Week 6: Java – Strings & 2D Arrays

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Q1. Username Encryption (Reverse + Uppercase)

Objective: Read a username, reverse it, convert to uppercase, and display the “encrypted” username.

import java.util.Scanner;

class UsernameEncryptor {
    static void userpass() {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter username: ");
        String user = sc.nextLine();

        System.out.println("Original Username: " + user);

        // Reverse and convert to uppercase
        String encrypted = new StringBuilder(user)
                                .reverse()
                                .toString()
                                .toUpperCase();

        System.out.println("Encrypted Username: " + encrypted);
    }

    public static void main(String args[]) {
        UsernameEncryptor.userpass();
    }
}

Key Points:

• StringBuilder allows reversing strings efficiently.
• Chaining .reverse() and .toUpperCase() is convenient.
• No explicit loop is needed for character manipulation.

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Q2. Image Rotation / Transpose of 2D Array

Objective: Input a 2D matrix representing an image and display its transpose (rows ↔ columns).

import java.util.Scanner;

class ImageRotation {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter number of rows: ");
        int rows = sc.nextInt();
        System.out.print("Enter number of columns: ");
        int cols = sc.nextInt();

        int[][] image = new int[rows][cols];

        // Input pixel values
        System.out.println("Enter pixel values:");
        for (int i = 0; i < rows; i++) {
            System.out.print("Row " + (i + 1) + ": ");
            for (int j = 0; j < cols; j++) {
                image[i][j] = sc.nextInt();
            }
        }

        // Transpose the matrix
        int[][] transposed = new int[cols][rows];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                transposed[j][i] = image[i][j];
            }
        }

        // Print the transposed matrix
        System.out.println("\nTransposed Image Matrix:");
        for (int i = 0; i < cols; i++) {
            for (int j = 0; j < rows; j++) {
                System.out.print(transposed[i][j] + " ");
            }
            System.out.println();
        }

        sc.close();
    }
}

Key Points:

• Transpose swaps rows and columns: transposed[j][i] = original[i][j].
• Useful for image processing or matrix operations.
• Always allocate the new array with dimensions [cols][rows].

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Week 6 Notes:

1. StringBuilder is efficient for reversing strings.
2. toUpperCase() converts all letters to uppercase.
3. 2D arrays are stored in row-major order in Java.
4. Transposing a matrix is a fundamental operation for image rotation or mathematical computations.

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