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Debugging Java programs with BlueJ

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• Debugging: finding errors in a runnable computer program
  • Facts:

    The Java compiler will help you find all the syntax errors in your Java program However, the Java compiler cannot help you find runtime errors

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  • What are run time errors:

    Run time errors are program erros that shows up when you run a program

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  • Types of run time errors:

    Fatal run time errors: A fatal run time error will can a running program to terminate immediately Example 1: using a NULL value in a reference variable public class Err1 { public static void main(String[] args) { double[] a = null; // NULL System.out.println( a[0] ); // Can't use a NULL-valued variable } } Example 2: array index out of range public class Err2 { public static void main(String[] args) { double[] a = {2.3, 3.4, 4.5}; // 3 elements System.out.println( a[4] ); // array index out of range } } Logical errors The program runs to completion, but does not produce the correct answer The cause of this kind of errors is very difficult to find...

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• Debuggers
  • Terminology:

    Debugger = an application that help programmers find run time errors in their programs.

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  • How can a debugger help you find run time errors:

    The debugger can locate (identify) the statement that causes a fatal error The debugger can let you make the program stop at some program location(s) The stop locations are called break points This feature allow you to reach the part of the program quickly that caused logical errors The debugger can let you execute one assignment statement at a time. Executing one statement is called single-stepping This feature allow you to identify the statement that cause logical errors The debugger can let you print the value of (accessible) variables This feature allow you to examine the outcome of suspected statements that cause logical errors

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• How to find fatal errors using BlueJ
  • Feature of a debugger:

    When a Java program is run using a , the execution will stop immediately when a fatal error is found Furthermore, the debugger will highlight the statement that caused the fatal error

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  • Example:

    Java program: public class Err1 { public static void main(String[] args) { double[] a = null; // NULL System.out.println( a[0] ); // Can't use a NULL-valued variable } } When run in BlueJ:

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• Preparing to use BlueJ to finding run time errors
  • We will now learn 3 techniques that are useful in finding run time errors:

    Step a break point in a Java program Remove a break point from a Java program Display the value of a variable Single step through the program.

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  • We will use the Euclid algorithm to illustrate the debugging techniques.

    Save the Euclid algorithm in some directory and open it using Open Non-BlueJ Project in BlueJ:

    The Euclid Java program: RIGHT click and Save

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• Setting break point
  • How to set a break point:

    RIGHT click on the class and select Open Editor to edit the class where you want to set a break point: The BlueJ editor will show the source code: Note: When the right column of the editor window is shaded, you need to compile the class before you can set break points !!! If necessary, compile the class by clicking on the Compile button: You should see the right column of the editor window become unshaded: Now you can set a break point by clicking in the left column at the position of the statement where you want to put a break point. Example: Before clicking (no break point) After clicking at the left column: The little STOP sign represents a break point in the program.

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• Removing a break point
  • Delete a break point:

    When you click on the STOP sign (that represents a break point), the break point is deleted.

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• Effect of break points
  • Run the main method in Euclid by right clicking on Euclid and select main():

    A window will pop up:

    Press OK to run.

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  • The program will stop at the break point:

  • In addition, the BlueJ debugger window will pop up:

    You can see the (accessible) variables A = 28 and B = 36 shown in the BlueJ debugger window.

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• Course of action when stopped at a break point
  • You can do 4 things (see the buttons in the figure above) when the program is stopped at a break point

    Terminate The Terminate command will terminate the program execution immediately Continue The continue command will continue the program execution. 2 things can happen after the program continues execution: The program stops when the program execution reaches a break point (It could be the same break point when the program is executing inside a loop) The program terminates (execution reaches the end of main) Step The Step command will cause the running program to execute one statement Example: click Step and the program execution will advance to this location: Click Step again and the program execution will advance to this location: And the value of the variable B has changed in the debugger window: due to the execution of the statement B = B − A;. Note: If the statement executed by the Step command contains one or more method invocations (calls), the Step command will complete all method invocations before stopping the program execution. Step Into The Step Into command will also cause the running program to execute one statement However, if the statement contains a method invocation, then the program execution will enter the called method and stops for further instruction.

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• Debugging a program
  • Fact:

    Debugging cannot be taught It's some what like art: I can teach you how to work with debugging tools But I cannot teach you how to debug your program --- because every error is different

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  • I can give you some advice on how to find errors in a program:

    How do I know if a program is correct In general, there is no way to know for sure if a program is completely correct There are simple ways to find out if a program is incorrect: Let the program compute the solution for some simple cases where you have the correct answer (We call that testing your program) If the program gives wrong answers, you must find out where the logic is wrong How to find the location of the program where you made a logic error in your programming: There is no simple answer to this question The hard way that will always work is: Trace (single step) the execution of the program using an example input that produced a wrong answer Examine how each statement changes the values of the relevant variables See which statement update the variables in the wrong manner

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• Divide and conquer
  • Prime directive in debugging:

    Do NOT debug the whole program at once Debug one method at a time

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  • How to test one method at a time using BlueJ:

    Static method Right click on the program class file Select the static method Enter the desired parameters Click OK Instance method Right click on the program class file Select the new ClassName tab (the first in the drop down menu) Enter a name for this instance Click OK You will see a new object appear at the bottom of the BlueJ window. Right click on this object and select an instance method to run.