Java Arrays

What is an Array?

An array is a container that holds a fixed number of values of the same type. Instead of creating separate variables for each value, you can store them all in one array.

Think of an array like a row of lockers. Each locker has a number (index) and can store one item. All lockers in the row hold the same type of items.

• Arrays store multiple values in a single variable
• All elements must be of the same data type
• Array size is fixed once created
• Array indexing starts from 0 (first element is at index 0)

Declaring and Creating Arrays

There are two ways to declare an array in Java. Let's see how to create an array of integers.

public class ArrayExample {
    public static void main(String[] args) {
        // Method 1: Declare and create separately
        int[] numbers;
        numbers = new int[5];
        
        // Method 2: Declare and create together
        int[] marks = new int[5];
        
        // Method 3: Declare, create, and initialize
        int[] scores = {85, 90, 78, 92, 88};
        
        System.out.println("Array created successfully!");
        System.out.println("First score: " + scores[0]);
    }
}

Array created successfully!
First score: 85

Explanation:

• int[] numbers - Declares an array variable
• new int[5] - Creates an array with space for 5 integers
• {85, 90, 78, 92, 88} - Initializes array with values directly
• scores[0] - Accesses the first element (index 0)

Accessing Array Elements

You can access array elements using their index number inside square brackets. Remember, indexing starts from 0.

public class AccessArray {
    public static void main(String[] args) {
        String[] fruits = {"Apple", "Banana", "Orange", "Mango"};
        
        // Accessing elements
        System.out.println("First fruit: " + fruits[0]);
        System.out.println("Second fruit: " + fruits[1]);
        
        // Modifying an element
        fruits[2] = "Grapes";
        System.out.println("Modified third fruit: " + fruits[2]);
        
        // Getting array length
        System.out.println("Total fruits: " + fruits.length);
    }
}

First fruit: Apple
Second fruit: Banana
Modified third fruit: Grapes
Total fruits: 4

Explanation:

• fruits[0] - Accesses the first element (Apple)
• fruits[2] = "Grapes" - Changes the value at index 2
• fruits.length - Returns the total number of elements in the array

Looping Through Arrays

You can use loops to access all elements of an array without writing repetitive code.

public class LoopArray {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40, 50};
        
        // Using traditional for loop
        System.out.println("Using for loop:");
        for (int i = 0; i < numbers.length; i++) {
            System.out.println("Element at index " + i + ": " + numbers[i]);
        }
        
        // Using enhanced for loop (for-each)
        System.out.println("\nUsing for-each loop:");
        for (int num : numbers) {
            System.out.println(num);
        }
    }
}

Using for loop:
Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40
Element at index 4: 50

Using for-each loop:
10
20
30
40
50

Explanation:

• for (int i = 0; i < numbers.length; i++) - Traditional loop using index
• for (int num : numbers) - Enhanced for-each loop, simpler and cleaner
• For-each loop automatically iterates through all elements without needing indices

Common Array Operations

Common Array Mistakes to Avoid

• ArrayIndexOutOfBoundsException: Trying to access an index that doesn't exist (e.g., accessing index 5 in a 5-element array)
• Forgetting array size is fixed: You cannot add more elements than the declared size
• Not initializing array elements: If you don't assign values, numeric arrays contain 0, boolean contains false, and object arrays contain null
• Starting from index 1: Remember arrays start at index 0, not 1