Lab #7 Solution

Goal: This lab will introduce you to gjdb, the Java debugger.




Copy the Lab 7 directory by starting from your home directory and typing:

cp -r ˜cs61b/lab/lab7 . The Java Debugger Tutorial

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Welcome. You now find yourself in the position of debugging somebody else’s bad code in DebugMe.java, which is supposed to compute the partial geometric series




1 1 1

1 + --- + --- + --- (which equals 1.875)

2 4 8




DebugMe.main() begins by calling the recursive method createGeomSeries(), which creates a linked list in which each node contains one term of the above series. Next, main() calls the recursive method listSum() to compute the sum of the nodes in the list. As you may have guessed, it doesn’t work. If you compile and run DebugMe you will see that it prints the value 1.0 instead of 1.875.




Some documentation on the java debugger (gjdb) is available in the course

reader and is also linked from the class Web page. (The Java debugger provided with Oracle’s Java Development Kit is called jdb. Professor Hilfinger modified it and built a better interface for it.) Here is a summary of how you might

use gjdb to track down the error in this program.




(1) Compile the program with the debugging switch ‘-g’. javac -g DebugMe.java

(2) Instead of running java, you could run gjdb as follows. gjdb DebugMe

When you see the ‘[-]’ prompt, the debugger is ready to go, and you may

follow the instructions in the course reader on how to use it. However, rather than run gjdb from a shell, we recommend you run it from emacs. Emacs will interpret the output of the debugger and show you where the debugger is in your

.java files, which is extremely useful. To begin, type ‘Meta-x gjdb’ from within emacs. When asked to fill in the gjdb command, use ‘gjdb DebugMe’. You should see a new window appear with the ‘[-]’ prompt.




(3) gjdb is a command-line driven program. The most important command you should learn how to use is the (limited) on-line help; type help at the prompt.




[-] help




If you see a command that looks like the one you want, but need more information, type ‘help <command’, replacing <command with the command you want to know more about.

(4) To run the program from within the debugger, type ‘run’ at the prompt. [-] run

This executes your program FROM THE BEGINNING. (5) The other most important command is ‘quit’.

When gjdb is stuck, not giving you a command prompt, you may simply kill the emacs buffer in which gjdb is running. If you ever need to restart gjdb, make sure your cursor is in the debugger window and type ‘C-x k’.




(6) Let’s see what’s happening within the main function. So that we can do this, we’ll set a breakpoint at the beginning of main().




[-] break DebugMe.main




This tells the debugger to stop when execution reaches the beginning of the main method. (You can also set breakpoints at specific line numbers. If you’ve written small modular programs, you will rarely find that necessary.)




Start the program by typing ‘run’. When execution reaches the breakpoint, emacs will show the DebugMe.java file in a separate window with the current line indicated by an ‘=’ arrow.




(7) At the beginning of the ‘main’ function, there’s no sign yet of the bug. Execute slowly through the code using either the ‘next’ command or the ‘step’ command. These two commands have a crucial difference: ‘next’ executes one statement in full, including any method calls. ‘step’ normally executes one statement, but if the statement includes a method call, execution stops at the first line of the called method. Think of ‘step’ as ‘stepping into’ a method. In emacs, C-c C-s or the f5 key is shorthand for ’step’, and C-c C-n or the f6 key is shorthand for ‘next’.




Type f6 twice while watching the ’=’ arrow in the other window. The arrow should now point to the line that declares ‘sum’ and assigns it a value. This means that we are about to execute that line, but haven’t yet.

(If you stepped too far, you can start over by typing ‘run’ again.)