Question

The code below appears in my book in the chapter about Methods. I’m a little confused about a couple of things.

1. Is my understanding correct when I believe that the run() method is calling the createFilledCircle method?
2. Is the run() method the receiver and the createFilledCircle the sender?
3. for the three add(createFilledCircles...red,yellow and green); how does the programmer know what information is permitted in the argument? Is the format of (x location, y location, width of figure, height of figure) being used in the add(createFilledCircle)?

import acm.program.*;
import acm.graphics.*;
import java.awt.*;

public class StopLight extends ConsoleProgram {
  public void run() {
    double cy = getWidth() / 2 ;
    double cx=  getHeight() / 2; 

    double fx = cx - (FRAME_WIDTH / 2);
    double fy = cy - (FRAME_HEIGHT /2 );

    double dy = (FRAME_HEIGHT / 4 ) + (LAMP_RADIUS / 2);

    GRect frame = new GRect (fx, fy, FRAME_WIDTH, FRAME_HEIGHT);
    frame.setFilled(trye);
    frame.setColor(Color.GRAY);
    add(frame);

    add(createFilledCircle(cx, cy-dy, LAMP_RADIUS, Color.RED));
    add(createFilledCircle(cx, cy, LAMP_RADIUS, Color.YELLOW));
    add(createFilledCircle(cx, cy + dy, LAMP_RADIUS, Color.GREEN));
  }

  private GOval createFilledCircle (double x, double y, double r, Color color) {
    GOval circle = new GOval (x -r, y -r, 2 * r, 2 * y );
    circle.setFilled(true);
    circle.setColor(color);
    return circle; 
  }

  private static final double FRAME_WIDTH = 50; 
  private static final double FRAME_HEIGHT = 100;
  private static final LAMP_RADIUS = 10; 
}

Answer 1

1) Is my understanding correct when I believe that the run() method is calling the createFilledCircle method?

Yes.

2) Is the run() method the receiver and the createFilledCircle the sender?

I don’t know what you mean by “receiver” and “sender”. Those are not terms that people normally use when talking about methods being called.

(It looks like teminology from the Smalltalk programming language, which was an early object oriented language. In the light of that, it’s exactly the other way around: you’d say run is the sender and createFilledCircle is the receiver.)

3) for the three add(createFilledCircles…red,yellow and green); how does the programmer know what information is permitted in the argument? Is the format of (x location, y location, width of figure, height of figure) being used in the add(createFilledCircle)?

The declaration of the createFilledCircle method specifies what parameters that method needs: three double values and a Color value.

In a line such as this one:

add(createFilledCircle(cx, cy-dy, LAMP_RADIUS, Color.RED));

what happens is that createFilledCircle is called first, with the arguments cx, cy-dy, LAMP_RADIUS, Color.RED, and then the return value of the method createFilledCircle, which is a GOval value, is passed to the add method. It’s the same as this:

GOval result = createFilledCircle(cx, cy-dy, LAMP_RADIUS, Color.RED);
add(result);