ArrayList size() method - Syntax and Examples

Learn about the Java ArrayList's size() method, its syntax, parameters, return value, and practical examples.

This reference explains ArrayList size() method - Syntax and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

The size() method in Java's ArrayList is a simple but essential tool. It tells you how many elements are currently stored within the list. Think of it like asking, 'How many items are in my shopping basket?' This information can be crucial for iterating through the list correctly, checking if it's empty, or performing calculations based on its contents.

public int size()

The size() method doesn't accept any parameters. It simply retrieves the current number of elements.

The size() method returns an integer representing the number of elements currently in the ArrayList.

This example demonstrates how to create an ArrayList and use the size() method to determine its initial size, then add elements and check the updated size.

import java.util.ArrayList;

public class SizeExample1 {
    public static void main(String[] args) {
        // Create an ArrayList of Strings
        ArrayList<String> names = new ArrayList<>();

        // Get the initial size
        int initialSize = names.size();
        System.out.println("Initial size: " + initialSize);

        // Add some elements
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        // Get the updated size
        int currentSize = names.size();
        System.out.println("Current size: " + currentSize);
    }

Initial size: 0
Current size: 3

In this example, we first create an empty ArrayList called names. The initial size is 0 because there are no elements in the list. We then add three names to the list and check the size again, which now correctly reflects the number of elements (3).

This example shows how you can use the size() method to determine whether an ArrayList is empty.

import java.util.ArrayList;

public class SizeExample2 {
    public static void main(String[] args) {
        // Create an ArrayList of integers
        ArrayList<Integer> numbers = new ArrayList<>();

        // Check if the list is empty
        if (numbers.size() == 0) {
            System.out.println("The list is empty.");
        } else {
            System.out.println("The list is not empty.");
        }
    }

The list is empty.

Since the numbers ArrayList is initially empty, its size is 0. The if statement evaluates to true, and the message "The list is empty." is printed.

This example demonstrates how you can use the size() method within a loop to safely iterate over all elements in an ArrayList. It's important to know the size so that your loop doesn’t go out of bounds.

import java.util.ArrayList;

public class SizeExample3 {
    public static void main(String[] args) {
        // Create an ArrayList of characters
        ArrayList<Character> letters = new ArrayList<>();
        letters.add('A');
        letters.add('B');
        letters.add('C');

        // Iterate through the list using size()
        for (int i = 0; i < letters.size(); i++) {
            System.out.println("Element at index " + i + ": " + letters.get(i));
        }
    }

Element at index 0: A
Element at index 1: B
Element at index 2: C

The loop iterates from i = 0 to i < letters.size(), ensuring that we access each element within the bounds of the ArrayList. Using `letters.size()` in the loop condition prevents an IndexOutOfBoundsException.

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Related explanations: Java Data Types - Primitive and Non-Primitive, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide.