Exceptions

1An exception is an object that names a failure

An exception is an object thrown when a method cannot keep its promise. SoldOutException is not a printed seat — it is the fact that there is no seat.

The call stack unwinds until a catch for that type runs. If none does, the program stops and prints a trace.

Figure. take cannot give a seat, so it throws. book must catch or declare.

How it works

1. An exception is an object that names a failureAn exception is a failure object. It is not a magic return value of -1.

2throw starts it; throws warns the caller

throw new SoldOutException("hall full"); creates the object and starts the unwind.

throws SoldOutException on the method tells the compiler — and the next author — that this method may pass that failure out instead of handling it.

Figure. throws SoldOutException sits on take and warns the caller. throw new SoldOutException("hall full") is the statement in the body that starts the unwind.

How it works

1. throw starts it; throws warns the callerthrow is the statement. throws is the method's warning.

throw in the body; throws on the method

void take(String seat) throws SoldOutException {
    if (full) {
        throw new SoldOutException("hall full");
    }
}

3try marks the risk; catch handles a type

try { ... } catch (SoldOutException e) { ... } runs the try block and, if that type is thrown, runs the matching catch instead of crashing.

A catch is chosen by type. A parent type catch can swallow children. Order more specific types first.

Figure. try marks hall.take. If SoldOutException is thrown, the matching catch runs instead of a crash. A parent-type catch can swallow children, so more specific types come first.

Handle sold-out; let other failures travel

try {
    hall.take("A12");
} catch (SoldOutException e) {
    System.out.println(e.getMessage());
}

Coding lab. Catch a sold-out hall runs in the app, with checks on your output.

4finally runs on the way out

finally { ... } runs after try, whether the try finished, threw, or returned. It is the place to close a file or a connection the try opened.

It is not a second catch. It does not handle the exception. It always gets a turn before the method leaves.

Figure. desk.unlock() in finally runs after try, whether take finished or SoldOutException was caught. finally does not handle the exception; it always gets a turn before the method leaves.

How it works

1. finally runs on the way outfinally always runs. It is cleanup, not handling.

unlock runs after success or sold-out

try {
    hall.take("A12");
} catch (SoldOutException e) {
    System.out.println("full");
} finally {
    desk.unlock();
}

5Checked is forced; unchecked is not

A checked exception extends Exception but not RuntimeException. The compiler requires you to catch it or declare throws. IOException is the usual example.

An unchecked exception is a RuntimeException (or Error). NullPointerException and IllegalArgumentException are unchecked. You may catch them; the compiler will not force you.

No diagram — the idea is carried by the prose or the Java listing.