Java ArrayList class

The java.util.ArrayList class is a very useful class in the Collections framework. It is basically a resizable-array implementation of the List interface. ArrayList extends AbstractList class and implements List Interface.

An ArrayList in Java represent a resizable list of objects. It is widely used because of the functionality and flexibility it offers. Most of the Java developers choose ArrayList over arrays as it is a very good alternative of traditional arrays. We can easily add, remove, find, sort and replace elements in this list.

ArrayList Class Constructors

Following is the list of constructors provided by the Java ArrayList class.

ArrayList Class Methods

Apart from the methods inherited from its super classes and super interfaces, ArrayList defines the following methods

NOTE: ArrayList class does not have any capacity() method unlike Vector class. So, ArrayList has to use ensureCapacity() method which increases the capacity of its instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.

Example 1

The following program illustrates several of the methods supported by the ArrayList class.

ArrayListDemo1.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;

public class ArrayListDemo1 {

    public static void main(String[] args) {
        // Create an ArrayList of different types (non-generic style)
        ArrayList list1 = new ArrayList();
        
        // Add elements to ArrayList
        list1.add(10);
        list1.add(45.37);
        list1.add(true);
        list1.add('O');
        list1.add("JAVA");
        System.out.println("List1: " + list1);
        System.out.println("Size: " + list1.size());
        
        // Ensure that ArrayList can hold up to 50 elements 
        list1.ensureCapacity(50);
        System.out.println("ArrayList list1 can now store up to 50 elements."); 
        
        // Create an ArrayList of Integers using Generics
        ArrayList<Integer> list2 = new ArrayList<Integer>(Arrays.asList(74, 49, 25, 34, 57));
        // Add Elements
        list2.add(62); 
        list2.add(16);
        // Remove Elements
        list2.remove(6); // remove element at index 6
        list2.remove(new Integer(74)); // remove element 74
        
        System.out.println("\nList2: " + list2);
        System.out.println("Size: " + list2.size());
        // Sort the array list
        Collections.sort(list2);
        System.out.println("Sorted ArrayList: " + list2);
        
        // Create an ArrayList of Strings with initial capacity 20
        ArrayList<String> list3 = new ArrayList<String>(20);
        
        // Add new elements to list2
        list3.add("Book");
        list3.add("Diary");
        list3.add("Pen");
        System.out.println("\nList3: " + list3);
        System.out.println("Size: " + list3.size());
        
        // Add more elements to list2
        list3.add("Eraser");
        list3.add(3, "Pencil");
        
        System.out.println("Modified list3 elements are:~ ");
        // Iterate through the elements of ArrayList
        Iterator it = list3.iterator();
        while (it.hasNext()) {
            System.out.print(it.next() + " ");
        }
        System.out.println();
        
        // Create an array of String type
        String[] arr = {"Dog", "Cat", "Horse"};
        // Create an ArrayList from an array
        ArrayList<String> animals = new ArrayList<>(Arrays.asList(arr));
        
        // Create another ArrayList
        ArrayList<String> extra_animals = new ArrayList<>(Arrays.asList("Cow", "Goat"));
        
        // Create ArrayList using Collection object as argument of constructor
        ArrayList<String> list4 = new ArrayList<>(animals); //Java SE 8 style
        // Appends all of the elements in the specified collection to the end of the list4
        list4.addAll(extra_animals); 
        System.out.println("\nArrayList list4 elements are:~ ");
        list4.forEach(System.out::println);
        System.out.println();
        
        // Create a new array of String type
        String[] array = new String[list4.size()];
        // Convert ArrayList into an array
        list4.toArray(array);
        
        System.out.print("Array for list4: ");
        // Access the elements using get() method
        for(int i=0; i<array.length; i++) {
            System.out.print(list4.get(i)+ " ");
        }
        System.out.println();
        
        list4.removeAll(list4);
        System.out.println("List4 after removal: " + list4);
    }
}

Output:

List1: [10, 45.37, true, O, JAVA]
Size: 5
ArrayList list1 can now store up to 50 elements.

List2: [49, 25, 34, 57, 62]
Size: 5
Sorted ArrayList: [25, 34, 49, 57, 62]

List3: [Book, Diary, Pen]
Size: 3
Modified list3 elements are:~ 
Book Diary Pen Pencil Eraser

ArrayList list4 elements are:~ 
Dog
Cat
Horse
Cow
Goat

Array for list4: Dog Cat Horse Cow Goat 
List4 after removal: []

Example 2

The following program creates an ArrayList of objects of class type Book (user-defined class).

ArrayListDemo2.java

import java.util.ArrayList;
import java.util.Iterator;

class Book {
    String title, author;
    int id, pages, year;
    double price;

    public Book(String title, String author, int id, int pages, int year, double price) {
        this.title = title;
        this.author = author;
        this.id = id;
        this.pages = pages;
        this.year = year;
        this.price = price;
    }

    @Override
    public String toString() {
        return "(Title: " + title + "; Author: " + author + "; Id: " + id + "; Pages: " + pages + "; Yesr: " + year + "; Price: " + price + ")";
    }
}

public class ArrayListDemo2 {

    public static void main(String[] args) {
        // Creating an ArrayList of objects of class type Book
        ArrayList<Book> ab = new ArrayList<Book>();
        ab.add(new Book("Let Us C", "Kanetkar", 101, 500, 1998, 225.25));
        ab.add(new Book("Java Complete Reference", "Schildt", 102, 1000, 1996, 455.75));
        ab.add(new Book("Basic Electrical", "Thereja", 103, 600, 1995, 325.95));
        ab.add(new Book("Digital Logic", "Mano", 104, 495, 1993, 275.95));
        ab.add(new Book("Discrete Mathematics", "Rosen", 105, 1300, 1991, 695.95));

        System.out.println("ArrayList Contents:~ ");
        System.out.println("===================");
        Iterator ite = ab.iterator();
        int i = 0;
        while (ite.hasNext()) {
            System.out.println("Book " + (i + 1) + " = " + ite.next() + " ");
            i++;
        }
        System.out.println();

    }
}

Output:

ArrayList Contents:~ 
===================
Book 1 = (Title: Let Us C; Author: Kanetkar; Id: 101; Pages: 500; Yesr: 1998; Price: 225.25) 
Book 2 = (Title: Java Complete Reference; Author: Schildt; Id: 102; Pages: 1000; Yesr: 1996; Price: 455.75) 
Book 3 = (Title: Basic Electrical; Author: Thereja; Id: 103; Pages: 600; Yesr: 1995; Price: 325.95) 
Book 4 = (Title: Digital Logic; Author: Mano; Id: 104; Pages: 495; Yesr: 1993; Price: 275.95) 
Book 5 = (Title: Discrete Mathematics; Author: Rosen; Id: 105; Pages: 1300; Yesr: 1991; Price: 695.95)