1. Using a Debugger
1-Jan-19

2. About Debuggers Bugs happen
Sometimes you can figure the problem out right away
Sometimes you have to track down the problem
A Debugger can be a big help
Sometimes it’s exactly the tool you need
Sometimes it isn’t
Debuggers are all basically pretty much alike
“If you know one, you know them all”
BlueJ comes with a nice, simple debugger that gives you all the most important functionality
Unfortunately, BlueJ’s debugger is crash-prone

3. What a debugger does
A debugger lets you step through your code line by line, statement by statement
At each step you can examine the values of all your variables
You can set breakpoints, and tell the debugger to just “continue” (run full speed ahead) until it reaches the next breakpoint
At the next breakpoint, you can resume stepping

4. Setting a breakpoint
Click in the left margin to set or remove breakpoints

5. Reaching a breakpoint
Run your program normally; it will automatically stop at each breakpoint

6. The Debugger window
When you reach a breakpoint, the debugger window will open
Or, you can open it yourself from the menus

7. Parts of the Debugger window
You can usually ignore Threads
Your variables and their values, as it says
This is how you got here

8. Viewing objects
Double-click it to get this
This isn’t very helpful

9. Debugger commands
Step ahead one statement; but if it calls a method, step into the method
Quit debugging and just run, until you hit another breakpoint
Step ahead one statement
Quit

10. Stepping into a method Here we are in the new method
This is how we got here

11. Cautions The BlueJ debugger is very flaky—use with care
Debuggers can be very helpful, but they are no substitute for thinking about the problem
Often, print statements are still the better solution

12. The End