Learning Processing: A Beginner's Guide to Programming Images ...

432 Learning Processing
23.5
Other Useful Java Classes: Rectangle
Th e second helpful Java class we will examine is the Rectangle class:
http://java.sun.com/j2se/1.4.2/docs/api/java/awt/Rectangle.html .
A Java Rectangle specifi es an area in a coordinate space that is enclosed by the Rectangle object’s top-left point
( x , y ), its width, and its height. Sound familiar? Of course, the Processing function rect( ) draws a rectangle with
precisely the same parameters. Th e Rectangle class encapsulates the idea of a rectangle into an object.
A Java Rectangle has useful methods, for instance, contains( ). contains( ) off ers a simple way of checking if
a point or rectangle is located inside that rectangle, by receiving an x and y coordinate and returning true
or false, based on whether the point is inside the rectangle or not.
Here is a simple rollover implemented with a Rectangle object and contains( ) . See Example 23.3 .
Example 23-3: Using a java.awt.Rectangle object
Rectangle rect1, rect2;
void setup() {
size(200,200);
rect1 = new Rectangle(25,25,50,50);
rect2 = new Rectangle(125,75,50,75);
}
void draw() {
background(255);
stroke(0);
if (rect1.contains(mouseX,mouseY)) {
fill(200);
} else {
fill(100);
}
The contains() function is used to determine
if the mouse is located inside the rectangle.
rect(rect1.x, rect1.y, rect1.width,rect1.height);
This sketch uses two
Rectangle objects.
The arguments for
the constructor
(x,y,width,height) are
documented in the Java
reference: http://java.sun.
com/j2se/1.4.2/docs/api/
java/awt/Rectangle.html.
// Repeat for the second Rectangle
// (of course, we could use an array or ArrayList here!)
if (rect2.contains(mouseX,mouseY)) {
fill(200);
} else {
fill(100);
}
rect(rect2.x, rect2.y, rect2.width,rect2.height);
}
fi g. 23.5
A Rectangle object only
knows about the variables
associated with a rectangle.
It cannot display one. So
we use Processing’s rect()
function in combination with
the Rectangle’s data.
Let’s have some fun and combine the ArrayList “ particle ” example with the Rectangle “ rollover. ” In
Example 23-4, particles are made every frame and pulled down by gravity toward the bottom of the
window. If they run into a rectangle, however, they are caught. Th e particles are stored in an ArrayList
and a Rectangle object determines if they have been caught or not. (Th is example contains answers to
Exercise 23-2.)