Tricky Targets Exercise

Purpose: A reality check on the Tricky program in the lecture notes (i.e. are the instructors telling the truth?).

It is worthwhile to verify that the Java compiler identifies the targets that the memory model says it should.

Download the zip file with all the java files and unzip it. Or copy all the following files into a new directory:

• Tricky.java
• I.java
• P.java
• A.java
• B.java

Exercises:

1. Trace the execution of the main method (using the memory model presented in class). Explain what output you expect. Imagine commenting out any statement which you think will cause compilation errors, and continue with your tracing. Read the comments in the code regarding the style rule for referencing static members and the use of references of the form this.mem and super.mem.

2. Verify your tracing. The output from running this code is out.txt. Compare this with the results of your tracing.

3. Build and run the code. It should compile and run without error.

4. Check that the output of the program agrees with ours in out.txt.

5. Experiment with the code. If you remove the widening cast on line 45 of Tricky.java and rerun the program, you find the output changes from before. Explain why.

   In the distributed source code, several groups of lines have been commented out. Try removing some of these comments. Do you understand why the program no longer compiles, or if it does compile, can you explain the new output?

   Try other variations that you are curious about. It is very easy to edit, build and rerun...

6. Use the DrJava debugger to step through the execution of this code and verify the memory model.