Extending Classes and Inheritance - Beginning Java 2 SDK - page 265

You might also want to create a PolyLine object from an array of coordinates. The constructor to do
this would be:
// Construct a polyline from an array of coordinates
public PolyLine(double[][] coords) {
if(coords != null) {
// Create a one point list
start = new ListPoint(new Point(coords[0][0], coords[0][1]));
// First is start
end = start; // as well as end
// Now add the other points
for(int i = 1; i < coords.length ; i++)
addPoint(coords[i][0], coords[i][1]);
}
}
How It Works
The PolyLine class has the data members start and end that we saw in the diagram. These will
reference the first and last points of the list, or null if the list is empty. Storing the end point in the list
is not essential since we could always find it by going through the list starting with start. However,
having a reference to the last point saves a lot of time when we want to add a point to the list. The
constructor accepts an array of Point objects and starts the process of assembling the object, by
creating a list containing one ListPoint object produced from the first element in the array. It then
uses the addPoint() method to add all the remaining points in the array to the list.
Adding a point to the list is deceptively simple. All the addPoint() method does is create a
ListPoint object from the Point object passed as an argument, sets the next member of the old end
point in the list to refer to the new point and finally stores a reference to the new end point in the
member end .
The method toString() will return a string representing the PolyLine object as a list of point
coordinates. Note how the next member of the ListPoint objects controls the loop that runs through
the list. When the last ListPoint object is reached, the next member will be returned as null , and
the while loop will end. We can now give the PolyLine class a whirl.
Try It Out - Using PolyLine Objects
We can create a simple example to illustrate how to use the PolyLine class:
public class TryPolyLine {
public static void main(String[] args) {
// Create an array of coordinate pairs
double[][] coords = { {1., 1.}, {1., 2.}, { 2., 3.},
{-3., 5.}, {-5., 1.}, {0., 0.} };
// Create a polyline from the coordinates and display it
PolyLine polygon = new PolyLine(coords);
System.out.println(polygon);