Lab 4: Writing simple methods

The primary goal of this lab is to get practice with basic methods. The logic is very simple in this lab; the point is to become familiar with the syntax of methods and how to document them. This lab also gives you an opportunity for practice with the Mercurial revision control system. You have been previously provided with an overview of Mercurial, which is also accessible here.

1. Setup

Clone the repository for this lab from the shared class directory:

$ hg clone /cslab/class/csci235/labs/lab4

You can check the status of your working copy with

$ hg status

$ hg log

Now fire up your editor and make an awful mess of ArrayManip.java, being sure you save the results. If you try the status command again, the file will be flagged as "modified". Now try

$ hg revert ArrayManip.java

The "revert" will also leave behind a file ArrayManip.java.orig containing your mess. That would give you a chance to salvage something from it, if you needed to. If you now try

$ hg status

2. Time travel

The command hg log lists the available revisions. The first line for each revision starts with changeset, followed by two numbers. The first number (before the colon) is the convenient way to identify a revision; what follows the colon is a less convenient way to do the same thing. Notice that each revision indicates who made it and when, and it has a line summarizing the change.

You can set your working copy to any revision in the log. So do

$ hg update -r 2

Look at ArrayManip.java with an editor, and go to the end of the file. You'll find the methods we wrote in class, but I have

• placed them in alphabetical order, and
• added a documentation comment before each.

Look closely at the comments before the methods. Notice that each of these comments begin with "/**" and includes one or more lines with @ to indicate a tag. This style of documenting methods allows special tools to generate the kind of documentation that you see for the library. Writing comments like these is an important part of making your program intelligible to the people that will read it.

After you have looked it over, travel back to the latest revision with

$ hg update -r 3

3. Overview

You are going to be working on ArrayManip.java, filling in some methods, writing new methods, and modifying the cases within the main() switch statement to use the methods you are writing. Before you start making changes, take a few more minutes to review the code. Compared to the revision you were looking at previously, it has some additional methods and comments about methods. They are in alphabetical order, to make it easier to find the one you are looking for.

I have provided you with the complete method fillRandom(), the skeleton for isSorted(), and comments or place-holders for several more methods.

The comments before fillRandom() and isSorted are examples of how we will document (describe) each method we write. Here is the comment for isSorted():

    /**
     * Test whether an array is sorted (in ascending order).
     * @param a The array to test.
     * @return Whether a is sorted.
     */

Note how it is formatted and that it starts with '/**'. The first line of text is a sentence stating what the method should do. The next two lines have tags; they describe (in order) the parameters of the method and the value that it returns.

The comment for fillRandom() is similar. There is no @return tag, because the return type is void. The POSTCONDITION line states what is true after the method returns. Some methods will have a PRECONDITION line to say what must be true before the method is called (see the comment for enlarge()).

4. What to do

Open ArrayManip.java in an editor and add your names to the @author tag in the header comment. After you change the file, use hg status to verify that just the file you want shows up as modified, and then commit your change to the repository with

hg commit -m "Filled in our names"

$ hg log

Make the following changes, one at a time. As you finish each, test to make sure it works correctly, and commit the change to your repository (check status and commit). Do as many of the changes as you have time for.

1. Fill in case 10 to use the fillRandom() method.
2. Fill in case 8 to call the isSorted() method and print whether the array is in order. An array is sorted when every element is greater than or equal to the element preceding it.
3. Write the method enlarge() to do the work for case 5. The comment has already been written. You can then change case 5 by replacing the last six lines before the break with

   a = enlarge(a, newLength);
   

4. Write a method makeReverse() that takes an array as a parameter and returns a new array that has the same elements in reverse order. Fill in case 7 to use makeReverse().
5. Write the heading and body of the method changeLength() that makes a new array that is either larger or smaller than the one passed to it. Case 6 should prompt for a new size (as case 5 does) and then use this method to make the change.
6. Write a method reverse() that reverses the elements in the array passed to it without creating a new array. The heading for that method will be

   public static void reverse(int[] a)
   

   Change case 7 to use this method in place of makeReverse().

5. Turn in

Remove your .class file. Then create a script file in which you compile and run your program. Do enough operations to demonstrate each of the methods you wrote. After you have run the program, include hg log before you end the script.

Use handin to turn in your source file and the script file as lab4. Do not turn in a printed copy.

5. Cloning a repository to your account

You can copy the repository (with all of its history) to your account as a Mercurial clone. Assuming that your account name is mortimer.snerd and that you already have a subdirectory named cs235, you'd do the command

$ hg clone . ssh://mortimer.snerd@cslab25/cs235/lab4/

The script file is not in your repository. If it is named typescript you can copy it with the command

$ scp typescript mortimer.snerd@cslab25:cs235/lab4/