Java ArrayList

In Java, we use the ArrayList class to implement the functionality of resizable-arrays.

It implements the List interface of the collections framework .

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 their capacity when we add or remove elements from them. 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<Type> arrayList= new ArrayList<>();

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

// create Integer type arraylist
ArrayList<Integer> arrayList = new ArrayList<>();

// create String type arraylist
ArrayList<String> 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<String> 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<String> 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().

// add JavaScript at index 1
languages.add(1, "JavaScript");

// add C++ at index 3
languages.add(3, "C++");

import java.util.ArrayList;
import java.util.HashSet;

class Main {
  public static void main(String[] args) {

    // create a set
    HashSet<String> vowels = new HashSet<>();
    vowels.add("a");
    vowels.add("e");
    vowels.add("i");

    // create an arraylist
    ArrayList<String> alphabets = new ArrayList<>();

    // add all elements of set to arraylist
    alphabets.addAll(vowels);
    System.out.println("ArrayList: " + alphabets);
  }
}

// Output: ArrayList: [a, e, i]

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<String> 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<String> 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().

4. Remove ArrayList Elements

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<String> 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.

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<String> 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,

import java.util.ArrayList;

class Main {
  public static void main(String[] args) {
    ArrayList<String> 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 + ", ");
    }
  }
}

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

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<String> languages = new ArrayList<>(Arrays.asList(arr));
    System.out.println("\nArrayList: " + languages);
  }
}

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

Arrays.asList(arr)

import java.util.ArrayList;
import java.util.Arrays;

class Main {
  public static void main(String[] args) {

    // create and initialize ArrayList
    ArrayList<String> languages = new ArrayList<>(Arrays.asList("Java", "Python", "C"));
    System.out.println("ArrayList: " + languages);

  }
}

// Output: ArrayList: [Java, Python, C]

import java.util.ArrayList;

class Main {
  public static void main(String[] args) {
    ArrayList<String> 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);
  }
}

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

import java.util.ArrayList;
import java.util.List;

class Main {
  public static void main(String[] args) {

    // create arraylist using List
    List<String> languages = new ArrayList<>();
    languages.add("Java");
    languages.add("Python");
    languages.add("C");

    System.out.println("ArrayList: " + languages);
  }
}

// Output: ArrayList: [Java, Python, C]