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6.6: Tracing With a Debugger

  • Page ID
    15310
  • A great way to visualize the flow of execution, including how parameters and arguments work, is to use a debugger. Most debuggers make it possible to:

    1. Set a breakpoint, a line where you want the program to pause.
    2. Step through the code one line at a time and watch what it does.
    3. Check the values of variables and see when and how they change.

    For example, open any program in DrJava and move the cursor to the first line of main. Press Ctrl+B to toggle a breakpoint on the current line; it should now be highlighted in red. Press Ctrl+Shift+D to turn on Debug Mode; a new pane should appear at the bottom of the window. These commands are also available from the Debugger menu, in case you forget the shortcut keys.

    When you run the program, execution pauses at the first breakpoint. The debug pane displays the call stack, with the current method on top of the stack, as shown in Figure 15.6.1. You might be surprised to see how many methods were called before the main method!

    Screenshot of the DrJava debugger. Execution is currently paused on the first line of printTwice. There is a breakpoint on the first line of main.
    Figure \(\PageIndex{1}\): Screenshot of the DrJava debugger. Execution is currently paused on the first line of printTwice. There is a breakpoint on the first line of main.

    To the right are several buttons that allow you to step through the code at your own pace. You can also press Automatic Trace to watch DrJava run your code one line at a time.

    Using a debugger is like having the computer proofread your code out loud. When the program is paused, you can examine (or even change) the value of any variable using the Interactions Pane.

    Tracing allows you to follow the flow of execution and see how data pass from one method to another. You might expect the code do one thing, but then the debugger shows it doing something else. At that moment, you gain insight about what may be wrong with the code.

    You can edit your code while debugging it, but we don’t recommend it. If you add or delete multiple lines of code while the program is paused, the results can be confusing.

    See http://drjava.org/docs/user/ch09.html for more information about using the debugger feature of DrJava.

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