Adding Breakpoints

To pause the execution of an application, place a breakpoint marker in the line of code you want execution to stop. Listing 6-1 shows the definition of the Debug class in the Debug project.

Listing 6-1  Debug.java file of Debug project

import java.util.*;

public class Debug {

    public static void main (String args[]) {

        System.out.println("<Debug.main> Hello, World!");

        int a_number = 1;

        int another_number = 10;// 1

        method(a_number, another_number);

        System.out.println("<Debug.main> a_number = " + a_number);

        System.out.println("<Debug.main> another_number = " + another_number);

}

    public static void method(int number1, int number2) {

        Person person = new Person("Kathy", "Yates", "female", "brown");

        String a_string = person.firstName() + " likes debugging code.";

        System.out.println("<Debug.method> person = " + person);

    }

}

To add a breakpoint to the line numbered 1, click the line’s left margin in the editor. You can also set the insertion point in the line and choose Debug > Add Breakpoint at Current Line. Figure 6-1 shows the result.

To remove a breakpoint, click the breakpoint marker, drag the marker out of the margin, or choose Debug > Remove Breakpoint at Current Line.

To disable a breakpoint, Command-click the breakpoint marker or choose Debug > Disable Breakpoint at Current Line.

Stepping Through Lines of Code

To build and debug the Debug project, choose Build > Build and then choose Debug > Debug Executable, or click the Build and Debug toolbar button. Figure 6-2 shows the result, in which the highlighted line is about to be executed.

To step to the next line of code choose Debug > Step Over or click the Step Over toolbar button, as shown in Figure 6-3. Because the line executed is not a method call, clicking the Step Into toolbar button would give the same result.

To step into a method choose Debug > Step Into or click the Step Into toolbar button, as shown in Figure 6-4.

To step out of a method, (that is, to execute the rest of the lines in the current method and return to calling method), choose Debug > Step Out or click the Step Out toolbar button.

Viewing the Debug Information

The pop-up menu to the right of the Files tab (with main chosen) lists threads of execution. The list below it shows the call stack for the chosen thread. The pane to the right of the call stack pane, the variable pane, shows the names of the parameters and variables declared for the currently executing method in the chosen thread. It may also show the arguments used in the method invocation and the values of the local variables. Figure 6-5 shows the call stack of the main thread and parameters and local variables of a method.

Accessing the Contents of Objects

While you debug code, you may need to see the values of an object’s instance variables. Most programmers sprinkle System.out.println invocations throughout their code to accomplish this essential task. In Project Builder you can execute an object’s toString method to get the same effect.

Listing 6-2 shows a partial listing of the Person class. It contains an implementation of the toString method.

Listing 6-2  Person.java file

public class Person {

    private String firstName;

    private String lastName;

    private String gender;

    private String hairColor;

    public Person(String firstName, String lastName, String gender, String hairColor) {

        setFirstName(firstName);

        setLastName(lastName);

        setGender(gender);

        setHairColor(hairColor);

    }

    ...

    public String toString() {

        return "{FirstName: " + firstName() + "},{LastName: " + lastName() + "},{Gender: " + gender() + "},{HairColor: " + hairColor() + "}";

    }

}

Figure 6-6 depicts a debugging session in which the user chooses the Print Description to Console command through the contextual menu of person in the Variable list of the Debug pane.

Listing 6-3 shows the output generated.

Listing 6-3  Console output after executing Print Description to Console command on a Person object

Picked up _JAVA_OPTIONS: -Xdebug -Xnoagent -Djava.compiler=NONE -Xrunjdwp:transport=dt_local,server=y,address=8000

<Debug.main> Hello, World!

Printing description of person:

"{FirstName: Kathy},{LastName: Yates},{Gender: female},{HairColor: brown}"

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