ArrayList in Java

ArrayList, a class in the java.util package, implements dynamic arrays in Java, offering flexibility over traditional fixed-size arrays. It extends AbstractList and implements List, RandomAccess, and Serializable interfaces. ArrayLists grow or shrink dynamically, accommodating additions and removals of elements. While they are slower than built-in arrays, their resizable nature is advantageous in programs requiring frequent array manipulations. One key feature is that, unlike standard arrays, the size of an ArrayList need not be specified initially, though it's an available option, providing more flexibility in managing array sizes.

What is ArrayList in Java?

The ArrayList class is a part of Collection framework, extends AbstractList class and implements the List interface. This class is declared in java.util package. It is used for creating dynamic arrays that are resizable in nature. Apart from this, we can also use pre-defined methods of this class to manipulate the ArrayList elements.

Syntax:

Important Features of ArrayList Java

• Dynamic Resizing: ArrayList grows dynamically as we add elements to the list or shrinks as we remove elements from the list.
• Ordered: ArrayList preserves the order of the elements i.e. the order in which the elements were added to the list.
• Index based: ArrayList in Java supports random access from the list using index positions starting with ‘0’.
• Object based: ArrayList can store only Objects data types. They cannot be used for primitive data types (int, float, etc). We require a wrapper class in that case.
• Not Synchronized: ArrayList in Java is not synchronized, we can use the Vector class for the synchronized version. It means ArrayList operations are not thread-safe and multiple threads should not operate on the same ArrayList object at the same time.

How ArrayList Works?

ArrayList can dynamically adjust its size as elements are added or removed. While the internal workings of the ArrayList may seem complex, the basic concept is straightforward: it grows as needed when more elements are added.

When an ArrayList is created, its default capacity is 10 unless specified otherwise. The ArrayList automatically resizes itself when the number of elements exceeds its current capacity.

• If the current capacity is reached, the ArrayList expands its size by approximately 50% of the old capacity, allowing for the addition of new elements without the need for creating a new array.
• The resizing process ensures that there is always sufficient space to accommodate new elements, and this is managed internally.

For example, if the user creates an ArrayList of size 10,

After adding the 7th element to the list, the size will increase. Internally, a new ArrayList with a new capacity is created and the elements present in the old ArrayList are copied in the new ArrayList, as shown in the following image.

In Java 8 and later, the new capacity of the ArrayList is calculated to be 50% more than its old capacity.

new_capacity = old_capacity + (old_capacity >> 1)

In the above formula, the new capacity is calculated as 50% more than the old capacity. Here, (old_capacity >> 1) gives us half of the old capacity. For more details on how the Right Shift Operator works, follow this article.

For example, if the array size is 10 and it has reached the threshold value, we have to increase its capacity to add new elements. The new capacity will be 10 + (10 >> 1) => 10 + 5 => 15. Hence, the size is increased from 10 to 15.

Constructors in the Java ArrayList

Since ArrayList is a generic class, you can parameterize it with any type you want, and the compiler will ensure that, for example, you can't place Integer values inside a collection of Strings. In addition, while accessing elements from an ArrayList, you do not need to cast them.

1. ArrayList<>(): This constructor is the default one, it creates an empty ArrayList instance with default initial capacity i.e. 10.

Syntax:

2. ArrayList(int capacity): This constructor creates an empty ArrayList with initial capacity as mentioned by the user.

Syntax

3. ArrayList(Collection list): This constructor creates an ArrayList and stores the elements that are present in collection list.

Syntax

Syntax:

Methods in Java ArrayList

In Java, ArrayList class provides various predefined methods which can be accessed using object of ArrayList class. Let’s discuss a few important methods of the ArrayList class.

How to Create ArrayList in Java?

We can only add objects in the ArrayList but if we want to add primitive data types such as int, float, etc., we can use wrapper class for such cases. To create an ArrayList in Java, we first need to create an instance of the ArrayList class. This can be done by the following statement:

The following Java program creates an ArrayList with the help of constructors.

Output:

How to Add Elements to an ArrayList?

We can add elements to an ArrayList with the help of add() method. This method is overloaded, allowing it to do several operations based on various parameters. Here's what they are:

Syntax:

• add(Object o): This method is used to directly add the specified element at the end of the ArrayList.
• add(int index, Object o): This method is used to insert the specified element in the array list at the specified index.

The following Java program adds new elements to an ArrayList using the add() method:

Output:

How to Get Element from ArrayList?

We can randomly access elements from the ArrayList using the get() method.

Syntax:

• get(int index): In this method, we provide the index and it returns the element present at the specified index.

The following Java program accesses elements from an ArrayList using the get() method:

Output:

How to Sort ArrayList in Java?

We can sort an ArrayList using the sort() method of the Collection framework in Java.

Syntax:

• Collections.sort(ArrayList): As an argument, it takes the object of an ArrayList and sorts the ArrayList in the ascending order according to the natural ordering of its elements.

The following Java program sorts an ArrayList using the sort() method:

Output:

Change an element in ArrayList

In order to change an element in the ArrayList, we can use the set() method.

Syntax:

• set(int index, E element): In this method, we provide the index and the new element as arguments which in return replaces the element present at the specified index with the new element.

The following Java program changes an element in the ArrayList using set() method.

Output:

How to Remove Elements from ArrayList?

We can remove elements from an ArrayList with the help of remove() method. This method is also overloaded, allowing it to do several operations based on various parameters. Here's what they are:

Syntax:

• remove(Object o) : This method is used to directly remove the specified element from the ArrayList.
• remove(int index) : This method is used to remove the element present at the specified index from the ArrayList.

The following Java program deletes elements from the ArrayList using remove() method.

Output:

Iterating ArrayList

There are many ways of iterating through an ArrayList. In the above examples, we were displaying the elements just by calling the ArrayList instance which isn't the best approach to show the elements. The simple method to iterate through an ArrayList is a simple loop or an advanced one.

The following Java program iterates through an ArrayList using loops.

Using Loops

Output:

Explanation:

• While using simple for loop, we are accessing elements with the help of get() method by passing index positions.
• Whereas for each loops are mainly used for traversing items in a collection, so we are able to directly access the elements as per the implementation.

Using listIterator()

An iterator is an object that enables a programmer to traverse through collections such as ArrayList, HashSet, etc.

Syntax:

• public ListIterator<E> listIterator(): This method returns a list iterator over the elements in the ArrayList in proper sequence. The returned list iterator is fail-fast.

The following Java program iterates through an ArrayList using the listIterator() method.

Output:

Finding the length of the ArrayList

In order to find the length of an ArrayList, we can use the size() method.

Syntax:

• size(): It returns the number of elements present in an ArrayList.

The Java program given below demonstrates how to find the size of an ArrayList.

Output:

How to check if ArrayList is Empty in Java?

In order to find out if an ArrayList is empty or not, we can use the isEmpty() method.

Syntax:

• public boolean isEmpty(): This method returns true if an ArrayList is empty.

The following Java program demonstrates if an ArrayList is empty or not.

Output:

How to synchronize ArrayList in Java?

ArrayLists in Java are not synchronized, this means it must be synchronized externally if a thread modifies it structurally and multiple threads try to access it at the same time.

This can be done using the following methods:

• Collections.synchronizedList() method.
• Using CopyOnWriteArrayList (thread-safe variant of ArrayList)

The crucial thing to remember is that under this type of synchronization, the iterator must be in a synchronized block, as demonstrated in the example below.

Output:

The following Java program synchronizes an ArrayList externally using CopyOnWriteArrayList.

Output:

Java Array vs ArrayList

The key differences between Array and ArrayList are listed in the table below:

Benefits of ArrayList

• The fact that ArrayList is dynamic in size is one of its main advantages. We can increase and decrease the size of ArrayList dynamically.
• ArrayList has various predefined methods which help to manipulate the stored objects.
• In ArrayList, we can randomly insert and delete elements.
• We can add different types of objects into the ArrayList.
• In ArrayList, listIterator() allows us to traverse in both directions.

Related Topics

• How to Create 2D ArrayList in Java?
• How to Sort ArrayList in Java?
• ArrayList to Array Java
• ArrayList Vs LinkedList in Java
• List in Java