Module 9: Exceptions, Validation, and Debugging
Mini-Project: Safe Input Validator
Build a small validator that parses input, rejects bad values, and reports problems clearly.
Author
Java Learner Editorial Team
Reviewer
Technical review by Java Learner
Last reviewed
2026-04-17
Java version
Java 25 LTS
Learning goals
- Handle user input failures cleanly
- Use exceptions to enforce domain rules
- Practice debugging with deliberate bad input cases
Project goal: Read values such as age, email, or score input, validate them, and print useful error messages when they fail.
What to include: Parsing, range checks, one custom exception, and a helpful catch path that preserves the failure reason.
Stretch version: Store validation errors in a list so the user can fix several problems at once instead of one at a time.
Review standard: A good result should fail clearly, not silently, and should be easy to debug from the console output alone.
Runnable examples
Validation should fail with a clear reason
if (score < 0 || score > 100) {
throw new IllegalArgumentException("Score must be between 0 and 100.");
}Expected output
Invalid input is rejected immediately with a useful message.
Mini exercise
Validate one field first, such as age or score, before expanding to several inputs.
Summary
- Validation belongs close to the data boundary.
- Failures should be explicit and readable.
- This project turns exception handling into practical user-facing behavior.
Next step
The next intermediate module moves from failure handling to strings, files, regex, and practical core Java APIs.
Sources used