We will be using a Java Application Project for this example. Since we are just showcasing the capabilities of the debugger too, we won't be using anything complicated; a normal Java Application Project will suffice. If you're unsure on how to create one, please check the recipe Creating a Java Project using Wizard.

We are also going to need a Java class. If the Creating a Java Project using Wizard recipe was used, then a main class is already generated and that one can be used in our example. If a project already exists, then it is possible to follow the Create a class recipe in order to get a clean class for the work to proceed. The automatically generated class will be our main class, so the class name is Main.java.

If the Creating a Java Project using Wizard recipe is followed, the project name used for this recipe would be HowToDebugApp and all of the default settings present on the wizards should be left untouched.

We will need another class to demonstrate how to setup breakpoints in other objects.

The Person.java file will show up on the Java Editor.

Inside the Person class declaration, write:

int age;
String name;

Now lets refactor our class by encapsulating the fields:

Inside the main method, enter the following:

Person person1 = new Person();
Person person2 = new Person();
person1.setName("Rhawi");
person1.setAge(28);
person2.setName(null);
person2.setAge(32);

It is not good practice to set variables with null values but, for the purpose of this example, we will do it this one time.

A watch is exactly what the name says, it "watches" a specific variable for the entire lifetime of the application and displays its value in the variables view.

To add a watch, simply:

For our first breakpoint, click on the left-side bar, where line numbers are placed, specifically on the line shown below:

person2.setName(null);

A breakpoint will be added to the side bar and a long pinkish line will specify the breakpoint location.

Then add another breakpoint, but this time to Person.java, where the line is:

this.age = age;

Finally, let's debug our example. Open Main.java and press Control+Shift+F5.

Upon the debug mode execution, NetBeans stops the execution at the first breakpoint.

The breakpoint tells the IDE to temporarily halt the program and let the developer examine the contents of variables and watches at that point.

In our example, NetBeans will look like this:

It is possible to see our watch, shown with a blue diamond icon, and also the normal variables, shown with a green lozenge. It is possible to expand the watch or variables to check the contents of the object on runtime.

The execution point is highlighted in green. To jump one line of code in the execution, press F8. To continue press F5; the IDE will resume the debugging process until another breakpoint is found, at which point the execution will once again stop, so that the developer can have another chance to examine the execution. In our example, the execution will continue until the Person's set method for age is reached. To continue with the execution, just press F5 one more time and the program execution will continue until the end, since we do not have any other breakpoints.

To remove a breakpoint, simply click on it and it will disappear.

What if I want a breakpoint when my variable reaches a certain value? Watches are too complicated, is there something easier? Does NetBeans debug other types of applications?