Custom Exceptions

class MyException extends Exception {

  MyException(String message) {
    super(message);
  }

}

class InvalidAgeException extends Exception {

  InvalidAgeException(String message) {
    super(message);
  }

}

class Test {

  static void checkAge(int age) throws InvalidAgeException {

    if (age < 18) {
      throw new InvalidAgeException("Age must be 18 or above");
    }

    System.out.println("Valid age");

  }

  public static void main(String[] args) {

    try {
      checkAge(16);
    }
    catch (InvalidAgeException e) {
      System.out.println("Caught: " + e.getMessage());
    }

    System.out.println("Program continues...");

  }

}

// Output:
// Caught: Age must be 18 or above
// Program continues...

• 1 User-defined exception classes.
• 2 Extend Exception or RuntimeException.
• 3 Used for application-specific errors.
• 4 Improve code readability.

• 1 To represent business rules errors.
• 2 To make error handling more meaningful.
• 3 To improve debugging.
• 4 To separate system and business errors.

• 1 Checked custom exceptions (extend Exception).
• 2 Unchecked custom exceptions (extend RuntimeException).
• 3 Business logic exceptions.
• 4 Validation exceptions.

• 1 Create a class extending Exception.
• 2 Throw using throw keyword.
• 3 Handle using try-catch block.
• 4 Propagate using throws keyword.

• 1 Used in banking systems for invalid transactions.
• 2 Used in login systems for authentication failures.
• 3 Used in validation systems for business rules.
• 4 Used in APIs for custom error responses.
• 5 SaaS products use Custom Exceptions in Java in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply Custom Exceptions in Java with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use Custom Exceptions in Java carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the Custom Exceptions in Java rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Extending wrong base class for exception.
• 2 Not providing meaningful error messages.
• 3 Forgetting to handle custom exception.
• 4 Using custom exceptions for simple logic.
• 5 Skipping the small working example before adding framework code.
• 6 Ignoring null, empty, duplicate, and boundary inputs.
• 7 Mixing business logic, input handling, and output formatting in one place.
• 8 Using broad error handling that hides the real failure.
• 9 Forgetting to test the behavior after refactoring.
• 10 Adding clever code that future maintainers will struggle to read.

• 1 Extend Exception for checked or RuntimeException for unchecked.
• 2 Use meaningful and clear exception names.
• 3 Provide descriptive error messages.
• 4 Use custom exceptions only for business logic errors.
• 5 Start with clear requirements and one minimal working example.
• 6 Use meaningful names that explain business intent.
• 7 Keep examples small enough to debug line by line.
• 8 Validate input at every trust boundary.
• 9 Handle errors explicitly and preserve useful context.
• 10 Prefer simple control flow over deeply nested logic.
• 11 Separate domain logic from I/O and framework code.
• 12 Write tests for normal, boundary, and failure cases.
• 13 Review security assumptions before production use.
• 14 Measure performance before optimizing.
• 15 Document non-obvious decisions close to the code or in project notes.
• 16 Use official documentation when behavior is version-specific.
• 17 Keep dependencies current and remove unused code.
• 18 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 19 Log operational events without exposing sensitive data.
• 20 Design examples so learners can safely modify and rerun them.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place Custom Exceptions in Java inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates Custom Exceptions in Java.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice Custom Exceptions in Java with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.