Debugging

When completing the Exercise for Java Fundamentals, you probably ran into errors and then fixed them. This means that you did some debugging! Debugging is the process of finding those problems, often called bugs, in our code then fixing them. This section will cover effective strategies to debugging your robot code.

Did you know?

Using the word “bug” to refer to an error in a program was popularized by Grace Hopper, an American computer scientist. While her and her team was working on the Mark II computer, they were able to trace an error to moth that was stuck in the computer. They put the moth in their notebook with the note “First actual case of bug being found”.

Interpreting a Stacktrace

When an error happens, VS Code will tell you in the code itself or in the terminal. When the code runs and there is an error, a message will appear in the terminal. This message is called a stacktrace. A stacktrace traces or identifies the spot in the program where the error happened. This can have a lot of useful information about the type of error and how you can fix in.

Types of Errors

There’s a few different types of errors that you can run into when writing code. Let’s take a look!

Name	Timing	Symptoms
Syntax Error	Compile time	The compiler will display an error message, and your code will not finish building
Runtime Error	Run time	The program will display an error message
Logic Error	Run time	The code will run but not function as intended

Syntax Error

Before the program runs, there’s a program called a compiler that takes all the code you wrote and turns it into instructions a machine will understand. When there is an error with the syntax, the compiler will show an error. VS Code shows these errors by putting a red line under the part of code that it thinks is wrong, just like a typo in Google Docs.

This error is called a syntax error because these errors occur when you mess up the syntax of a language. We say this error happens at compile time because it happens when the compiler is translating the code, before it ever actually runs.

Note

When programming, a yellow line might appear under a part of code. Most of the time, this is not an error. It’s a warning and it typically does not affect the output of the code. The warning is just there to say that the code is not being used.

For instance, the following code would generate a syntax error:

Misspelling variable names is another form of syntax errors. The following code would also generate a syntax error

If you notice that the code has the red line under it, put the cursor over that line of code. VS Code will give more information about the error. Let’s look back at the previous misspelled variable name example, when putting the cursor over the error, it reads numer cannot be resolved to a variable. This means it’s looking for a variable called numer but it can’t find it.

Sometimes, errors are too subtle for the compiler to notice immediately, and it blames the wrong part of the code with a confusing error message. When this happens, look closely at the code, it’s syntax, and try to find what went wrong.

Runtime Error

The code might look like it has no bugs, but when the code runs, an error appears. When this happens, it’s a runtime error. Runtime errors are errors that can only be found when the program is running. One common types of runtime error is dividing by zero.

Let’s look at an example.

    int answer = 10 / 0;
    System.out.println(answer);

The example looks like it should run. However, if the code was to run, it would give a runtime error for Diving by zero. The compiler would give an error similar to Exception in thread "main" java.lang.ArithmeticException: / by zero at main.main(main.java:5) By reading the error message, it’s clear that the error is a divide by 0 error. This happens in int answer = 10/0; because it’s diving by zero. One way to fix this error would be to swap the 10 and 0.

Note

The stacktrace will tell you what line the error is on. For example, Exception in thread "main" java.lang.ArithmeticException: / by zero at main.main(main.java:5) has (main.java:5). That means the error is on line 5 of main.java.

Logic Error

There will be times when the code runs, has no errors but it doesn’t give the desired output. Typically, this does not mean that your syntax was wrong. Most of the time, the instructions or the logic in your code isn’t correct. This means that the code could have wrong conditions, formulas, or operators.

Computers don’t know what we are expecting them to do. They only know what we told them. If the instructions given are wrong, then the output will be wrong too.

Let’s look at simple example

int width = 2;
int height = 5;
int area = width + height;
System.out.println("Area of the rectangle is " + area);

This example finds the area of a rectangle. The width is 2 and the height is 5. If this code was to be ran, it would print out Area of the rectangle is 7, but that isn’t right. The area should be 10. The code runs with no errors, so why it the code doing the math wrong? When trying to find the area of a rectangle, the equation for the area is width times height. In the code, it isn’t multiplying the width and height, it adds the two. Therefore, changing the addition sign to a multiplication sign will fix the issue and it should print out Area of the rectangle is 10. This is a simple example of a logic issue. The code ran with no errors but, but had bad instructions.

Software bugs are often not as easy to spot as this example. Be sure to take time to read through the code and verify that the intended behavior matches what the code is saying. Adding System.out.println(); can also help break down what the code is doing at different spots in the program.

Helpful Tips

Errors will always appear in code, but there are things to do to help avoid errors appear as often.

• Check for typos in your variable names or in the code.
• Look out for missing semi-colons and make sure the syntax is correct.
• Use variable names that match their meaning.
• Create the variable first, don’t use variables that have not been made yet.

When there is an error, read through the error message carefully. If you’re stuck, use online resources! Looking up an error can help point you to the right direction on how to fix it.

• For some, changing VS Code to be in a more accessible font (i.e Comic Sans) can be helpful in reading code to find or avoid errors.