构造器该如何理解？ - 编程论坛

程序代码：

public class Point {
        double x, y;

        public Point() {
                System.out.println("这是无参构造器");
        }

        public Point(double _x, double _y){
                System.out.println("这是有参构造器");
                x = _x;
                y = _y;
        }

        public double getDistance(Point p) {
                double d = Math.sqrt((x-p.x)*(x-p.x)+(y-p.y)*(y-p.y));
                return d;
        }

        public static void main(String[] args) {
                Point p = new Point();
                Point p1 = new Point(3.0, 4.0);
                Point origin = new Point(0, 0);
                System.out.println(p1.getDistance(origin));
        }
}

网上解释说，创建对象时系统会对这个对象的实例进行初始化，如果想改变这种初始化，就可以通过自定义构造器实现。现在有几个问题不明白：
1.

public class Point {
double x, y;}

本身就有x和y，public Point(double _x, double _y)这一段有有啥意义呢？

2.public double getDistance(Point p)里面的Point p是public class Point还是public Point() 还是public Point(double _x, double _y)？