Craeting a custom exception class begins by extending the built-in Exception class. This allows you to define application-specific error conditions that can be thrown and caught programmatically.

The following implementation demonstrates a custom exception that leverages the parent class constructor to store error messages:

1 public class GradeException extends Exception {
2 
3     public GradeException(String errorMessage) {
4         super(errorMessage);
5     }
6 }

This custom exception can be utilized to validate whether user input represents a valid integer score. If validation fails, the exception is thrown with an appropriate message; otherwise, the program proceeds to determine the corresponding grade level.

When working with try-catch blocks, any exception thrown by the code must be explicitly propagated using the throw keyword before it can be caught by a catch block. The key requirement is that potentially problematic code must be enclosed within a try block. A single try block may be followed by multiple catch blocks, but once an exception matches one catch block, subsequent catch blocks will not execute.

 1 import java.util.Scanner;
 2 
 3 public class GradeProcessor {
 4     static Scanner input = null;
 5     
 6     static int validate(String value) throws GradeException {
 7         if (!value.matches("^\\-?[0-9]+$")) {
 8             throw new GradeException("Invalid input: integers only!");
 9         }
10         int number = Integer.valueOf(value);
11         if (number > 100 || number < 0) {
12             throw new GradeException("Score must be between 0 and 100!");
13         }
14         
15         return number;
16     }
17     
18     public static void main(String[] args) {
19         System.out.println("Enter an integer score:");
20         String userInput = null;
21         
22         input = new Scanner(System.in);
23         userInput = input.nextLine();
24         try {
25             int score = validate(userInput);
26             if (score >= 90 && score <= 100) {
27                 System.out.println("Grade: A - Excellent!");
28             } 
29             else if (score >= 80 && score < 90) {
30                 System.out.println("Grade: B - Good!");
31             } 
32             else if (score >= 70 && score < 80) {
33                 System.out.println("Grade: C - Average!");
34             }
35             else if (score >= 60 && score < 70) {
36                 System.out.println("Grade: D - Passing!");
37             }
38             else {
39                 System.out.println("Grade: F - Failing!");
40             }
41         }
42         catch (GradeException error) {
43             System.out.println(error.getMessage());
44         }
45     }
46 }

The complete try-catch structure optionally includes a finally block, which executes regardless of whether an exception occurred. However, the finally block will NOT execute under these specific circumstances:

1. An unhandled exception occurs within the finally block itself
2. System.exit(0) is called in preceding code
3. The thread executing the code terminates
4. The CPU is powered off

The following example demonstrates the addition of a finally block to the previous code:

 1 import java.util.Scanner;
 2 
 3 public class GradeProcessor {
 4     static Scanner input = null;
 5     
 6     static int validate(String value) throws GradeException {
 7         if (!value.matches("^\\-?[0-9]+$")) {
 8             throw new GradeException("Invalid input: integers only!");
 9         }
10         int number = Integer.valueOf(value);
11         if (number > 100 || number < 0) {
12             throw new GradeException("Score must be between 0 and 100!");
13         }
14         
15         return number;
16     }
17     
18     public static void main(String[] args) {
19         System.out.println("Enter an integer score:");
20         String userInput = null;
21         
22         input = new Scanner(System.in);
23         userInput = input.nextLine();
24         try {
25             int score = validate(userInput);
26             if (score >= 90 && score <= 100) {
27                 System.out.println("Grade: A - Excellent!");
28             } 
29             else if (score >= 80 && score < 90) {
30                 System.out.println("Grade: B - Good!");
31             } 
32             else if (score >= 70 && score < 80) {
33                 System.out.println("Grade: C - Average!");
34             }
35             else if (score >= 60 && score < 70) {
36                 System.out.println("Grade: D - Passing!");
37             }
38             else {
39                 System.out.println("Grade: F - Failing!");
40             }
41         }
42         catch (GradeException error) {
43             System.out.println(error.getMessage());
44         }
45         finally {
46             System.out.println("Validation process completed.");
47         }
48     }
49 }