WebReference.com - Part 3 of chapter 5 from Beginning Java 2 SDK 1.4 Edition, Wrox Press Ltd (2/5)

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Beginning Java 2 SDK 1.4 Edition

Try It Out--The Line Class

We can use Point objects in the definition of the class Line:

class Line {
  Point start;   // Start point of line
  Point end;     // End point of line
  // Create a line from two points
  Line(final Point start, final Point end) {
    this.start = new Point(start);
    this.end = new Point(end);
  }
  // Create a line from two coordinate pairs
  Line(double xStart, double yStart, double xEnd, double yEnd) {
    start = new Point(xStart, yStart);   // Create the start point
    end = new Point(xEnd, yEnd);         // Create the end point
  }
 
  // Calculate the length of a line
  double length() {
    return start.distance(end);  // Use the method from the Point class
  }
  // Convert a line to a string 
  public String toString() {
    return "(" + start+ "):(" + end + ")";    // As "(start):(end)"
  }                                           // that is, "(x1, y1):(x2, y2)"
}

You should save this as Line.java in the directory TryGeometry.

How it Works

You shouldn't have any difficulty with this class definition, as it is very straightforward. The class Line stores two Point objects as instance variables. There are two constructors for Line objects, one accepting two Point objects as arguments, the other accepting the (x, y) coordinates of the start and end points. You can see how we use the variable this to differentiate the class instance variables, start and end, from the parameter names in the constructor.

Note how the constructor that accepts Point objects works:

  // Create a line from two points
  Line(final Point start, final Point end) {
    this.start = new Point(start);
    this.end = new Point(end);
  }

With this implementation of the constructor, two new Point objects are created which will be identical to, but independent of, the objects passed to the constructor. If you don't think about what happens you might be tempted to write it as:

  // Create a line from two points
  Line(final Point start, final Point end) {
    this.start = start;              // Dependent on external object!!!
    this.end = end;                  // Dependent on external object!!!
  }

The important thing you should notice here, is that the way the constructor is implemented could cause problems that might be hard to track down. It's the same problem of an object variable being separate from the object to which it refers. In this version of the constructor no new points are created. The start and end members of the object refer to the Point objects passed as arguments. The Line object will be implicitly dependent on the Point objects that are used to define it. If these were changed outside the Line class, by using the move() method for example, this would 'silently' modify the Line object. You might consciously decide that this is what you want, so the Line object continues to be dependent on its associated Point objects, for instance in a drawing package. But, in general, you should avoid implicit dependencies between objects.

In the toString() method for the Line class, we are able to use the Point objects directly in the formation of the String representation of a Line object. This works because the Point class also defines a toString() method.

We've now defined two classes. In these class definitions, we've included the basic data that defines an object of each class. We've also defined some methods which we think will be useful, and added constructors for a variety of input parameters. Note how the Point class is used in the definition of the Line class. It is quite natural to define a line in terms of two Point objects, and the Line class is much simpler and more understandable than if it was defined entirely in terms of the individual x and y coordinates. To further demonstrate how classes can interact, and how you can solve problems directly, in terms of the objects involved, let's devise a method to calculate the intersection of two Line objects.

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Created: July 16, 2002
Revised: July 16, 2002

URL: https://webreference.com/programming/java/beginning/chap5/3/2.html