Arrays in Java

# CHAPTER 9

Arrays in Java #

1. Introduction #

2. Learning Objectives #

• Declare and initialize arrays.

• Access elements by index.

• Traverse arrays using loops.

• Work with multidimensional arrays.

• Use the Arrays utility class.

3. Declaring and Initializing Arrays #

// Method 1: Declare and allocate
int[] scores = new int[5];      // 5 elements, default 0

// Method 2: Declare and initialize
int[] marks = {90, 85, 78, 92, 88};

// Method 3: Declare, then allocate
int[] ages;
ages = new int[3];

4. Accessing Array Elements #

int[] fruits = {10, 20, 30, 40, 50};
System.out.println(fruits[0]);  // 10 (first)
System.out.println(fruits[4]);  // 50 (last)
fruits[2] = 99;                 // Modify element at index 2
System.out.println(fruits.length); // 5 (array size)

Index:   [0]  [1]  [2]  [3]  [4]
Values:   10   20   99   40   50

5. Traversing Arrays #

int[] data = {10, 20, 30, 40, 50};

// for loop
for (int i = 0; i < data.length; i++) {
    System.out.println("Index " + i + ": " + data[i]);
}

// for-each loop
for (int value : data) {
    System.out.println("Value: " + value);
}

6. Multidimensional Arrays #

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

System.out.println(matrix[0][0]); // 1
System.out.println(matrix[1][2]); // 6

// Traversal
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.print(matrix[i][j] + " ");
    }
    System.out.println();
}

7. The Arrays Utility Class #

import java.util.Arrays;

int[] nums = {5, 2, 8, 1, 9};

Arrays.sort(nums);                        // Sort ascending
System.out.println(Arrays.toString(nums)); // [1, 2, 5, 8, 9]

int index = Arrays.binarySearch(nums, 5);  // Search for 5
System.out.println("Found at index: " + index);

int[] copy = Arrays.copyOf(nums, nums.length); // Copy array
boolean equal = Arrays.equals(nums, copy);      // Compare arrays
Arrays.fill(nums, 0);                           // Fill with 0s

8. Mini Project: Student Marks System #

import java.util.Scanner;
import java.util.Arrays;

public class StudentMarks {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("How many students? ");
        int n = sc.nextInt();
        int[] marks = new int[n];

        for (int i = 0; i < n; i++) {
            System.out.print("Enter marks for Student " + (i + 1) + ": ");
            marks[i] = sc.nextInt();
        }

        int sum = 0, max = marks[0], min = marks[0];
        for (int mark : marks) {
            sum += mark;
            if (mark > max) max = mark;
            if (mark < min) min = mark;
        }

        System.out.println("\n=== RESULTS ===");
        System.out.println("Marks: " + Arrays.toString(marks));
        System.out.println("Total: " + sum);
        System.out.printf("Average: %.2f%n", (double) sum / n);
        System.out.println("Highest: " + max);
        System.out.println("Lowest: " + min);
        sc.close();
    }
}

9. Common Mistakes #

• ArrayIndexOutOfBoundsException: Accessing arr[5] when the array has 5 elements (max index is 4).

• Forgetting zero-based indexing: First element is arr[0], not arr[1].

• Arrays are fixed size: Once created, the size cannot change. Use ArrayList for dynamic sizing.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: What is the difference between an Array and an ArrayList?

• Q: How do you find the largest element in an array without sorting?

• Q: What is the time complexity of searching an unsorted array?

12. Summary #

13. Next Chapter Recommendation #