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Engineering LibreTexts

9.8: Debugging Code

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  • It is a good idea to read this book in front of a computer so you can try out the examples as you go. You can run many of the examples directly in DrJava’s Interactions Pane (see Appendix A). But if you put the code in a source file, it will be easier to try out variations.

    Whenever you are experimenting with a new feature, you should also try to make mistakes. For example, in the hello world program, what happens if you leave out one of the quotation marks? What if you leave out both? What if you spell println wrong? These kinds of experiments help you remember what you read. They also help with debugging, because you learn what the error messages mean. It is better to make mistakes now and on purpose than later on and accidentally.

    Debugging is like an experimental science: once you have an idea about what is going wrong, you modify your program and try again. If your hypothesis was correct, then you can predict the result of the modification, and you take a step closer to a working program. If your hypothesis was wrong, you have to come up with a new one.

    Programming and debugging should go hand in hand. Don’t just write a bunch of code and then perform trial and error debugging until it all works. Instead, start with a program that does something and make small modifications, debugging them as you go, until the program does what you want. That way you will always have a working program, and it will be easier to isolate errors.

    A great example of this principle is the Linux operating system, which contains millions of lines of code. It started out as a simple program Linus Torvalds used to explore the Intel 80386 chip. According to Larry Greenfield in The Linux Users’ Guide, “One of Linus’s earlier projects was a program that would switch between printing AAAA and BBBB. This later evolved to Linux.”

    Finally, programming sometimes brings out strong emotions. If you are struggling with a difficult bug, you might feel angry, despondent, or embarrassed. Remember that you are not alone, and most if not all programmers have had similar experiences. Don’t hesitate to reach out to a friend and ask questions!