Java ArrayList

An ArrayList in Java represents a resizable list of objects. We can add, remove, find, sort and replace elements in this list.

ArrayList is the part of the collections framework. It extends AbstractList which implements List interface. The List extends Collection and Iterable interfaces in hierarchical order.

1. ArrayList Features

ArrayList has the following features –

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

2. How ArrayList Works?

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

ArrayList can 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. Java Array vs ArrayList

An array is fixed size data structure where the size has to be declared during initialization. Once the size of an array is declared, it is not possible to resize the array without creating a new array.

Integer[] numArray = new Integer[5];

The ArrayList offers to remove this sizing limitation. An ArrayList can be created with any initial size (default 16), and when we add more items, the size of the arraylist grows dynamically without any intervention by the programmer.

ArrayList<Integer> numList = new ArrayList<>();

Many people refer to ArrayList as dynamic array.

4. Creating an ArrayList

4.1. How to create an ArrayList

To create ArrayList, we can call one of its constructors.

Given below program shows how to declare and initialize an arraylist in Java.

ArrayList list = new ArrayList();

List<Integer> numbers = new ArrayList<>(6); 

Collection setOfElements = ...;
List<Integer> numbers = new ArrayList<>(setOfElements); 

4.2. Generic ArrayList

A generic arraylist clearly mentions the type of objects, it will store. It helps in avoiding a lot of defects caused by incorrect typecasting.

//Non-generic arraylist - NOT RECOMMENDED !!
ArrayList list = new ArrayList();

//Generic Arraylist with default capacity
List<Integer> numbers = new ArrayList<>(); 

//Generic Arraylist with the given capacity
List<Integer> numbers = new ArrayList<>(6); 

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

4.3. ArrayList of primitive types

In array list, we are supposed to add only objects. But in case, we are required to add primitive data types such as int, float etc, we can use their wrapper classes for providing type information during arraylist initialization.

When we add the int or float value to array list, values are automatically upcasted.

In given example, we have created an array list of Integer values. When we add int value 1, it is automatically converted to new Integer(1).

List<Integer> numbers = new ArrayList<>(6); 

numbers.add(1); // This runs fine

4.4. Create and initialize ArrayList in single line

Generally, creating an arraylist in multi-step process. In first step, we create empty array list. In later steps, we populate the list with elements – one by one.

Using Arrays.asList() and constructor ArrayList(collection), we can combine these steps in single statement.

ArrayList<String> charList = new ArrayList<>(Arrays.asList(("A", "B", "C"));

5. Get element from ArrayList

To get an element from the ArrayList, we have two ways.

5.1. get(index)

If we know the index location in advance, then we can call the get(index) which returns the element present at index location.

Please remember that indexes start with zero.

ArrayList<String> alphabetsList = new ArrayList<>(Arrays.asList(("A", "B", "C"));

String aChar = alphabetsList.get(0);	// A

5.2. iterator.next()

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

The next() method returns the element at current index location and increment the index count by one. Call hasNext() method to check if there are more elements in the list to iterate.

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.

1
2
3
4
5
6

6. Iterating over an ArrayList

6.1. Iterator

Java example to iterate over an arraylist using the Iterator.

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());
}

6.2. For loop

Java example to iterate over an arraylist using for loop. When using for loop, we need to get the current element using the current index counter.

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

for(int i = 0; i < digits.size(); i++) 
{
    System.out.print(digits.get(i));
}

6.3. forEach loop

forEach loop works pretty much same to simple for loop. The only difference is that the JVM manages the counter initialization and increment. We get the next element in each iteration in the loop.

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

for(Integer d : digits) 
{
    System.out.print(d);
}

7. Finding the length of the ArrayList

To get the length of the arraylist, we use the size() method.

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

System.out.print( digits.size() );		// 6

8. Sorting an ArrayList

ArrayList sort() method sorts the list according to the order induced by the specified Comparator instance. All elements in the list must must be mutually Comparable.

public class AgeSorter implements Comparator<Employee> 
{
    @Override
    public int compare(Employee e1, Employee e2) {
        //comparison logic
    }
}

ArrayList<Employee> employees = new ArrayList<>();

employees.add(new Employee(...));
employees.add(new Employee(...));
employees.add(new Employee(...));

employees.sort(new NameSorter());

9. ArrayList Methods

ArrayList add() method example
ArrayList addAll() method example
ArrayList clear() method example
ArrayList clone() – How to clone an ArrayList
ArrayList contains() method example
ArrayList ensureCapacity() method example
ArrayList forEach() method example
ArrayList get() method example
ArrayList indexOf() method example
ArrayList lastIndexOf() method example
ArrayList listIterator() method example
ArrayList remove() method example
ArrayList removeAll() method example
ArrayList retainAll() method example
ArrayList replaceAll() method example
ArrayList removeIf() method example
ArrayList sort() method example
ArrayList spliterator() method example
ArrayList subList() method example
ArrayList toArray() method example

10. Java ArrayList Examples

10.1. Create arraylist

Initialize ArrayList
Iterate through ArrayList

10.2. Add elements and remove elements

Add element at particular index of ArrayList
Remove element from ArrayList
Add multiple items to ArrayList

10.3. Sort arraylist

Sort ArrayList
Sort ArrayList of Objects using Comparable and Comparator
Sort ArrayList of objects by multiple fields
Sort ArrayList of objects using Collections.sort() method

10.4. Get/Search

Get Sub List of ArrayList
Find the index of last index of the element in the ArrayList
Get the index of the element in the ArrayList
Get element from ArrayList
Check if element exists in ArrayList

10.5. Working with ArrayList

Compare two ArrayLists
Synchronize ArrayList
Swap two elements in ArrayList
Serialize ArrayList
Join two ArrayList
Make ArrayList Empty
Check whether ArrayList is empty or not
Replace the value of existing element in ArrayList
Remove duplicate elements in ArrayList

10.6. Conversions

Convert LinkedList to ArrayList
Convert Vector to ArrayList
Convert ArrayList to String Array
Convert Array to ArrayList
Convert HashSet to ArrayList

10.7. Difference between collections

ArrayList vs Vector
ArrayList vs LinkedList