Exception Handling: A Deeper Look - Java: How to Program Early Objects - page 458

Software Engineering Observation 11.9
An exception can be thrown without containing information about the problem that
occurred. In this case, simply knowing that an exception of a particular type occurred may
provide sufficient information for the handler to process the problem correctly.
Software Engineering Observation 11.10
Throw exceptions from constructors to indicate that the constructor parameters are not
valid—this prevents an object from being created in an invalid state.
Rethrowing Exceptions
Line 32 of Fig. 11.5 rethrows the exception . Exceptions are rethrown when a catch block,
upon receiving an exception, decides either that it cannot process that exception or that it
can only partially process it. Rethrowing an exception defers the exception handling (or
perhaps a portion of it) to another catch block associated with an outer try statement. An
exception is rethrown by using the throw keyword , followed by a reference to the excep-
tion object that was just caught. Exceptions cannot be rethrown from a finally block, as
the exception parameter (a local variable) from the catch block no longer exists.
When a rethrow occurs, the next enclosing try block detects the rethrown exception,
and that try block's catch blocks attempt to handle it. In this case, the next enclosing try
block is found at lines 8-11 in method main . Before the rethrown exception is handled,
however, the finally block (lines 37-40) executes. Then method main detects the
rethrown exception in the try block and handles it in the catch block (lines 12-15).
Next, main calls method doesNotThrowException (line 17). No exception is thrown
in doesNotThrowException 's try block (lines 49-52), so the program skips the catch
block (lines 53-56), but the finally block (lines 57-61) nevertheless executes. Control
proceeds to the statement after the finally block (line 63). Then control returns to main
and the program terminates.
Common Programming Error 11.4
If an exception has not been caught when control enters a finally block and the finally
block throws an exception that's not caught in the finally block, the first exception will
be lost and the exception from the finally block will be returned to the calling method.
Error-Prevention Tip 11.6
Avoid placing in a finally block code that can throw an exception. If such code is re-
quired, enclose the code in a try … catch within the finally block.
Common Programming Error 11.5
Assuming that an exception thrown from a catch block will be processed by that catch block
or any other catch block associated with the same try statement can lead to logic errors.
Good Programming Practice 11.1
Exception handling removes error-processing code from the main line of a program's code to
improve program clarity. Do not place try … catch … finally around every statement that
may throw an exception. This decreases readability. Rather, place one try block around a
significant portion of your code, follow the try with catch blocks that handle each possible
exception and follow the catch blocks with a single finally block (if one is required).