Java: Exceptions - More
Kinds of Exceptions
There are many exceptions, but they can be put
into two groups: checked exceptions and unchecked exceptions.
There is some controversy about which type you should use.
A discussion of some of the issues can be found at
Java theory and practice: The exceptions debate.
- Unchecked Exceptions -- These exceptions are usually something
that should have been prevented by more careful programming.
For example, you should never get
NullPointerException
or ArrayIndexOutOfBoundsException.
If you do, there is something wrong with your program, and you
need to fix it. You usually don't catch unchecked exceptions.
Instead, fix your program so it can't produce one of these.
However, NumberFormatException is the one exception of this type
that is usually caught.
- Checked Exceptions -- These are usually errors in the input data.
The programmer has no control over the input the user gives you,
eg, file names, .... If the user gives you a bad
value, it may cause an exception when you use it.
You need to check for bad input using a
try statement.
Exception hierarchy
[Not yet written]
Multiple catch clauses
[Not yet written]
Order of catch clauses
[Not yet written]
Printing the stack trace
[Not yet written]
Throwing exceptions
[Not yet written]
Danger from the intermediate layers - finally
Exceptions provide a good infrastructure for error processing.
The simplicity of throwing an exception at a deep level and catching
it at a high level may generate problems at the intermediate, skipped,
levels. Did any of these methods leave any part of a data structure
or resource in an inconsistent state?
Each place that this may be true of needs to enclose critical code
in a try...finally block.
Define libarary exceptions to tell programmer they should fix their code
If you write a library that is used by others, throw your own execeptions
if your code is called with illegal values, eg, something isn't initialized.
Your exceptions are a way to tell the applications programmer that they
should fix their code.
Defining your own exceptions
You can also create your own exceptions. [Not yet written]