1. Fundamentals of Software Development 1Slide 1 Correcting Run-Time Errors Small changes and the “What did you just change?” ruleSmall changes and the “What did you just change?” rule Look at the stackLook at the stack Use the debuggerUse the debugger Compare against other codeCompare against other code Look up Java documentationLook up Java documentation These are all recommended practices – not necessarily in order!

2. Fundamentals of Software Development 1Slide 2 Small changes and the “What did you just change?” rule Only make a few changes, then compile and test.Only make a few changes, then compile and test. –This hugely simplifies debugging. –Saves you lots of time in the end. –Gives you tremendous confidence in your code. The first corollary to this rule is this: – –The error is always in the last thing you did. – –Which, if you follow the rule, is easy to spot!

3. Fundamentals of Software Development 1Slide 3 Look at the stack Example Java run-time exception dump:Example Java run-time exception dump: --------------------Configuration: -------------------- This tests the method written, above. Your input is: asdf You entered : asdf Exception in thread "main" java.lang.NumberFormatException: For input string: "asdf" at java.lang.NumberFormatException.forInputString(NumberFormat Exception.java:48) at java.lang.NumberFormatException.forInputString(NumberFormat Exception.java:48) at java.lang.Integer.parseInt(Integer.java:447) at java.lang.Integer.parseInt(Integer.java:447) at java.lang.Integer.parseInt(Integer.java:497) at java.lang.Integer.parseInt(Integer.java:497) at LittleTest.main(LittleTest.java:49) at LittleTest.main(LittleTest.java:49) Process completed.  The last visible thing the program did – also a clue!  Here’s the Java run-time exception dump!  The top line number shown – always a strong clue! And – What kind of error was it? – In this case, a “NumberFormatException” – maybe “asdf” isn’t numeric?

4. Fundamentals of Software Development 1Slide 4 Use the debugger We’ll do a demo momentarily.We’ll do a demo momentarily. Key activities:Key activities: 1.Make sure “Include debug info” is set. (can be set at the time of creating a project or later in Configure… Options…JDK Tools… Default… Parameters in Configure… Options…JDK Tools… Default… Parameters also in Project… Project Properties… JDK Tools) also in Project… Project Properties… JDK Tools) 2.Likewise – be sure “View… Other Windows… Debug View” is on. 3.Set breakpoints of interest, to stop the program. 4.Start the debugger. 5.Watch the yellow carat – It’s where you are! 6.Dump selected values by highlighting fields, then “Dump”. 7.Step or Continue the program.

5. Fundamentals of Software Development 1Slide 5 Compare against other code My new Transformer class def starts with:My new Transformer class def starts with: –public class CountDowner implements StringTransformable But all the other Transformer classes start like:But all the other Transformer classes start like: –public class Capitalizer extends StringTransformer implements StringTransformable Maybe that’s why I’m getting this – ?Maybe that’s why I’m getting this – ? Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: CountDowner at StringTransformers.add(StringTransformers.java:46) at StringTransformers.add(StringTransformers.java:46)

6. Fundamentals of Software Development 1Slide 6 Look up Java documentation What’s that “Integer.parseInt(…” really do? (back in the Java dump on slide 3)What’s that “Integer.parseInt(…” really do? (back in the Java dump on slide 3) Where would you look in the Java Docs? Checking the documentation is not the last resort, but a good recommendation to end on!Where would you look in the Java Docs? Checking the documentation is not the last resort, but a good recommendation to end on! Class Integer,method parseInt( ) (Not that a preposition is ever good to end on…)