1. Java Exception Handling
2. Basic try-catch-finally Exception Handling in Java
3. Java Try With Resources
4. Catching Multiple Exceptions in Java 7
5. Exception Hierarchies
6. Checked or Unchecked Exceptions?
7. Exception Wrapping
8. Fail Safe Exception Handling
9. Pluggable Exception Handlers
10. Logging Exceptions: Where to Log Exceptions?
11. Validation - Throw Exceptions Early
12. Validation - Throw Exception or Return False?
13. Exception Handling Templates in Java
14. Exception Enrichment in Java
15. Execution Context

When receiving input that needs to be validated before it can be used, validate all input before using any of it. You should not change any state in the application or attached systems until all input data has been validated. That way you avoid leaving the application in a half valid state.

For instance, in a DAO method that inserts a user and an address into two different tables in a database, do like this:

  check if user already exists
  validate user
  validate address

  insert user
  insert address

Do not do like this:

  check if user already exists
  validate user
  insert user

  validate address
  insert address

If you do like this, and the address turns out to be invalid you will still have inserted the user in the system. What happens when the user corrects the address and retry registering? He will be told that his user already exists, right?

The problem is not only related to database insertions. In general, validate all input before using any of it, to avoid leaving the application in a half valid (=unknown) state.