Java SE 8 Programmer II: Exceptions and Assertions

Exceptions and Assertions tests checked versus unchecked exceptions, resource management, custom exceptions, and the correct use of assertions.

Core rules

• Throwable splits into Error (unrecoverable) and Exception. RuntimeException and its subclasses are unchecked; all other Exception subclasses are checked.
• A checked exception must be caught or declared. Unchecked exceptions need not be.
• try requires catch and/or finally; multi-catch uses | and the parameter is implicitly final. Catch blocks must be ordered from most specific to most general.
• Try-with-resources closes resources in reverse order of declaration; close failures are added as suppressed exceptions.
• An overriding method may declare fewer or narrower checked exceptions, never broader ones.
• Assertions use assert condition; or assert condition : detail;. They are disabled by default (-ea enables them), throw AssertionError, and must not perform business logic or validation.

Worked example

try (Reader r = new FileReader("a.txt"); Reader s = new FileReader("b.txt")) { ... } s closes first, then r. If the body throws and s.close() also throws, the body exception propagates and the close exception is attached via getSuppressed().

Traps

• finally runs even after return; a return in finally discards the original return value or exception.
• Catching Exception before IOException makes the IOException catch unreachable.
• Assertions are not for argument validation; they may be disabled in production.
• A custom checked exception must extend Exception or a checked subclass; extending RuntimeException makes it unchecked.
• Try-with-resources requires AutoCloseable; a class with only close() that does not implement it will not compile in that form.