Outline of Class 4: Describing Classes

Held: Friday, January 23, 1998

Administrivia

• The "exotic programming languages" discussion group starts meeting next Tuesday at 4:15 pm in the Math seminar room (Science 2413). This semester, the group will be discussing Haskell, an interesting functional language. Consider checking it out.
• A reminder that this coming Monday at noon we'll be showing a film on Java in Science 2413. Get a brown bag lunch and come.
• I've come up with a second lab. I'm not sure when it will be assigned, but if you want to get a head start, take a look at it.

A Complex Class

• Previously, we had begun to develop a simple Complex class to represent complex numbers.
• Recall that a class definition begins with the keywords public and class class and is followed by the name of the class and the body of the class, surrounded in curly braces.

  public class Complex
  {
  } // Complex
  

• Often, we begin thinking about classes in terms of their attributes. For complex numbers, the key attributes are the real and imaginary part (or the angle and radius).

    double real_part;
    double imaginary_part;
  

• Information hiding and modularity dictate that we keep attributes "internal" and only allow others to access them through methods. To keep an attribute private, use the keyword protected or the keyword private. (They mean different things; use protected until you understand the difference.)

    protected double real_part;
    protected double imaginary_part;
  

• Good programming style dictates that we comment everything (or almost everything) we include in our programs.
• We should include a summary description for each class as well as indication as to authorship. These should be a javadoc comment, beginning with a slash and two stars and ending with a star and a slash.

  /**
   * A simple implementation of complex numbers supporting a small
   * set of operations.
   *
   * @author Samuel A. Rebelsky
   * @version 1.0 of January 1998.
   */
  

• Each attribute should also have a comment. It will also be in javadoc format.

    /**
     * The real part of a complex number which is represented in
     * traditional "x + y*i" format.
     */
    protected double real_part;
    /**
     * The imaginary part of a complex number which is represented
     * in traditional "x + y*i" format.  
     */
    protected double imaginary_part;
  

• Because classes include many different "parts" (attributes, methods, some other things), I find it useful to include comments to visually separate the sections. For example,

  // +------------+--------------------------------------------------------
  // | Attributes |
  // +------------+
  

• Now lets consider the methods. You've already seen some illustrations of methods when writing simple programs. In general, a method has
  • A descriptive comment
  • A protection level (public)
  • A return value (void if it returns nothing)
  • A name
  • A parenthesized argument list
  • A body
• Here's a method to get the real part of an imaginary number.

    /**
     * Get the real part of this complex number.
     */
    public double getReal()
    {
      return real_part;
    } // getReal()
  

• Traditionally, you include accessors to access/get attributes and modifiers to set attributes.
  • Not all classes support modification. For example, the Integer and Boolean classes that Java supplies do not allow you to modify the underlying integer.
• We might set the value of a complex number with

    /**
     * Set/change the value of this complex number to x + y*i.
     */
    public void setValue(double x, double y) 
    {
      real_part = x;
      imaginary_part = y;
    } // setValue()
  

• Note that we can have accessors and modifiers that work with "simulated" attributes.

    /**
     * Get the radius of the vector corresponding to this complex number.
     */
    public double getRadius()
    {
      return Math.sqrt(real_part*real_part + imaginary_part*imaginary_part);
    } // getRadius()