Debugging and Android Setup

First, we test the format of your code using checkstyle, a program that checks it against a set of rules about how Java code should be formatted. We do this because writing readable code is as important as writing correct code.

For example, the following code has a checkstyle error—can you spot what it is?

checkstyle produces error messages that look like this (note that this is not for the snippet above):

Note that the error message identifies the line number (3) where the error occurs, along with a brief human-readable description of the problem. Given the error message above, can you come up with a snippet that could have produced this error?

Second, we try to compile your code using the Java compiler. This is step that we’ll discuss more soon in class, but certain kinds of mistakes can be caught at this point. We sometimes refer to errors identified at this stage as compiler errors or syntax errors. Note that if the Java compiler can’t compile your code we can’t run it and continue with testing, so we stop at this point.

For example, the following code will produce a compiler error—can you spot it?

The Java compiler can produce all kinds of error messages depending on what you did wrong. Understanding how to interpret them will greatly improve your ability to fix errors and perform well on the homework, quizzes, and MPs. Here are a few examples of common mistakes. Note that, like the checkstyle errors above, each identifies the line number where the error took place and provide some information about the problem that was encountered. For each, see if you can determine a piece of code that could produce that error.

Finally, we run your code using a test suite. At that point your program will run, but it might not be correct. For example, we may have asked you to add x and y but if your code instead computes x * y, then it doesn’t match our specification. Our test suite will try and determine this and, if your code doesn’t behave correctly, we’ll provide some information about what went wrong.

Usually how we do this is to compare the behavior of your program to what we had expected. For example, let’s say that we instructed you to write a snippet of code that printed North Pole at the appropriate latitude and longitude and not print anything otherwise. You submitted the following code:

What we would do is test your code with different values of latitude. First we’d say, let’s see if it’s right if latitude is equal to 0.0. In that case it does print North Pole, so that’s good. OK, now what happens if we set latitude to 1.0? We didn’t tell you to not print anything otherwise, but you printed Not at North Pole. So you might get an error message something like this:

These error messages might look different depending on the problem you are working on. But in general they describe a mismatch between our expectations of how your code should behave based on our instructions and the reality of how it worked when we ran it.