Mehran Sahami Handout #20A CS 106A October 17, 2007

Mehran Sahami Handout #20A
CS 106A October 17, 2007
Solution to Section #3
Portions of this handout by Eric Roberts and Patrick Young.
1. True/False questions
a) The value of a local variable named i has no direct relationship with that of a
variable named i in its caller.
Answer: True
b) The value of a parameter named x has no direct relationship with that of a variable
named x in its caller.
Answer: True
2. Tracing method execution
The output of Hogwarts.java is:
snitch: x = 4004, y = 1001
quaffle: x = 2003, y = 1, z = 1001
bludger: x = 1001, y = 2001, z = 2003
– 2 –
3. Random circles
/*
* File: RandomCircles.java
* ------------------------
* This program draws a set of 10 circles with different sizes,
* positions, and colors. Each circle has a randomly chosen
* color, a randomly chosen radius between 5 and 50 pixels,
* and a randomly chosen position on the canvas, subject to
* the condition that the entire circle must fit inside the
* canvas without extending past the edge.
*/
import acm.program.*;
import acm.graphics.*;
import acm.util.*;
public class RandomCircles extends GraphicsProgram {
/** Number of circles */
private static final int NCIRCLES = 10;
/** Minimum radius */
private static final double MIN_RADIUS = 5;
/** Maximum radius */
private static final double MAX_RADIUS = 50;
public void run() {
for (int i = 0; i < NCIRCLES; i++) {
double r = rgen.nextDouble(MIN_RADIUS, MAX_RADIUS);
double x = rgen.nextDouble(0, getWidth() - 2 * r);
double y = rgen.nextDouble(0, getHeight() - 2 * r);
GOval circle = new GOval(x, y, 2 * r, 2 * r);
circle.setFilled(true);
circle.setColor(rgen.nextColor());
add(circle);
}
}
/* Private instance variable */
private RandomGenerator rgen = RandomGenerator.getInstance();
}
– 3 –
4. Drawing lines
/*
* File: RubberBanding.java
* ------------------------
* This program allows users to create lines on the graphics
* canvas by clicking and dragging with the mouse. The line
* is redrawn from the original point to the new endpoint, which
* makes it look as if it is connected with a rubber band.
*/
import acm.graphics.*;
import acm.program.*;
import java.awt.event.*;
/** This class allows users to drag lines on the canvas */
public class RubberBanding extends GraphicsProgram {
public void run() {
addMouseListeners();
}
/** Called on mouse press to create a new line */
public void mousePressed(MouseEvent e) {
double x = e.getX();
double y = e.getY();
line = new GLine(x, y, x, y);
add(line);
}
/** Called on mouse drag to reset the endpoint */
public void mouseDragged(MouseEvent e) {
double x = e.getX();
double y = e.getY();
line.setEndPoint(x, y);
}
/* Private instance variables */
private GLine line;
}