokesh]

3.2. Iterate

Use iterator() or listIterator() to get the reference of iterator instance. This iterator can be used to iterate the elements in the arraylist.

ArrayList<Integer> digits = new ArrayList<>(Arrays.asList(1,2,3,4,5,6));

Iterator<Integer> iterator = digits.iterator();

while(iterator.hasNext()) {
	System.out.println(iterator.next());
}

Program output.