class Point {
    double x, y, z;
   
    Point(double _x, double _y, double _z) {
        x = _x;
        y = _y;
        z = _z;
    }
   
    void setX(double _x) {
        x = _x;
    }
   
    double getDistance(Point p) {
        return (x - p.x)*(x - p.x) + (y - p.y)*(y - p.y) + (z - p.z)*(z - p.z);
    }

}

public class TestPoint {
    public static void main(String[] args) {
        Point p = new Point(1.0, 2.0, 3.0);
        Point p1 = new Point(0.0, 0.0, 0.0);
        System.out.println(p.getDistance(p1));
        
        p.setX(5.0);
        System.out.println(p.getDistance(new Point(1.0, 1.0, 1.0)));
    }
}


以上是程序的代码，问题在这里“double getDistance(Point p) ”，这里的形式参数为什么可以用Point p？？谁能帮我详细讲解一下吗？

还有啊，这段代码为什么会出错呢？
class xyz {
    double x,y,z;
    xyz (double _x,double _y,double _z){
    x=_x;
    y=_y;
    z=_z;
    }
    double deal (double _x,double _y,double _z){
    return (_x*_x+_y*_y+_z*_z);
    }

}


public class point {
    public static void main(String[] args) {
        xyz point2 =new xyz(3,3,3);
        xyz point1 =new xyz(3,3,3);
        System.out.print(point2.deal(point1));
     }

}