First BlueJ Day, Houston, Texas, 1st March 2006 1

Debugging in BlueJ

Davin McCall

First BlueJ Day, Houston, Texas, 1st March 2006 2

Overview

• What is debugging?

• Object inspector

• Method invocation

• “Traditional” BlueJ debugger window

• Demo

• Questions

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Debugging…

• Locate the source of a bug• Run through the part of the program

where we think the bug is occurring, watch program behaviour

• In BlueJ this is normal operation!! (to some extent) - inspectors, method invocation

• BlueJ also has a (simple) more traditional debugger

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Debugging (cont’d)

• Set “breakpoints” which will halt the program execution when reached

• Once execution is halted, can continue execution line by line (“stepping”), either following or stepping over function calls

• Inspect values of variables in currently executing methods (the stack). Inspected objects shown in familiar inspector windows

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Stepping

• Stepping through code …

• Might let us see what is going wrong

• Also serves as a good way to demonstrate control structure behavior, program flow, etc

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BlueJ as a debugger

• Even without the debugger window, BlueJ offers unique features which can be used for debugging and testing

• Construct objects on the fly

• Call arbitrary methods interactively

• Inspect objects and classes to see field values

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Conclusion

• BlueJ provides basic debugging functionality

• Some of BlueJ’s core features (interactive invocation and inspection) are also useful for debugging

• Debugging techniques can serve another purpose: demonstrating code behaviour