Writing a Java application with class structure

Before starting this assignment, you should ensure you can access your account on the CSIT host computer, sirah. I recommend you also familiarize yourself with the editors and Java development environment on this computer. You may, if you prefer, develop the current homework using Java compilers on other systems, possibly using a programming environment such as J++ or Cafe (if you use any of these programming environments, do not use them to generate Java code for you automatically). But you will need the sirah account to submit your homework. Also note that for later homeworks, some use of the editors and/or the JDK system on sirah and associated hosts will almost certainly be required.

The Assignment

   int nrows ;    // The number of rows
   int ncols ;    // The number of columns
   float [][];    // Array holding the elements of the matrix

You should also write (at least) the following public methods.

• A set method with three arguments i, j, x, which assigns floating point value x to the (i, j) element of the matrix.
• A get method with two arguments i, j, which returns the value of the (i, j) element of the matrix.
• inquiry methods getNumRows and getNumCols which respectively return the numbers of rows and columns.
• An add method which adds together two matrices and returns a new matrix containing the sum of the two matrices, element by element
• A multiply method which multiplies together the two matrices according to the proper rules of matrix multiplication, and returns a new matrix containing the product of the two matrices.
• A toString method which returns a formatted String representation of all the elements, laid out in a way suitable for displaying on a terminal. (This string might include newline characters, "\n", to separate rows).

The methods add and multiply may be implemented either as instance methods or as static methods on the Matrix class: you should decide which you think is most suitable and explain your reasons.

You must also write a test program for this class: design an application that prompts the user to type in the shape and element values for two matrices. Create instances of the Matrix class for each one. Combine the two matrices using both add and multiply, and use the toString method to print out a description of both results.

Show a couple of runs of the program (for small matrices, e.g. number of rows and columns may be two or three). Be sure to demonstrate a case of successfully combining matrices, and also one which fails because the add or multiply method gives an exception. Make sure your program works for "non-square" matrices--i.e. number of rows not equal to number of columns.

Definitions for matrix arithmetic:

c = a + b

    c.getNumRows() == a.getNumRows()
    c.getNumCols() == a.getNumCols()

    c.get(i, j) == a.get(i, j) + b.get(i, j)

    a.getNumRows() == b.getNumRows()
    a.getNumCols() == b.getNumCols()

c = a * b

    c.getNumRows() == a.getNumRows()
    c.getNumCols() == b.getNumCols()

    c.get(i, j) == Sum over k of terms: a.get(i, k) * b.get(k, j)

    a.getNumCols() == b.getNumRows()

Submission of your Homework

1. A description of your program, preferably in Microsoft Word, HTML, or plain text format,
2. all Java source code, and
3. file or files showing sample output from one or more runs. This may be captured by cutting and pasting from a command-line window.

For this assignment, there will be a total of 20 points. Your submission will be assessed on:

• Functional and clearly written code. Of course the code should meet the specification given above. It should also be written in a style that makes it clear to the grader that it is really correct! In particular, adopt a good indentation policy, avoid long lines and long method definitions. Break any lines longer than 80 characters. Consider breaking up any method definition longer than, say, 50 lines into smaller methods with simpler functionality. For a good set of stylistic rules for Java programs, see Sun's Java coding conventions.
• Clear and well-focussed documentation. Do not write a very long document (1000 well-chosen words would probably be more than enough), but give an overall description of the purpose of the software, cover the principal class interface(s), sketch any relevant implementation details, and briefly describe how to use the test program.
• Meaningful comments in the code. No marks for vacuous commentary, but include short descriptions for each class and method. Using javadoc comments is not required, but it is encouraged, and may gain extra credit (javadoc-generated HTML can be submitted in addition to but not in place of the conventional documentation described in the previous bullet). Also give brief implementation comments next to non-obvious pieces of code. Bear in mind that in general the better written the code, the less the need should be for implementation comments!
• Some points will be assigned on the basis of apparent effort and uniqueness.

• For information on all standard API classes, see the Java 2 API specification.
• Simple example of keyboard input in Java.