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