a) 

1. compiler error - incompatible types int and String

fix:

int x = 5;
int y = 10; 

int sum = x + y;

System.out.println("Sum: " + sum);

2. first compiler error - if for loop does not execute sum cannot be printed out as it was not initialized 

fix:

Scanner stdin = new Scanner(System.in);
int n = stdin.nextInt();
int sum = 0;
for (int i = 1; i <= n; i++) {
    sum = 0; 
    sum = sum + i;
}

System.out.println("sum = " + sum);

THEN logical error - sum is reset on every iteration of the loop

fix: 

Scanner stdin = new Scanner(System.in);
int n = stdin.nextInt();
int sum = 0;
for (int i = 1; i <= n; i++) {
    sum = sum + i;
}

System.out.println("sum = " + sum);

b)

import java.util.Scanner;

public class PositiveNumberCheck {
    public static void main(String[] args) {
        // Create a Scanner object to read input
        Scanner scanner = new Scanner(System.in);

        // Declare and initialize the variables x and y
        System.out.print("Enter value for x: ");
        int x = scanner.nextInt();

        System.out.print("Enter value for y: ");
        int y = scanner.nextInt();

        // 1. Check if both x and y are positive
        if (x > 0 && y > 0) {
            System.out.println("they are both positive");
        } else {
            System.out.println("they are not both positive");
        }

        // 2. Check if at least one of x or y is positive
        if (x > 0 || y > 0) {
            System.out.println("at least one is positive");
        } else {
            System.out.println("neither one is positive");
        }

        // 3. Check if either x or y is not not positive
        if (!(x > 0 && y > 0)) { // This effectively checks if either x or y is positive
            System.out.println("it is not positive");
        } else {
            System.out.println("it is positive"); // This means both are positive
        }

        // Close the Scanner
        scanner.close();
    }
}

c) 

import java.util.Scanner;

public class BookInfo {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in); //allows us to read from kybd
        int bookID = input.nextInt();
        //while loop
        while (bookID != -1) {
            input.nextLine(); //input buffer; reads newline operator
            String title = input.nextLine();
            String author = input.nextLine();
            int genreNum = input.nextInt();
            String genre = "Unconfirmed"; //default
            String recommendedAge = "N/A"; //default
            System.out.println("\n");
            switch (genreNum) {
                case 1 -> {
                    genre = "Fiction";
                    recommendedAge = "7+";
                }
                case 2 -> {
                    genre = "Non-Fiction";
                    recommendedAge = "10+";
                }
                case 3 -> {
                    genre = "Sci-Fi";
                    recommendedAge = "13+";
                }
                case 4 -> {
                    genre = "Mystery";
                    recommendedAge = "15+";
                }
                case 5 -> {
                    genre = "Romance";
                    recommendedAge = "16+";
                }
                default -> {
                }
            }
           
            System.out.printf("Book ID: %d%n", bookID); //int
            System.out.printf("Title: %s%n", title); //String
            System.out.printf("Author: %s%n", author); //String
            System.out.printf("Genre: %s%n", genre); //String
            System.out.printf("Recommended age: %s%n", recommendedAge); //String
            System.out.println("\n\n\n");

            bookID = input.nextInt(); //read again to avoid infinite loop
        }

        input.close();
    }

}