Using an ArrayList

We are actually going to be using ArrayList in two different ways here.

First, notice that a Polyline can contain an arbitrary number of points, and these points have to be stored in the Polyline object. In order to implement the Polyline class, we use an ArrayList to store the points. You can see the code for Polyline.java here.

Take a brief look at the relevant parts of the code:

First we declare an ArrayList of Point objects:

public class Polyline
{
  private ArrayList<Point> points;

In the constructor, we create the ArrayList, which is initially empty.

  public Polyline(String givenColor, int givenWidth)
  {
    color = givenColor;
    width = givenWidth;
    points = new ArrayList<Point>();
  }

Then the addPoint method just has to put the new point at the end of the list:

  public void addPoint(Point point)
  {
    points.add(point);
  }

An ArrayList is useful here because we don't know in advance, when constructing a Polyline object, how many points will eventually be added to it.