Java Arrays

Beginner ~25 min read

An array is a container object that holds a fixed number of values of a single type. Unlike variables that hold just one value, an array can hold dozens, hundreds, or even thousands of values under a single name.

Anatomy of an Array

Think of an array as a row of mailboxes. Each mailbox has an index number, and you can put data inside each one.

Fixed Size: Once created, you cannot change the size of an array.
Same Type: All elements must be of the same data type (e.g., all int).
Zero-Indexed: The first element is at index 0, not 1.

Output

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ArrayIndexOutOfBoundsException

This is the most common error with arrays. If an array has a size of 5, valid indices are 0, 1, 2, 3, 4. accessing arr[5] will crash your program!

Ways to Initialize Arrays

There are a few different ways to create and fill arrays, depending on whether you know the values upfront or just the size.

// 1. Size only (Default values: 0, null, false)
int[] nums = new int[5];

// 2. Values directly (Array Literal)
String[] names = {"Alice", "Bob", "Charlie"};

public class Main {
    public static void main(String[] args) {
        System.out.println("--- Array Initialization Ways ---");

// Way 1: With 'new' keyword (Fixed size, default values)
        int[] arr1 = new int[3];
        System.out.println("arr1[0] (Default int): " + arr1[0]); // 0

// Way 2: Array Literal (Values known upfront)
        String[] fruits = { "Apple", "Banana", "Cherry" };
        System.out.println("Fruit 1: " + fruits[0]);
        System.out.println("Fruit 2: " + fruits[1]);

// Way 3: Anonymous Array (Common in method arguments)
        printArray(new int[] { 1, 2, 3, 4, 5 });

System.out.println("\n--- Default Values ---");
        boolean[] bools = new boolean[2];
        System.out.println("boolean default: " + bools[0]); // false

String[] texts = new String[2];
        System.out.println("String default: " + texts[0]); // null
    }

public static void printArray(int[] arr) {
        System.out.print("Printed Array: ");
        for (int x : arr) {
            System.out.print(x + " ");
        }
        System.out.println();
    }
}

Output

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Looping Through Arrays

To process all elements in an array, we typically use a for loop.

int[] scores = {10, 20, 30};

// Standard For Loop
for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

// Enhanced For-Each Loop (Clean & Readable)
for (int score : scores) {
    System.out.println(score);
}

Summary

Arrays store multiple values of the same type in a contiguous memory block.
Arrays have a fixed size that cannot be changed.
Array indices start at 0.
Use array.length to find out how many items an array can hold.

What's Next?

You've mastered single-dimensional arrays. But what if you need a grid, a matrix, or a table of data? That's where Multi-dimensional Arrays come in. Let's explore 2D arrays in the next lesson.

Previous Strings

Next 2D Arrays →