hw02 From Exceptional to Enhanced Cat

This homework assignment is designed to familiarize students with exceptions and file I/O in Java.

Prerequisite Knowledge

• Basic knowledge of Java exceptions, including checked exceptions, unchecked exceptions, and the use of try-catch, throw, and throws.
• Familiarity with program command-line arguments in Java.

Course-Specific Learning Outcomes

• LO2.b: Define, throw, and propagate exceptions appropriately in a software solution.

Questions

In your notes, clearly answer the following questions. These instructions assume that you are logged into the Odin server.

NOTE: For each step, please provide in your notes the full command that you typed to make the related action happen along with an explanation of why that command worked. Some commands require multiple options. It is important to not only recall what you typed but also why you typed each of them. If context is necessary (e.g., the command depends on your present working directory), then please note that context as well. You won't need to submit your notes in your final submission. However, if done properly, your exercise notes will serve as a helpful study guide for the exam.

Exercise Steps

Checkpoint 1 Steps

1. Use Git to clone the repository for this exercise onto Odin into a subdirectory called cs1302-hw02:

   $ git clone --depth 1 https://github.com/cs1302uga/cs1302-hw02.git
   

2. Change into the cs1302-hw02 directory that was just created and look around. There should be multiple Java files somewhere in the directory structure. You may want to execute the find command on the src directory for a quick, easy-to-read view of the directory contents.

   • What are the fully qualified names for the classes contained in the Java files?
   • What is the path to the default package for source code relative to the cs1302-hw02 directory?

3. The directory you downloaded contains a Java implementation of the Unix cat utility. Remember, the commands you have been executing in Unix are just programs that were installed by the system administrators. MyCat.java works similarly to cat but was written by your instructors and will be compiled by you. Before compiling, use the Unix cat utility to print the contents of MyCat.java to the terminal. Write the command you used to do this in your notes.

4. Read through the Java code in MyCat.java and Printer.java. Note, there is a dependency between the two files. Based on the dependencies, which .java file must be compiled first?

5. From the cs1302-hw02 directory, try to compile each Java file separately, specifying bin as the default package for compiled code.

   Note In this step, you will encounter a compile-time (syntax) error related to exceptions and exception handling. Hint: the error should not be a "cannot find symbol" error. If it is, you will need to adjust your compilation command.

   Answer the following in your notes about the compile-time error:

   • In what file is the error?
   • On what line in the source code is the error?
   • What specifically is causing this error?

   Fix that specific compile-time error and recompile the code. If you notice any logical errors in the code, don't worry about fixing them at this time.

6. Execute the find src command from directly within your cs1302-hw02 directory. You should see the following output:

   src
   src/cs1302
   src/cs1302/exceptions
   src/cs1302/exceptions/MyCat.java
   src/cs1302/exceptions/Printer.java
   

7. Execute the find bin command from directly within your cs1302-hw02 directory. If everything was compiled properly, you should see the following output:

   bin
   bin/cs1302
   bin/cs1302/exceptions
   bin/cs1302/exceptions/Printer.class
   bin/cs1302/exceptions/MyCat.class
   

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Checkpoint 2 Steps - Using and Enhancing Your Cat

1. Answer the following questions about MyCat.java in your notes:

   • What is the name of the variable that stores the first command-line argument?

   • What method is called if you run with MyCat with a single - as the only command-line argument?

     • Test it out! execute the code from your cs1302-hw02 directory using the following command:

       $ java -cp bin cs1302.exceptions.MyCat -

     • Notice that the program is blocked waiting for you to type. Go ahead and type a few words.
     • When you're finished, you can trigger the end of file (a.k.a. the EOF) by pressing C-d.
     • Try running the unix cat command with a single -. Notice how it behaves the same way.

   • What method is called if you pass in the name of a regular file?

     • Test it out! execute the code from your cs1302-hw02 directory by passing in the relative path to the Printer.java file using the following command:

       $ java -cp bin cs1302.exceptions.MyCat src/cs1302/exceptions/Printer.java

     • Test the Unix `cat utility with the same input. Cool, huh?

2. Interesting Side Note: Take a close look at the following command:

   $ java -cp bin cs1302.exceptions.MyCat - < src/cs1302/exceptions/Printer.java

   Notice that this execution has a single command-line argument (-). However, we are using redirecting the input to come from Printer.java! So, the program will output the contents of the file even though the internal Scanner object will be created with System.in!

3. Take a few moments to understand the three methods in Printer.java. Note that both printStdInLines and printFileLines both call printLines. The difference is in how the Scanner object is created. Although the Scanner objects are created differently, the tasks of printing lines of a file and printing lines from standard input are nearly identical!

   Also notice how moving the methods to print lines into the Printer class greatly simplified the logic in our MyCat program. As of now, MyCat doesn't even need to directly contain any loops!

4. From the cs1302-hw02 directory, run the MyCat program with no command-line arguments. A run-time exception should occur. Before attempting to fix the exception, answer the following questions about the exception in your notes:

   • What is the name of the exception?
   • Why did the exception occur?
   • Is this exception a checked or an unchecked exception? How can you tell?

5. There are multiple ways to fix the run-time exception that you encountered in the last step. You are allowed to change the code in MyCat by moving lines around, deleting existing code, adding code, etc. Just make sure you fix the problem in such a way that the following criteria are met whenever the exception occurs:

   • The program does not crash.
   • The exception message is displayed to standard error (using System.err.println instead of System.out.println). To print the exception message, you can call the toString() method on the exception object reference given in the catch statement.

   When displaying the exception message, something like the following will suffice (replacing <message> with the actual exception message generated from calling toString on the exception object):

   MyCat: <message>
   

6. From the cs1302-hw02 directory, run the MyCat program with no command-line arguments. If implemented properly, you should see the output below if no command-line arguments are given:

   MyCat: java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
   

   What's the difference between this execution of the program and the one performed two steps earlier? Take a minute to think about why catching the exception is beneficial before moving on.

   Answer: When you execute this code and you see the new error message, the code is not crashing. In fact, it has recovered from the crash. If you were to add additional code below the try/catch block, you would see that code execute after the error message is printed. Before we added our try/catch block, the code crashed when no command-line argument was given. So, despite the fact that the error message looks similar in both cases, the result is actually quite different.

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Checkpoint 3 Steps - Further Enhancing Your Cat

1. Now, let's add some more functionality to the MyCat program. Change the code so that one or more command-line arguments are accepted. The expected behavor is that MyCat should print the files, in order, to standard output, effectively concatenating the contents of the supplied files.

2. With this change, your program may no longer generate an ArrayIndexOutOfBoundsException if the user doesn't provide any command-line arguments. However, we still want to provide a helpful message to let the user know how to properly use the program. Update your code so that when the user provides no command-line arguments, the program outputs: Usage: MyCat [filename]...

3. From the cs1302-hw02 directory, use your enhanced MyCat program to display the contents of the following three files all passed in at once:Printer.java, standard input ("-"), and MyCat.java in that order! If your program does not currently allow "-" to be specified for arbitrary file names in the list of command-line arguments, then modify it to accomodate that feature.

4. Run your enhanced MyCat program by passing in two filenames as command-line arguments. Make sure the first file does not exist in the file system. Your program should catch the FileNotFoundException, print the appropriate message, and still print the contents of the second file (assuming it exists).

5. Update the comments in the source code to reflect any functionality that has been added since the beginning of this exercise.

6. Try additional test cases to test the robustness of your application. If you come up with a good idea for a test case, feel free to share it on Piazza!

7. Verify that all of your code passes the checkstyle audit using the command check1302 src. Note: if you receive any error messages as a result of running this command, you can find more information about the error and how to fix it in the 1302 Style Guide.

8. Generate the API documentation website for all of the code in the cs1302 package. Host the documentation on Odin using cs1302-hw02-doc as the name for your symbolic link.

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Submission Steps

Each student needs to individually submit their own work.

1. Create a plain text file called SUBMISSION.md directly inside the cs1302-hw02 directory with the following information:

   1. Your name and UGA ID number; and
   2. Full URL for your hosted API website

   Here is an example of the contents of SUBMISSION.md.

   Sally Smith (811-000-999)
   https://webwork.cs.uga.edu/~your_username/cs1302-hw02-doc/
   

2. Change directories to the parent of cs1302-hw02 (e.g., cd .. from cs1302-hw02). If you would like to make a backup tar file, the instructions are in the submissions steps for hw01. We won't repeat those steps here and you can view them as optional.

3. Use the submit command to submit this exercise to csci-1302:

   $ submit cs1302-hw02 csci-1302
   

   Read the output of the submit command very carefully. If there is an error while submitting, then it will displayed in that output. Additionally, if successful, the submit command creates a new receipt file in the directory you submitted. The receipt file begins with rec and contains a detailed list of all files that were successfully submitted. Look through the contents of the rec file and always remember to keep that file in case there is an issue with your submission.

   Note: You must be on Odin to submit.

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Copyright © Michael E. Cotterell, Bradley J. Barnes, and the University of Georgia. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License to students and the public and licensed under a Creative Commons Attribution-NonCommercial 4.0 International License to instructors at institutions of higher education. The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.