Arrays in Java

An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created, and after creation, its length is fixed (cannot grow or shrink dynamically).

Each item in an array is called an element, and each element is accessed by its numerical index. Array indexing in Java starts at 0.

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1. Single-Dimensional Arrays

Declaring and Initializing Arrays

There are a few ways to declare and initialize arrays in Java:

// Method 1: Declaration followed by allocation
int[] numbers = new int[5]; // Creates an array of size 5, default values are 0

// Method 2: Declare and initialize with values directly
int[] ages = {18, 20, 25, 30, 40};

// Method 3: Using the new keyword with values
String[] fruits = new String[]{"Apple", "Banana", "Orange"};

Accessing and Modifying Elements

You can access and update elements using their index enclosed in square brackets [].

int[] numbers = {10, 20, 30, 40};

// Accessing elements
System.out.println("First element: " + numbers[0]); // Output: 10
System.out.println("Third element: " + numbers[2]); // Output: 30

// Modifying elements
numbers[1] = 50; // Changes 20 to 50
System.out.println("Updated second element: " + numbers[1]); // Output: 50

Finding Array Length

You can find the number of elements in an array using the .length property.

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println("Number of cars: " + cars.length); // Output: 4

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2. Iterating Through Arrays

To process or display all elements in an array, you can loop through them.

Using a Standard for Loop

int[] marks = {95, 80, 89, 92, 85};

for (int i = 0; i < marks.length; i++) {
    System.out.println("Mark at index " + i + ": " + marks[i]);
}

Using an Enhanced for-each Loop

If you do not need to keep track of the indices, the enhanced for loop is cleaner and more readable.

String[] colors = {"Red", "Green", "Blue"};

for (String color : colors) {
    System.out.println("Color: " + color);
}

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3. Multi-Dimensional Arrays

A multi-dimensional array is an array of arrays. The most common multi-dimensional array is the two-dimensional array (often thought of as a table/matrix with rows and columns).

Declaring and Initializing a 2D Array

// Method 1: Specify size [rows][columns]
int[][] matrix = new int[3][4]; // 3 rows and 4 columns

// Method 2: Initialize with values directly
int[][] coordinates = {
    {1, 2, 3}, // Row 0
    {4, 5, 6}, // Row 1
    {7, 8, 9}  // Row 2
};

Accessing Elements in a 2D Array

Specify both the row index and the column index:

int[][] coordinates = {
    {1, 2, 3},
    {4, 5, 6}
};

System.out.println(coordinates[0][1]); // Row 0, Column 1 -> Output: 2
System.out.println(coordinates[1][2]); // Row 1, Column 2 -> Output: 6

Iterating over a 2D Array

To loop through a 2D array, you must use nested loops:

int[][] coordinates = {
    {1, 2},
    {3, 4}
};

for (int i = 0; i < coordinates.length; i++) { // Loop through rows
    for (int j = 0; j < coordinates[i].length; j++) { // Loop through columns
        System.out.print(coordinates[i][j] + " ");
    }
    System.out.println(); // New line after each row
}
// Output:
// 1 2
// 3 4

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Common Pitfall: ArrayIndexOutOfBoundsException

If you try to access an index that is negative, or greater than or equal to the array’s length, Java will throw an ArrayIndexOutOfBoundsException.

int[] numbers = {1, 2, 3};

// ❌ Throws ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
System.out.println(numbers[3]);

[!WARNING] Remember that the last valid index in any array is always length - 1.

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Next Steps ➡️

Now that you can store lists of data in arrays, the next step is to dive into Object-Oriented Programming (OOP) in Java, starting with Classes and Objects.