从Point.getX()和.getY()中生成double/int

Question

我正在尝试用Java Point计算2个点之间的距离，它运行良好，直到我说例如：double point1x=point1.getX（）；。因为当我这样做的时候，我会收到一个错误。有人能帮我找出代码中的问题吗？

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

问题就在这里：

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

您正在定义类ClickAndCalc的成员变量。这些成员变量是在创建类的新实例时初始化的，也就是调用p1.getX()、p1.getY()、p2.getX()和p2.getY()的时刻。然而，在这一点上，p1和p2是null，所以您得到了一个NullPointerException。

只有在确定坐标后，也就是说，在框架上单击两次后，才能调用计算p1和p2之间的距离。最简单的解决方案是将代码移动到mousePressed方法：

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 + ".");
    }
    ... 
}