Lab 2: Getting Comfy With Objects

Java lets us program with objects. We use classes, one per file, to describe how these objects work -- a blueprint, if you will. Here is the code for a simple class that represents a 2-dimensional point:

public class Point2d {
    
    // Some private data members
    private int xCoordinate;
    private int yCoordinate;

    // Regular constructor:  takes two arguments.
    public Point2d(int x, int y) {
	this.xCoordinate = x;
	this.yCoordinate = y;
    }

    // No-argument constructor:  defaults to a point at the origin.
    public Point2d() {
	this.xCoordinate = 0;
	this.yCoordinate = 0;
    }

    // "Accessor" methods
    public int getX() { return xCoordinate; }
    public int getY() { return yCoordinate; }

    // "Mutator" methods
    public void setX(int value) { this.xCoordinate = x; }
    public void setY(int value) { this.yCoordinate = y; }
}

This code must be saved in a file called Point2d.java, which is provided for your consumption. You might save a copy of it to your working directory and build off of it for this lab.

Recall that we can instantiate our class anywhere else in our code by calling any of the constructors we've defined, like so:

    Point2d myPoint = new Point();            // creates a point at (0,0)
    Point2d myOtherPoint = new Point(5,3);    // creates a point at (5,3)
    Point2d aThirdPoint = new Point();

Be careful: myPoint != aThirdPoint! This is because the comparison operator (and its opposite) compare the two object references. Each of myPoint and aThirdPoint refers to a different instance of the Point2d class. It is true in this case that the private data members are the same, and as humans we would probably agree that these two Point2d's are "equal."

To test for object equality and not reference equality, define a method of the Point2d class called equals which takes another Point2d as an argument and explicitly compares the internal data members:

    public boolean equals(Point2d other) {
	if (this.xCoordinate != other.getX()) return false;
	if (this.yCoordinate != other.getY()) return false;
	
	// okay, these are "equal"
	return true;
    }

Your task:

1. Create a new class Point3d to represent, you guessed it, points in three dimensional Euclidean space. It should be possible to:
   • create a new Point3d described by any three floating-point values,
   • create a new Point3d at (0.0, 0.0, 0.0) by default,
   • access and mutate all three values individually, and
   • compare two Point3d's for geometric equality using equals().

   It should not be possible to:

   • directly access the internal data members of any Point3d object.

2. Furthermore, add an additional method distanceTo which takes as an argument another Point3d, computes a floating-point approximation of the straight-line distance between the two points, and returns that value.

3. Create a second class called Lab2 that exists primarily to house the static method main. Remember that main must be public, have a void return type, and accept an array of Strings as an argument. Inside this class, add some functionality:
   • Input three ordered triples from the user, each representing the coordinates of one point in 3-space. Generate three Point3d's from this data. (For now, assume the user will not enter invalid data.) Print a prompt before inputting each of the triples.
   • Write a second static method computeArea which takes three Point3d's and computes the area within the triangle bounded by them. (You may wish to use Heron's formula.) Return this area as a float or double, of course.
   • Use the data and code you gathered and wrote above to determine the area and print that out for the user's consumption. Before you call computeArea, however, test for value equality amongst any of the three Point3d's. If any of them are "equal", state this and do not compute an area.

4. Compile both of your source files together like so:

   > javac Point3d.java Lab2.java
   

   and then run your Lab2 program, testing it with several sample triangles.

5. As a way of standardizing the submit procedure, please place your lab 2 source code in the directory

    ~/cs11/java/lab2
   

   on the CS cluster and make sure it is world-readable. (If you don't know, it is.)