Shiffman Daniel. Learning processing
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432 Learning Processing
23.5 Other Useful Java Classes: Rectangle
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. 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);
}
rect(rect1.x, rect1.y, rect1.width,rect1.height);
// 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);
}
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. e particles are stored in an ArrayList
and a Rectangle object determines if they have been caught or not. ( is example contains answers to
Exercise 23-2.)
fi g. 23.5
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.
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.
The contains() function is used to determine
if the mouse is located inside the rectangle.
Java 433
Example 23-4: Super fancy ArrayList and rectangle particle system
// Declaring a global variable of type ArrayList
ArrayList particles;
// A "Rectangle" will suck up particles
Rectangle blackhole;
void setup() {
size(200,200);
blackhole = new Rectangle(50,150,100,25);
particles = new ArrayList();
smooth();
}
void draw() {
background(255);
// Displaying the Rectangle
stroke(0);
fill(175);
rect(blackhole.x, blackhole.y, blackhole.width,blackhole.height);
// Add a new particle at mouse location
particles.add(new Particle(mouseX,mouseY));
// Loop through all Particles
for (int i = particles.size() – 1; i > = 0; i – – ){
Particle p = (Particle) particles.get(i);
p.run();
p.gravity();
p.display();
if (blackhole.contains(p.x,p.y)) {
p.stop();
}
if (p.finished()) {
particles.remove(i);
}
}
}
// A simple Particle Class
class Particle {
float x;
float y;
float xspeed;
float yspeed;
float life;
// Make the Particle
Particle(float tempX, float tempY) {
x = tempX;
y = tempY;
xspeed = random(–1,1);
yspeed = random(–2,0);
life = 255;
}
fi g. 23.6
If the Rectangle contains the location of the
Particle, stop the Particle from moving.
434 Learning Processing
// Move
void run() {
x = x + xspeed;
y = y + yspeed;
}
// Fall down
void gravity() {
yspeed + = 0.1;
}
// Stop moving
void stop() {
xspeed = 0;
yspeed = 0;
}
// Ready for deletion
boolean finished() {
life – = 2.0;
if (life < 0) return true;
else return false;
}
// Show
void display() {
stroke(0);
fill(0,life);
ellipse(x,y,10,10);
}
}
Exercise 23-3: Write a Button class that includes a Rectangle object to determine if the
mouse has clicked inside the button’s area. ( is is an extension of Exercise 9-8 .)
23.6 Exception (Error) Handling
In addition to a very large library of useful classes, the Java programming language includes some features
beyond what we have covered while learning Processing . We will explore one of those features now:
exception handling .
Programming mistakes happen. We have all seen them.
java.lang.ArrayIndexOutOfBoundsException
java.io.IOException: openStream() could not open file.jpg
java.lang.NullPointerException
The Particle has a “life” variable which
decreases. When it reaches 0 the Particle
can be removed from the ArrayList.
Java 435
It is sad when these errors occur. Really, it is. e error message prints out and the program sputters to a
halt, never to continue running again. Perhaps you have developed some techniques for protecting against
these error messages. For example:
if (index < somearray.length) {
somearray[index] = random(0,100);
}
e above is a form of “ error checking. ” e code uses a conditional to check that the index is valid
before an element at that index is accessed. e above code is quite conscientious and we should all aspire
to be so careful.
Not all situations are as simple to avoid, however, and this is where exception handling comes into play.
Exception handling refers to the process of handling exceptions , out of the ordinary events (i.e., errors)
that occur during the execution of a program.
e code structure for exception handling in Java is known as try catch . In other words, “ Try to run some
code. If you run into a problem, however, catch the error and run some other code. ” If an error is caught
using try catch, the program is allowed to continue. Let’s look at how we might rewrite the array index
error checking code try catch style.
try {
somearray[index] = 200;
} catch (Exception e) {
println( "Hey, that’s not a valid index! ");
}
e above code catches any possible exception that might occur. e type of error caught is the generic
Exception. However, if you want to execute certain code based on a specifi c exception, you can, as the
following code demonstrates.
try {
somearray[index] = 200;
} catch (ArrayIndexOutOfBoundsException e) {
println ( "Hey, that’s not a valid index! ");
} catch (NullPointerException e) {
println( "I think you forgot to create the array! ");
} catch (Exception e) {
println( "Hmmm, I dunno, something weird happened! ");
e.printStackTrace();
}
e above code, nevertheless, does nothing more than print out custom error messages. ere are
situations where we want more than just an explanation. For example, take the examples from Chapter 18
where we loaded data from a URL path. What if our sketch had failed to connect to the URL? It would
have crashed and quit. With exception handling, we can catch that error and fi ll the array with Strings
manually so that the sketch can continue running.
The code that might produce an error
goes within the “try” section. The
code that should happen if an error
occurs goes in the “catch” section.
Different “catch” sections
catch different types of
exceptions. In addition, each
exception is an object and
can therefore have methods
called on it. For example:
e.printStackTrace();
displays more detailed
information about the
exception.
436 Learning Processing
String[] lines;
String url = " http://wwww.rockinurl.com " ;
try {
lines = loadStrings(url);
} catch (Exception e) {
lines = new String[3];
lines[0] = " I could not connect to " + url + " !!! " ;
lines[1] = " But here is some stuff to work with " ;
lines[2] = " anyway. " ;
}
println(lines);
23.7 Java Outside of Processing
Here we are, the very last section of this book. You can can take a break now if you want. Maybe go
outside and have a walk. A little jog around the block even. It will be good for you.
OK, back now? Great, let’s go on.
As we close out this chapter and the book, we will cover one fi nal topic: what to do if and when you
decide you want to start coding outside of Processing .
But why would I ever want to do this?
One instance that would merit another programming environment is in the case of making an application
that does not involve any graphics! Perhaps you need to write a program that takes all of your fi nancial
info from spreadsheets and logs it in a database. Or a chat server that runs in the background on your
computer. While both of these could be written in the Processing environment, they do not necessarily
need any of the features of Processing to run.
In the case of developing larger and larger projects that involve many classes, the Processing environment
can become a bit diffi cult to use. For example, what if your project involves, say, 20 classes. Creating a
Processing sketch with 20 tabs (and what if it were 40? or 100?) can be hard to manage, let alone fi t on
the screen. In this case, a development environment designed for large-scale Java projects would be better.
Since Processing is Java, you can still use all of the functions available in Processing in other development
environments by importing the core libraries.
So, what do you do? First, I would say do not rush. Enjoy Processing and take the time to feel comfortable
with your own coding process and style before getting yourself wrapped up in the myriad of issues and
questions that come along with programming in Java.
If you feel ready, a good fi rst step is to just take some time browsing the Java web site:
http://java.sun.com/
Start with one of the tutorials:
http://java.sun.com/docs/books/tutorial/
If a problem occurs connecting
to the URL, we will just fi ll the
array with dummy text so that
the sketch can still run.
Java 437
e tutorials will cover the same material found in this book, but from a pure Java perspective.
e next step would be to just try compiling and running a “ Hello World ” Java program.
public class HelloWorld {
public static void main(String[] args) {
System.out.println( "Hello World. I miss you Processing. ");
}
}
e Java site has tutorials which explain all the elements in a Hello World program, as well as provide
instructions on how to compile and run it on the “ command line. ”
http://java.sun.com/docs/books/tutorial/getStarted/index.html
Finally, if you are feeling ambitious, go and download Eclipse:
http://www.eclipse.org/
fi g. 23.7 A Processing sketch in Eclipse
438 Learning Processing
Eclipse is a development environment for Java with many advanced features. Some of those features will
give you a headache and some will make you wonder how you ever programmed without Eclipse. Just
remember, when you started with Processing in Chapter 2, you were probably up and running with your
fi rst sketch in less than fi ve minutes. With something like Eclipse , you could need a much longer time to
get started. See Figure 23.7 .
Visit this book’s web site for links and tutorials about how to run a Processing sketch within the Eclipse
environment.
anks for reading. Feedback is encouraged and appreciated, so come on down to http://www.
learningprocessing.com and sound off !
Appendix 439
Appendix: Common Errors
is appendix off ers a brief overview of common errors that occur in Processing , what those errors mean
and why they occur. e language of error messages can often be cryptic and confusing, mostly due to
their origins in the Java programming language.
For these error messages, we will use the following conventions:
Variables in the error messages will be named “ myVar ” .
Arrays in the error messages will be named “ myArray ” .
If the error is specifi cally related to an object variable, the variable will be named “ myObject ” .
If the error is related to a class, the class will be named “ ing ” .
If the error is related to a function, the function will be named “ function ” .
ERROR PAGE NO.
unexpected token: _____ 440
Found one too many { characters without a } to match it. 440
No accessible fi eld named “ myVar ” was found in type “ Temporary_####_#### ” 441
e variable “ myVar ” may be accessed here before having been defi nitely assigned a
value.
442
java.lang.NullPointerException 442
Type “ ing ” was not found. 444
Perhaps you wanted the overloaded version “ type function (type $1, type $2, … ); ”
instead?
445
No method named “ fnction ” was found in type “ Temporary_####_#### ” . However,
there is an accessible method “ function ” whose name closely matches the name
“ fnction ” .
445
No accessible method with signature “ function (type, type, … ) ” was found in type
“ Temporary_3018_2848 ” .
445
java.lang.ArrayIndexOutOfBoundsException 446
Error:
unexpected token: something
Translation:
I’m expecting a piece of code here that you forgot, most likely some punctuation, like a semicolon,
parenthesis, curly bracket, and so on.
is error is pretty much always caused by a typo. Unfortunately, the line of code that the error points to
can be misleading. e “ something ” is sometimes correct and the actual error is caused by line before or
after. Any time you forget or include an improper comma, semicolon, quote, parenthesis, bracket, and so
on, you will get this error. Here are some examples:
ERROR CORRECTED
void setup() {
for (int i = 0; i < 10; i + + ) {
if (i > 5) {
line(0,i,i,0);
println( " " );
}
}
void setup() {
for (int i = 0; i < 10; i + + ) {
if (i > 5) {
line(0,i,i,0);
println( " i is greater than 5 " );
}
}
}
Error:
Found one too many { characters without a } to match it.
Translation:
You forgot to close a block of code, such as an if statement, a loop, a function, a class, and so on.
Any time you have an open curly bracket in your code ( “ { ” ), you must have a matching close curly bracket
( “ } ” ). And since blocks of code are often nested inside each other, it is easy to forget one by accident,
resulting in this error.
440 Learning Processing
ERROR CORRECTED
int val = 5 int val = 5;
if (x < 5 {
ellipse(0,0,10,10);
}
if (x < 5) {
ellipse (0,0,10,10);
}
for (int i = 0, i < 5,i + + ) {
println(i);
}
for (int i = 0; i < 5; i + +)
println(i);{
Missing semicolon.
Missing close parenthesis.
For loop should be separated
by semicolons, not commans.
Missing a close curly bracket!
i is greater than 5
Appendix 441
Error:
No accessible fi eld named “ myVar ” was found in type “ Temporary_7665_1082 ”
Translation:
I do not know about the variable “ myVar. ” It is not declared anywhere I can see.
is error will happen if you do not declare a variable. Remember, you can’t use a variable until you have
said what type it should be. You will get the error if you write:
myVar = 10;
instead of:
int myVar = 10;
Of course, you should only declare the variable once, otherwise you can run into trouble. So this is
perfectly OK:
int myVar = 10;
myVar = 20;
is error can also occur if you try to use a local variable outside of the block of code where it was
declared. For example:
if (mousePressed) {
int myVar = 10;
}
ellipse(myVar,10,10,10);
Here is a corrected version:
int myVar = 0;
if (mousePressed) {
myVar = 10;
}
ellipse(myVar,10,10,10);
Or if you have declared a variable in setup( ) , but try to use it in draw( ) .
void setup() {
int myVar = 10;
}
void draw() {
ellipse(myVar,10,10,10);
}
Corrected:
int myVar = 0;
void setup() {
myVar
= 10;
}
ERROR! myVar is local to the if statement so
it cannot be accessed here.
OK! The variable was declared.
OK! The variable is declared outside of the
if statement.
ERROR! myVar is local to the setup() so it
cannot be accessed here.