ArrayList in ArrayList Java

The ArrayList class of the Java collections framework provides the functionality of resizable-arrays.

It implements the List interface.

Java ArrayList Implementation

Java ArrayList Vs Array

In Java, we need to declare the size of an array before we can use it. Once the size of an array is declared, it's hard to change it.

To handle this issue, we can use the ArrayList class. It allows us to create resizable arrays.

Unlike arrays, arraylists can automatically adjust its capacity when we add or remove elements from it. Hence, arraylists are also known as dynamic arrays.

Creating an ArrayList

Before using ArrayList, we need to import the java.util.ArrayList package first. Here is how we can create arraylists in Java:

ArrayList arrayList= new ArrayList[];

Here, Type indicates the type of an arraylist. For example,

// create Integer type arraylist ArrayList arrayList = new ArrayList[]; // create String type arraylist ArrayList arrayList = new ArrayList[];

In the above program, we have used Integer not int. It is because we cannot use primitive types while creating an arraylist. Instead, we have to use the corresponding wrapper classes.

Here, Integer is the corresponding wrapper class of int. To learn more, visit the Java wrapper class.

Example: Create ArrayList in Java

import java.util.ArrayList; class Main { public static void main[String[] args]{

// create ArrayList ArrayList languages = new ArrayList[];

// Add elements to ArrayList languages.add["Java"]; languages.add["Python"]; languages.add["Swift"]; System.out.println["ArrayList: " + languages]; } }

Output

ArrayList: [Java, Python, Swift]

In the above example, we have created an ArrayList named languages.

Here, we have used the add[] method to add elements to the arraylist. We will learn more about the add[] method later in this tutorial.

Basic Operations on ArrayList

The ArrayList class provides various methods to perform different operations on arraylists. We will look at some commonly used arraylist operations in this tutorial:

• Add elements
• Access elements
• Change elements
• Remove elements

1. Add Elements to an ArrayList

To add a single element to the arraylist, we use the add[] method of the ArrayList class. For example,

import java.util.ArrayList; class Main { public static void main[String[] args]{ // create ArrayList ArrayList languages = new ArrayList[];

// add[] method without the index parameter languages.add["Java"];

languages.add["C"]; languages.add["Python"]; System.out.println["ArrayList: " + languages]; } }

Output

ArrayList: [Java, C, Python]

In the above example, we have created an ArrayList named languages. Here, we have used the add[] method to add elements to languages.

To learn more, visit the Java ArrayList add[].

We can also pass an index number as an additional parameter to the add[] method to add an element at the specified position. For example,

// add JavaScript at index 1 languages.add[1, "JavaScript"]; // add C++ at index 3 languages.add[3, "C++"];

We can also add all elements of a collection [set, map] to an arraylist using the addAll[] method. For example,

import java.util.ArrayList; import java.util.HashSet; class Main { public static void main[String[] args] { // create a set HashSet vowels = new HashSet[]; vowels.add["a"]; vowels.add["e"]; vowels.add["i"]; // create an arraylist ArrayList alphabets = new ArrayList[];

// add all elements of set to arraylist alphabets.addAll[vowels];

System.out.println["ArrayList: " + alphabets]; } } // Output: ArrayList: [a, e, i]

To learn more, visit Java ArrayList addAll[].

2. Access ArrayList Elements

To access an element from the arraylist, we use the get[] method of the ArrayList class. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { ArrayList animals = new ArrayList[]; // add elements in the arraylist animals.add["Cat"]; animals.add["Dog"]; animals.add["Cow"]; System.out.println["ArrayList: " + animals];

// get the element from the arraylist String str = animals.get[1];

System.out.print["Element at index 1: " + str]; } }

Output

ArrayList: [Cat, Dog, Cow] Element at index 1: Dog

In the above example, we have used the get[] method with parameter 1. Here, the method returns the element at index 1.

To learn more, visit the Java ArrayList get[].

We can also access elements of the ArrayList using the iterator[] method. To learn more, visit Java ArrayList iterator[].

3. Change ArrayList Elements

To change elements of the arraylist, we use the set[] method of the ArrayList class. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { ArrayList languages = new ArrayList[]; // add elements in the array list languages.add["Java"]; languages.add["Kotlin"]; languages.add["C++"]; System.out.println["ArrayList: " + languages];

// change the element of the array list languages.set[2, "JavaScript"];

System.out.println["Modified ArrayList: " + languages]; } }

Output

ArrayList: [Java, Kotlin, C++] Modified ArrayList: [Java, Kotlin, JavaScript]

In the above example, we have created an ArrayList named languages. Notice the line,

language.set[2, "JavaScript"];

Here, the set[] method changes the element at index 2 to JavaScript.

To learn more, visit the Java ArrayList set[].

To remove an element from the arraylist, we can use the remove[] method of the ArrayList class. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { ArrayList animals = new ArrayList[]; // add elements in the array list animals.add["Dog"]; animals.add["Cat"]; animals.add["Horse"]; System.out.println["ArrayList: " + animals];

// remove element from index 2 String str = animals.remove[2];

System.out.println["Updated ArrayList: " + animals]; System.out.println["Removed Element: " + str]; } }

Output

ArrayList: [Dog, Cat, Horse] Updated ArrayList: [Dog, Cat] Removed Element: Horse

Here, the remove[] method takes the index number as the parameter. And, removes the element specified by the index number.

To learn more, visit the Java ArrayList remove[].

We can also remove all the elements from the arraylist at once. To learn more, visit

• Java ArrayList removeAll[]
• Java ArrayList clear[]

Methods of ArrayList Class

In the previous section, we have learned about the add[], get[], set[], and remove[] method of the ArrayList class.

Besides those basic methods, here are some more ArrayList methods that are commonly used.

Methods Descriptions

size[]	Returns the length of the arraylist.
sort[]	Sort the arraylist elements.
clone[]	Creates a new arraylist with the same element, size, and capacity.
contains[]	Searches the arraylist for the specified element and returns a boolean result.
ensureCapacity[]	Specifies the total element the arraylist can contain.
isEmpty[]	Checks if the arraylist is empty.
indexOf[]	Searches a specified element in an arraylist and returns the index of the element.

If you want to learn about all the different methods of arraylist, visit Java ArrayList methods.

Iterate through an ArrayList

We can use the Java for-each loop to loop through each element of the arraylist. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { // creating an array list ArrayList animals = new ArrayList[]; animals.add["Cow"]; animals.add["Cat"]; animals.add["Dog"]; System.out.println["ArrayList: " + animals]; // iterate using for-each loop System.out.println["Accessing individual elements: "];

for [String language : animals] { System.out.print[language]; System.out.print[", "]; }

} }

Output

ArrayList: [Cow, Cat, Dog] Accessing individual elements: Cow, Cat, Dog,

Some of the major differences between ArrayList and LinkedList in Java are:

ArrayList	LinkedList
Implements List interface	Implements List, Queue, and Deque interfaces.
Stores a single value.	Stores 3 values: data, previous and next address
Provides the functionality of a resizable array.	Provides the functionality of doubly-linked list

We can convert the ArrayList into an array using the toArray[] method. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { ArrayList languages = new ArrayList[]; // add elements in the array list languages.add["Java"]; languages.add["Python"]; languages.add["C++"]; System.out.println["ArrayList: " + languages]; // create a new array of String type String[] arr = new String[languages.size[]];

// convert ArrayList into an array languages.toArray[arr];

System.out.print["Array: "]; // access elements of the array for [String item : arr] { System.out.print[item + ", "]; } } }

Output

ArrayList: [Java, Python, C++] Array: Java, Python, C++,

In the above example, the toArray[] method converts the languages arraylist to an array and stores it in arr. To learn more, visit Java ArrayList toArray[].

We use the asList[] method of the Arrays class. To use asList[], we must import the java.util.Arrays package first. For example,

import java.util.ArrayList; import java.util.Arrays; class Main { public static void main[String[] args] { // create an array of String type String[] arr = { "Java", "Python", "C++" }; System.out.print["Array: "]; // print array for [String str : arr] { System.out.print[str]; System.out.print[" "]; }

// create an ArrayList from an array ArrayList languages = new ArrayList[Arrays.asList[arr]];

System.out.println["\nArrayList: " + languages]; } }

Output

Array: Java Python C++ ArrayList: [Java, Python, C++]

In the above program, we first created an array arr of the String type. Notice the expression,

Arrays.asList[arr]

Here, the asList[] method converts the array into an arraylist.

We use the Arrays.asList[] method to create and initialize an arraylist in a single line. For example,

import java.util.ArrayList; import java.util.Arrays; class Main { public static void main[String[] args] {

// create and initialize ArrayList ArrayList languages = new ArrayList[Arrays.asList["Java", "Python", "C"]];

System.out.println["ArrayList: " + languages]; } } // Output: ArrayList: [Java, Python, C]

We use the toString[] method of the ArrayList class to convert an arraylist into a string. For example,

import java.util.ArrayList; class Main { public static void main[String[] args] { ArrayList languages = new ArrayList[]; // add elements in the ArrayList languages.add["Java"]; languages.add["Python"]; languages.add["Kotlin"]; System.out.println["ArrayList: " + languages];

// convert ArrayList into a String String str = languages.toString[];

System.out.println["String: " + str]; } }

Output

ArrayList: [Java, Python, Kotlin] String: [Java, Python, Kotlin]

Here, the toString[] method converts the whole arraylist into a single string. To learn more, visit Java ArrayList toString[].

We can also create an arraylist using the List interface. It's because the ArrayList class implements the List interface. Let's see an example,

import java.util.ArrayList; import java.util.List; class Main { public static void main[String[] args] {

// create arraylist using List List languages = new ArrayList[];

languages.add["Java"]; languages.add["Python"]; languages.add["C"]; System.out.println["ArrayList: " + languages]; } } // Output: ArrayList: [Java, Python, C]