Defining Classes - Beginning Java 2 SDK - page 195

Try It Out - Calculating the Intersection of Two Lines
We can use these results to write the additional method we need for the Line class. Add the following
code to the definition in Line.java :
// Return a point as the intersection of two lines -- called from a Line object
Point intersects(final Line line1) {
Point localPoint = new Point(0, 0);
double num = (this.end.y - this.start.y)*(this.start.x - line1.start.x) -
(this.end.x - this.start.x)*(this.start.y - line1.start.y);
double denom = (this.end.y - this.start.y)*(line1.end.x - line1.start.x) -
(this.end.x - this.start.x)*(line1.end.y - line1.start.y);
localPoint.x = line1.start.x + (line1.end.x - line1.start.x)*num/denom;
localPoint.y = line1.start.y + (line1.end.y - line1.start.y)*num/denom;
return localPoint;
}
Since the Line class definition refers to the Point class, the Line class can't be compiled without the
other being available. When you compile the Line class the compiler will compile the other class too.
How It Works
The intersects() method is called from one Line object, and takes another Line object as an argument.
In the code, the local variables num and denom are the numerator and denominator in the expression for t
in the diagram. We then use these values to calculate the x and y coordinates for the intersection.
If the lines are parallel, the denominator in the equation for t will be zero, something
you should really check for in the code. For the moment, we will ignore it and end up
with coordinates that are Infinity if it occurs.
Note how we get at the values of the coordinates for the Point objects defining the lines. The dot
notation for referring to a member of an object is just repeated when you want to reference a member of
a member. For example, for the object line1 , the expression line1.start refers to the Point
object at the beginning of the line. Therefore line1.start.x refers to its x coordinate, and
line1.start.y accesses its y coordinate.
Now we have a Line class, which we can use to calculate the intersection point of two Line objects.
We need a program to test the code out.
Try It Out - The TryGeometry Class
We can demonstrate the two classes we have defined, with the following code in the method main() :
public class TryGeometry {
public static void main(String[] args) {
// Create two points and display them
Point start = new Point(0.0, 1.0);