Tutorial 7 - Debugging

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This tutorial describes techniques about how to debug your program and figure out what is wrong with it. First we will start off with some terminology about debugging, and then I will show some of the debugging features Netbeans has to offer.

Part 1: Terminology

• Breakpoint - a line of code where you want to "pause" the execution of a program. For example, if you want to pause the program at line 14, place a breakpoint there.
• Continue - will continue the execution of the program until the next breakpoint or until the program terminates

Part 2: Features

• Call stack - allows you to see the current stack of method calls. For example, if you put a breakpoint on line 15 in method foo(), you will see the method that called foo(), and then the method that called that method, and then further down the line...
• Watches window - allows you to "watch" the value of a variable. For example, if you have a variable named "x", you can type x inside the watch window and watch the value as you go from breakpoint to breakpoint.
• Mouse Over Variable - allows you to mouse over any variable within the scope of a breakpoint to see the value of that variable

Part 3: Brief Introduction

For a brief introduction, we will put a breakpoint inside a method, run Netbeans in debug mode and take a look at the variables at a breakpoint. First, use the Tutorial project from the previous tutorials and place a breakpoint inside the printName method of the Person class. You can place a breakpoint by clicking on the left of that line. You will notice a pink square will appear where you click and the whole line will be highlighted pink. Clicking on a breakpoint you have already set will deactivate it.

To start debugging, press the "Debug Main Project" button. It is the button that is highlighted red in the picture below.

After running it in debug mode, you will notice the program will pause at that line. You will know it is paused because the line will now be colored green. Additionally, if you look at the call stack at the bottom of the screen, you will notice the list of methods that lead to this breakpoint. The main method in the Driver class called printName in the Person class.

Now that we are at a breakpoint, we can watch any variable available to us at this moment. Unfortunately, this is a boring tutorial program and the only variable available to us is the "name" variable. Click on the "Watches" tab at the bottom. Now click on Run > New Watch at the top menu. Type in "name" to watch the name variable and press the OK button.

After pressing enter you will notice that the name variable is in the Watches window. You will also notice that the value for this variable at this current moment is "Mike".

You can also achieve the same effect by just mousing over the "name" variable when that breakpoint is reached. See the picture below.

When you are done, you can press the continue button at the top. The program will finish executing as normal.

This feature is meant to help you figure out what the values of variables are without having to put a lot of print statements all over your program. Of course there are many more advanced debugging features such as step-ins, step-outs, bookmarking, etc... You can learn more about debugging by searching it on the internet or reading the help menu in Netbeans.