Thread: ArrayList for 2DPoints

I just started my second quarter of Java. I'm in Java 2 now anyways I was given this as my first assignment:

Implement a Class Polygon that contains an array List of Point2D.Double objects. Support methods

public void add(Point2D.Double aPoint)
public void draw(Graphics g2)

Draw the polygon by joining adjacent points with a line, and then closing it up by joining the end and start points.
Write a graphical application that draws a square and a pentagon using two Polygon Objects.

I don't have much experience with ArrayLists so I'm pretty lost but this is what I have so far...

private ArrayList<Point2D.Double> myPolygon;
 
public void add(Point2D.Double aPoint) {
System.out.println(aPoint); // Outputs Point2D.Double[20.0, 10.0]
myPolygon.add(aPoint); // NullPointerException gets thrown here
}
 
and for my main class:
 
Polygon poly = new Polygon();
 
Point2D.Double a = new Point2D.Double(20,10);
poly.add(a);
 
Point2D.Double b = new Point2D.Double(30,50);
poly.add(b);
 
Point2D.Double c = new Point2D.Double(40,60);
poly.add(c);
 
Point2D.Double d = new Point2D.Double(70,90);
poly.add(d);
 
 
Line2D.Double segmenta = new Line2D.Double(a, b);
Line2D.Double segmentb = new Line2D.Double(b, c);
Line2D.Double segmentc = new Line2D.Double(c, d);
Line2D.Double segmentd = new Line2D.Double(d, a);

I'm pretty lost so any help is appreciated. I'm still waiting on my book to arrive so I don't have much to work from right now.