Checked vs unchecked exceptions

Everything thrown extends Throwable, which has two branches:

• Error — serious JVM problems (OutOfMemoryError, StackOverflowError). Applications normally do not catch these.
• Exception — conditions a program may handle.
• Checked exceptions: Exception and its subclasses except RuntimeException. The compiler forces you to catch them or declare them with throws (e.g. IOException, SQLException).
• Unchecked exceptions: RuntimeException and its subclasses (NullPointerException, IllegalArgumentException, ArrayIndexOutOfBoundsException). No compile-time requirement.

class SeatUnavailableException extends Exception {           // checked: callers must plan for it
    SeatUnavailableException(String seat) { super("Seat " + seat + " is taken"); }
}
void book(String seat) throws SeatUnavailableException {
    if (seat == null) throw new IllegalArgumentException("seat is required");   // unchecked: caller bug
    if (taken.contains(seat)) throw new SeatUnavailableException(seat);
    taken.add(seat);
}

Rule of thumb: use a checked exception for an expected, recoverable situation the caller should handle; use an unchecked one for programming errors (bad arguments, broken invariants). Many modern libraries lean towards unchecked exceptions to avoid long throws lists.