Arrays and Records — Collecting Values

Student: OK, so we can make collections of strings. I’m not impressed.

Prof: Oh?

Student: I started this conversation wanting to make big collections of like, lots of images or course rosters like you keep suggesting.

Prof: I see, that makes sense. Let me think of a good first example… OK how about this. We’ll have a class that stores an array of points, representing a bunch of positions on a coordinate plane. In, say, sports analytics, this could be positions of players on a field. In a physics experiment, it could be positions of particles.

record Point(double x, double y) { }
record Positions(Point[] points) { }

Student: OK, so now you’ve used Point[] as the type of a field?

Prof: That’s right. Why don’t you try constructing some example points and a Positions instance?

Student: Sure.

record Point(double x, double y) { }
record Positions(Point[] points) { }
Point p1 = new Point(3.0, 4.0);
Point p2 = new Point(7.0, 8.0);
Point p3 = new Point(5.0, -1.0);
Positions posns = new Positions(p1, p2, p3);

Student: Huh, that didn’t work. Why not?

Prof: How many fields does Positions have?

Student: Three?

Prof: Setting aside your example for a minute, just look at record Positions(Point[] points). How many fields?

Student: Oh, just one. Ohhhh so I have to make an array and give just one array to the constructor? Like this:

record Point(double x, double y) { }
record Positions(Point[] points) { }
Point p1 = new Point(3.0, 4.0);
Point p2 = new Point(7.0, 8.0);
Point p3 = new Point(5.0, -1.0);
Point[] parray = { p1, p2, p3 };
Positions posns = new Positions(parray);

Prof: You got it. OK so let’s think of some interesting methods to write for Positions. Can you think of any?

Student: Um… maybe like which points are the closest to each other or furthest?

Prof: Great. Let’s do “which point is closest to a given point”, how about that? That could make sense for a lot of tasks, like which player is closest to a goal, or which particle is closest to some stimulus in an experiment.

Student: Sure, I buy that.

Prof: Let’s assume Point defines double distanceTo(Point other); we’ve seen things like that before, and focus on this new method in the Positions record. What do you think the method header looks like?

Student: OK, I need a name and the arguments. So probably the name is like closestTo and then the argument is a Point?

Prof: And you need one more thing – the return type.

Student: Oh, probably it returns a point, like the one it found that is closest.

Prof: Great. What’s a good placeholder value for the method?

Student: Maybe just like… the first point in the array?

Prof: Sure – go ahead and let’s get that in. Maybe at the same time write a few examples of calling it with comments for what you expect?

record Point(double x, double y) {
  double distanceTo(Point other) { … }
}
record Positions(Point[] points) {
  Point closestTo(Point target) {
    return points[0];
  }
}
Point p1 = new Point(3.0, 4.0);
Point p2 = new Point(7.0, 8.0);
Point p3 = new Point(5.0, -1.0);
Point[] parray = { p1, p2, p3 };
Positions posns = new Positions(parray);
Point closestOrigin = posns.closestTo(new Point(0.0, 0.0));
Point closesttTopRight = posns.closestTo(new Point(9.0, 9.0));

Prof: Excellent! So both of these are always returning the 3.0, 4.0 point for now because that’s the first one. Now we need to fill it in.

Student: Awesome!

Practice

Make order a PizzaOrder that holds both pizzas: fill in the type of the pizzas field, define an array of the pizzas on the empty line, and pass it to the constructor.

record Pizza(double inches, int costCents) { }
record PizzaOrder( pizzas) { }
Pizza big = new Pizza(16, 1000);
Pizza small = new Pizza(10, 500);

PizzaOrder order = new PizzaOrder();