making a double/int out of Point.getX() and .getY()

Question

i'm trying to calculate the distance between 2 points with Java Point, it works fine till i say for example: double point1x = point1.getX();. cause when i do, i receive a error. can somebody help me find the problem in my code?

public class ClickAndCalc extends JFrame implements MouseListener {

public ClickAndCalc() throws IOException{
    addMouseListener(this);
}

public static void Window() throws IOException {
    String path = "kaakfoto.jpg";                       //imported image
    File file = new File(path);
    BufferedImage image = ImageIO.read(file);
    int width = image.getWidth();
    int height = image.getHeight();
    JLabel label = new JLabel(new ImageIcon(image));    //add the image to the JLabel

    System.out.println("height: " + height);            //print out the height and width of the image
    System.out.println("width: " + width);

    ClickAndCalc frame = new ClickAndCalc();            //make a new frame
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(label);
    frame.setSize(width+3, height+26);                  //set the size of the frame the same as the size of the image
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

int i=0;
Point p1, p2, p3;

public void mouseClicked(MouseEvent e) {}

public void mousePressed(MouseEvent e) {    //when clicked add point for 3 times

    if(i==0) {
        p1 = e.getPoint();
        System.out.println("coordinates Point1: X=" + p1.getX() + ". Y=" + p1.getY());
    }
    else if(i==1) {
        p2 = e.getPoint();
        System.out.println("coordinates Point2: X=" + p2.getX() + ". Y=" + p2.getY());
    }
    else if(i==2) {
        p3 = e.getPoint();
        System.out.println("coordinates Point3: X=" + p3.getX() + ". Y=" + p3.getY());
    }
    i++;
}

public void mouseReleased(MouseEvent e) {}

public void mouseEntered(MouseEvent e) {}

public void mouseExited(MouseEvent e) {}

//down here is some mistake i can't find

double distanceAB(double x1, double y1, double x2, double y2) {
    return Math.sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));
}

double  x1 = p1.getX(),     //convert getX and getY to doubles
        y1 = p1.getY(),
        x2 = p2.getX(),
        y2 = p2.getY(),
        distanceAB;{

distanceAB = distanceAB(x1, y1, x2, y2);        //calculate the distance between point A and B
System.out.println("the distance between the points AB is " + distanceAB + ".");
}

public static void main(String[] args) throws IOException {
    Window();                                   //add the window (frame, label, image)
}

}

Answer 1

The problem is here:

double  x1 = p1.getX(),     //convert getX and getY to doubles
        y1 = p1.getY(),
        x2 = p2.getX(),
        y2 = p2.getY(),
        distanceAB;

You are defining member variable of the class ClickAndCalc. These member variables are initialized when a new instance of the class is created, and that is the moment p1.getX(), p1.getY(), p2.getX() and p2.getY() are called. At this point, however, p1 and p2 are null, so you get a NullPointerException.

You can call calculate the distance between p1 and p2 only when you've determined the coordinates, that is, after you've clicked twice on the frame. The easiest solution is to move the code to the mousePressed method:

public void mousePressed(MouseEvent e) {    //when clicked add point for 3 times

    if (i==0) {
    ...
    }
    else if (i==1) {
        p2 = e.getPoint();
        System.out.println("coordinates Point2: X=" + p2.getX() + ". Y=" + p2.getY());

        double x1 = p1.getX();
        double y1 = p1.getY();
        double x2 = p2.getX();
        double y2 = p2.getY();
        double distanceAB = distanceAB(x1, y1, x2, y2);
        System.out.println("the distance between the points AB is " + distanceAB + ".");
    }
    ... 
}