Calculating the distance between two points

Question

I need to create a class which calculates the distance between two points. I am stuck and I am a total beginner. Here are my classes:

package org.totalbeginner.tutorial;

public class Point {

    public double x;
    public double y;

    Point(double xcoord, double ycoord){
        this.x = xcoord;
        this.y = ycoord;
    }

    public double getX() {
        return x;
    }

    public double getY() {
        return y;
    }    
}

The second class.

package org.totalbeginner.tutorial;

public class Line {

    double x;
    double y;

    Point p1 = new Point(2.0,2.0);
    Point p2 = new Point(4.0,4.0);
    Point mp = new Point(x,y);

    public void midpoint() {
        x = (p1.getX() + p2.getX()) / 2;
        y = (p1.getY() + p2.getY()) / 2;
    }
}

I am not sure how to get a point object (the middle point) between both defined points.

I can create point objects but I am not sure how to return a point object through my midpoint() method that lies between those two point objects.

Answer 1

The distance between two points (x1,y1) and (x2,y2) on a flat surface is:

    ____________________
   /       2          2
 \/ (y2-y1)  + (x2-x1)

But, if all you want is the midpoint of your two points, you should change your midpoint function to:

public Point midpoint (Point p1, Point p2) {
    return new Point ((p1.getX() + p2.getX()) / 2, (p1.getY() + p2.getY()) / 2);
}

This will return a brand new point object with the points set to the middle of the given two points (without having to concern yourself with any other math). And, since your second class is a line, you only need the two end points to describe it, so I'd make some minor changes.

First Point.java:

class Point {
    double x, y;
    Point (double xcoord, double ycoord) {
        this.x = xcoord;
        this.y = ycoord;
    }
    public double getX() { return x; }
    public double getY() { return y; }
}

Then Line.java:

public class Line {
    Point p1, p2;
    Line (Point point1, Point point2) {
        this.p1 = point1;
        this.p2 = point2;
    }
    public Point midpoint() {
        return new Point ((p1.getX()+p2.getX())/2, (p1.getY()+p2.getY())/2);
    }
    public double abstand() {
        return Math.sqrt(
            (p1.getX() - p2.getX()) *  (p1.getX() - p2.getX()) + 
            (p1.getY() - p2.getY()) *  (p1.getY() - p2.getY())
        );
    }
    static public void main (String args[]) {
        Line s = new Line (new Point(2.0, 2.0), new Point(5.0, 6.0));
        Point mp = s.midpoint();
        System.out.println ("Midpoint = (" + mp.getX() + "," + mp.getY() + ")");
        double as = s.abstand();
        System.out.println ("Length   = " + as);
    }
}

These two files, when compiled and run with the endpoints 2,2 and 5,6 (the hypotenuse of a classic 3/4/5 right-angled triangle), generate the correct:

Midpoint = (3.5,4.0)
Length   = 5.0

Answer 2

Simple Pythag... root(dx^2 + dy^2)

Math.sqrt(Math.pow((p2.getX() - p1.getX()), 2) + Math.pow((p2.getY() - p1.getY()), 2))

Answer 3

 X
 +
 |\
 | \
a|  \c
 |   \
 |    \
 +-----+ 
    b   Y

Imagine X and Y are your points on a flat surface. Then a is X.y - Y.y and b is Y.x - X.x . The length of c is their distance, and is the length of the hypotenuse of that triangle. It is calculated using

sqrt(a^2 + b^2);

Since you see we are squaring a and b, the sign of them isn't relevant - it will come down to the same. So this method always works, where ever the points lie.

Lookup the Pythagorean theorem

Answer 4

Do you really need the distance, or are you trying to just get the midpoint? Because from your code snippet, it kind of looks like you just want to create a new point that is half-way between two existing points.

If you're really just after the midpoint, you don't really need an entire 2nd class (i.e., 'Line') to accomplish that. Since the thing you are trying to find is also a point, it makes sense to add a constructor to your existing Point class, like so ..

Point(Point a, Point b)
{
  x = (a.x + b.x) / 2;
  y = (a.y + b.y) / 2;
}

.. then, elsewhere let's say you already have a couple of points you want to use this on, you use the constructor thus:

Point p1 = new Point(2,2);
Point p2 = new Point(4,4);
Point midpoint = new Point(p1, p2);

and if you really want distance between two points, that's not really an attribute of either point, so it makes sense to use a static method for that, like so

public static double distance(Point a, Point b)
{
  double dx = a.x - b.x;
  double dy = a.y - b.y;
  return Math.sqrt(dx * dx + dy * dy);
}

and back in the calling code, you can use it this way:

Point p1 = new Point(2,2);
Point p2 = new Point(4,4);
System.out.println("Distance between them is " + Point.distance(p1, p2));

Answer 5

You can use a Maths function for this:

public Point midpoint() {
    //Calculate the difference between the old and new x/y
    double dx = p1.getX() - p2.getX();
    double dy = p1.getY() - p2.getY();

    double newX = Math.pow(dx, 2D);
    double newY = Math.pow(dz, 2D);
    return new Point(newX, newZ);
}

Math.pow handles the issues with negative values and etc. For you, using Math.pow gives you a safe method because it has a lot of checks built inside.

Answer 6

You can use the Pythagorean Theorem, as other said. Here is a visually demostration from the Wolfram Demostration Project.

alt text http://demonstrations.wolfram.com/DistanceBetweenTwoPoints/HTMLImages/index.en/popup_5.jpg

Answer 7

In your second class, it looks like you're trying to set the values of x and y that are used to construct your mp variable. All your formulas are correct, but you need to consider the order that everything is executed. In the code as it is, it's creating the x and y variables, which start out as 0, then the various Points. x and y are still 0, so mp is set to Point(0, 0).

What you probably want to do is change the return type of midpoint to Point, so that when you call that function, you get back a Point. Then you can create a new Point object with the values you calculate. It should look more like this:

public Point midpoint() {
    // calculate the middle x and y
    double x = (p1.getX() + p2.getX()) / 2;
    double y = (p1.getY() + p2.getY()) / 2;
    // now make a new Point with those values, and return it
    return new Point(x, y);
}