The abstract supertype of values indicating exceptional conditions. An exception may be raised using the throw statement, and handled using the catch clause of the try statement. An instance of Throwable may be passed from throw to catch.

void tryToDoIt() {
    if (canDoIt()) {
        doIt();
    }
    else {
        throw CantDoIt(); //the Throwable
    }
}

try {
    tryToDoIt();
}
catch (CantDoIt e) {
    e.printStackTrace();
}

An instance of Throwable represents a problem, typically an unexpected failure. Either:

• a unrecoverable error in the program, especially an AssertionError, or
• a transient, and possibly-recoverable Exception.

The use of the exceptions facility to manage expected failures, that is, failures that are usually handled by the immediate caller of an operation, is discouraged. Instead, the failure should be represented as a return value of the operation being called.

For example, nonexistence of a file should not result in an exception. Instead, an openFile() operation should return the type File?, where a null return value indicates nonexistence. On the other hand, failure to read from an already open file could result in an Exception.