CS1027b Computer Science Fundamentals II
General lab instructions to help labs run smoothly
- Read through the lab instructions before coming to the lab.
- Do the pre-lab preparation.
Overview and preparation
We will work with exceptions and practice debugging
For preparation, review the lecture notes on Java exceptions.
Don't forget to submit your work on owl.
Exercise 1: Experimenting with Exceptions
In this exercise, you will experiment with some examples of code
that generates exceptions and answer some questions on the
examples. Download the Exception Examples 1, 2, 6, 7, 8, 9, 10, 11,
and 12 from the Sample Code section of the course webpage. It may be
simplest if you put them all in one project, and run them separately
as you progress through this Exercise. Note that some of them will
(purposely) not compile.
- Run examples 1 and 2. They both have an "unchecked exception": one
that you do not explicitly need to check for in the code, but will be
generated if the conditions for the exception occur.
- What is the condition that generates the exception in each of these programs?
- Which of the two examples allows the program to continue even if the exception is generated?
- Run examples 6 and 7. They both have the same exception, but
notice that the try-catch statement is in a different method in each
of the examples. This is an example of propagating the exception, as
mentioned in the exception lecture notes. (Notice also that these
examples show how print statements can be included to allow us to see
where we are in the execution of the program, when the variable debug
has the value true.)
- What is the statement that actually generates the exception in example 6? In example 7?
- In example 7, does execution of the method
change continue after the statement that generated the exception?
- What happens of you change the statement containing
debug = true to
debug = false ?
- Now run example 8. Notice that both the
and the method
change have a try-catch statement. In which method(s)
is the exception actually caught in this case?
- Run example 12. This is an example that contains multiple possible
exceptions. Try running it for an illegal input such as the letter A,
and then for a legal integer input such as the value 6. When handling
an exception, how many of each of the following clauses can be present
in a try/catch statement?
- number of try clauses possible?
- number of catch clauses possible?
- Try to run example 9. Note that it does not compile. This is an
example of an exception that is known as a "checked" exception, and
the method header must state that it is throwing an exception. Now
run examples 10 and 11 using illegal input such as abc. What
is the "checked" exception that is generated in examples 9, 10 and
- Add a loop to example 11, that asks the user for another
input for as long as the input is not valid. You will need to use a
while loop and a boolean variable that tells the program whether a
valid input has yet been entered.
Eclipse comes with a debugger which allows you to single-step through your code one statement at a time, trace variables, stop the execution at any time, and find errors in this way. In this exercise you will use the debugger to trace through the program
- Download the file
DebuggingExercise.java and create a
DebuggingExercise.java and try to run it. You
will see that it compiles but does not run.
- Check that you have a Debug button in the top right corner of your
Eclipse window, beside the Java button. If you don't,
select Window - Open Perspective - Debug.
- Click on the Debug button to change to the Debug view. You
can, at any time, switch back to the Java view by clicking on the Java
button in the top right corner.
- In the window displaying your code, you will now add a breakpoint
to the line
for (int j=1; j<=6; j++). You do this by
right-clicking in this line at the very left, and then clicking Toggle
Breakpoint. You can remove a breakpoint at any time in the exact
- When executing your program, the debugger will stop every time
this line is reached. Run your program in the debug mode by clicking
on Run - Debug (alternatively, F11).
The variables will appear in the variable window and the program stops
upon reaching the breakpoint for the first time. Now you can use F5
to execute your program one instruction at a time (called
single-stepping) or F8 to continue with the execution until the next
breakpoint is reached (in a loop the next breakpoint might be the same
one again). Note how the variables
are changing in the Variables window.
While you are doing this, expand some of the rows and columns of the
testArray in the Variables window, to see how the
variables are changing.
- Fix the error when you find it.