Using a Debugger

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Using a Debugger
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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 its exactly the tool you need
• Sometimes it isnt
• 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, BlueJs debugger is crash-prone

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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

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Setting a breakpoint
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Reaching a breakpoint
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The Debugger window

• When you reach a breakpoint, the debugger window
  will open
• Or, you can open it yourself from the menus

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Parts of the Debugger window
You can usually ignore Threads
This is how you got here
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Viewing objects
This isnt very helpful
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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
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Stepping into a method
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Cautions

• The BlueJ debugger is very flakyuse 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

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The End