Shiffman Daniel. Learning processing

422 Learning Processing

fill(255);

fill(255,0,255);

fill(0,0,255,150);

 e ability to deﬁ ne functions with the same name (but diﬀ erent arguments) is known as overloading .

With ﬁ ll( ) , for example, Processing does not get confused about which deﬁ nition of ﬁ ll( ) to look up, it

simply ﬁ nds the one where the arguments match. A function’s name in combination with its arguments

is known as the function’s signature —it is what makes that function deﬁ nition unique. Let’s look at an

example that demonstrates how overloading can be useful.

Let’s say you have a Fish class. Each Fish object has a location: x and y .

class Fish {

float x;

float y;

What if, when you make a Fish object, you sometimes want to make one with a random location and

sometimes with a speciﬁ c location. To make this possible, we could write two diﬀ erent constructors:

Fish() {

x = random(0,width);

y = random(0,height);

}

Fish(float tempX, float tempY) {

x = tempX;

y = tempY;

}

When you create a Fish object in the main program, you can do so with either constructor:

Fish fish1 = new Fish();

Fish fish2 = new Fish(100,200);

Overloading allows us to deﬁ ne two

constructors for the same object

(as long as these constructors take

different arguments).

If two constructors are deﬁ ned,

an object can be initialized using

either one.

Java 423

23 Java

“ I l o v e c o ﬀ ee, I love tea, I love the Java Jive and it loves me. ”

—Ben Oakland and Milton Drake

In this chapter:

– Processing is really Java.

– If we did not have Processing what would our code look like?

– Exploring the Java API.

– Some useful examples: ArrayList, and Rectangle.

– Exception (error) handling—try and catch.

– Beyond Processing ?

23.1 Revealing the Wizard

Perhaps, all along while reading this book, you have been thinking: “ Gosh, this is so awesome. I love

programming. I am so glad I am learning Processing . I can’t wait to go on and learn Java! ”  e funny thing

is, you do not really need to learn Java. Pulling back the curtain of Processing , what we will discover is that

we have been learning Java all along. For example, in Java, you:

• Declare, initialize, and use variables the same way.

• Declare, initialize, and use arrays the same way.

• Employ conditional and loop statements the same way.

• Define and call functions the same way.

• Create classes the same way.

• Instantiate objects the same way.

Processing , of course, gives us some extra stuﬀ for free (and simpliﬁ es some stuﬀ here and there) and this

is why it is a great tool for learning and for developing interactive graphics projects. Here is a list of some

of what Processing oﬀ ers that is not as easily available with just plain Java.

• A set of functions for drawing shapes.

• A set of functions for loading and displaying text, images, and video.

• A set of functions for 3D transformations.

• A set of functions for mouse and keyboard interaction.

• A simple development environment to write your code.

• A friendly online community of artists, designers, and programmers!

23.2 If we did not have Processing , what would our code look like?

In Chapter 2, we discussed the compilation and execution process—this is what happens when you

press the play button and turn your code into a window with graphics. Step 1 of this process involves

“ translating ” your Processing code into Java. In fact, when you export to applet, this same process

occurs and you will notice that along with the ﬁ le “ Sketchname.pde, ” a new ﬁ le will appear named

424 Learning Processing

“ SketchName.java. ”  is is your “ translated ” code, only translation is somewhat of a misnomer since very

little changes. Let’s look at an example:

// Randomly Growing Square

float w = 30.0; // Variable to keep track of size of rect

void setup() {

size(200, 200);

}

void draw() {

background(100);

rectMode(CENTER);

fill(255);

noStroke();

rect(mouseX,mouseY,w,w); // Draw a rect at mouse location

w + = random( – 1,1); // Randomly adjust size variable

}

Exporting the sketch, we can open up the Java ﬁ le and look at the Java source.

// Randomly Growing Square with Java Stuff

import processing.core.*;

import java.applet.*;

import java.awt.*;

import java.awt.image.*;

import java.awt.event.*;

import java.io.*;

import java.net.*;

import java.text.*;

import java.util.*;

import java.util.zip.*;

float w = 30.0f; // Variable to keep track of size of rect

public void setup() {

size(200, 200);

}

public void draw() {

background(100);

rectMode(CENTER);

fill(255);

noStroke();

rect(mouseX,mouseY,w,w); // draw a rect at mouse location

w + = random( – 1,1); // randomly adjust size variable

}

It is clear that little has changed, rather, some code has been added before and after the usual setup( ) and

draw( ) stuﬀ .

• Import Statements —At the top, there are a set of import statements allowing access to certain

libraries. We have seen this before when using Processing libraries. If we were using regular Java

What we are used to seeing, our

regular old Processing code.

The translated Java code has

some new stuff at the top, but

everything else stays the same.

public class JavaExample extends PApplet {

Java 425

instead of Processing , specifying all libraries would always be required. Processing , however, assumes a

base set of libraries from Java (e.g., java.applet.*) and from Processing (e.g., processing.core.* ) which is

why we do not see these in every sketch.

• public —In Java, variables, functions, and classes can be “ public ” or “ private. ” This designation

indicates what level of access should be granted to a particular piece of code. It is not something

we have to worry much about in the simpler Processing environment, but it becomes an important

consideration when moving on to larger Java programs. As an individual programmer, you are

most often granting or denying access to yourself, as a means for protecting against errors. We

encountered some examples of this in Chapter 22’s discussion about encapsulation.

• class JavaExample —Sound somewhat familiar? Java, it turns out, is a true object-oriented language.

There is nothing written in Java that is not part of a class! We are used to the idea of the Zoog class,

Car class, PImage class, and so on, but it is important to note that the sketch as a whole is a class,

too! Processing fills this stuff in for us so we do not have to worry about classes when we are first

learning to program.

• extends PApplet —Well, after reading Chapter 22, we should be quite comfortable with what this

means. This is just another example of inheritance. Here, the class JavaExample is a child of the class

PApplet (or, equivalently, PApplet is the parent of JavaExample). PApplet is a class developed by

the creators of Processing and by extending it, our sketch has access to all of the Processing goodies—

setup( ), draw( ) , mouseX , mouseY , and so on. This little bit of code is the secret behind how almost

everything works in a Processing sketch.

Processing has served us so well because it eliminates the need to worry about the above four elements, all

the while providing access to the beneﬁ ts of the Java programming language.  e rest of this chapter will

show how we can begin to make use of access to the full Java API. (We brieﬂ y began this journey when

we worked with String parsing in Chapters 17 and 18.)

23.3 Exploring the Java API

 e Processing reference quickly became our best friend forever while learning to program.  e Java API

will start oﬀ more as an acquaintance we bump into from time to time.  at acquaintance might turn into

a really excellent friend someday, but for now, small doses will be just ﬁ ne.

We can explore the full Java documentation by visiting:

http://java.sun.com/

 ere, we can click over to API speciﬁ cations:

http://java.sun.com/reference/api/index.html

and ﬁ nd a selection of versions of Java. On a Mac, Processing will run with the selected version of Java

(run Java Preferences, found, e.g., in: /Applications/Utilities/Java/J2SE 5.0/). On a PC, the Processing

“ standard ” download comes with Java version 1.4.2 (the Windows expert version allows you to install

your own version of Java). Versions of Java will change in the future with updated info at processing.org.

In any case, while there are diﬀ erences between the version of Java, for what we are doing, they will not

be terribly relevant, and we can look at the API for J2SE 1.4.2 for pretty much whatever we want to do.

See Figure 23.1 .

426 Learning Processing

ﬁ g. 23.1 http://java.sun.com/j2se/1.4.2/docs/api/

And so, very quickly, you will ﬁ nd yourself completely lost. And that is OK.  e Java API is huge.

Humongous. It is not really meant to be read or even perused . It is really pure reference, for looking up

speciﬁ c classes that you know you need to look up.

For example, you might be working on a program that requires sophisticated random number

generation beyond what random( ) can do, and you overheard a conversation about the class “ Random ”

and thought “ Hey, maybe I should check that out! ”  e reference page for a speciﬁ c class can be

found by scrolling down the “ All Classes ” list or else by selecting the right package (in this case the

package java.util). A package, much like a library, is a collection of classes (the API organizes

them by topic).  e easiest way to ﬁ nd a class, though, is to just type the name of the class and Java

(i.e., “ Java Random ” ) into google.  e documentation page is just about always the ﬁ rst result.

See Figure 23.2 .

Java 427

Just as with the Processing reference, the Java page includes an explanation of what the class does, the

Constructors for creating an object instance, and available ﬁ elds (variables) and methods (functions). Since

Random is part of the java.util package (which Processing imports by assumption), we do not need to

explicitly write an import statement to use it.

Here is some code that makes a Random object and calls the function nextBoolean( ) on it to get a

random true or false.

Example 23-1: Using java.util.Random instead of random()

Random r;

void setup() {

size(200,200);

r = new Random();

}

ﬁ g. 23.2

Declaring a Random object and

calling the constructor as found

at http://java.sun.com/j2se/1.4.2/

docs/api/java/util/Random.html

428 Learning Processing

void draw() {

boolean trueorfalse = r.nextBoolean();

if (trueorfalse) {

background(0);

} else {

background(255);

}

Exercise 23-1: Visit the Java reference page for Random and use it to get a random integer

between 0 and 9. http://java.sun.com/j2se/1.4.2/docs/api/java/util/Random.html .

23.4 Other Useful Java Classes: ArrayList

In Chapter 6, we learned how to use an array to keep track of ordered lists of information. We created

an array of N objects and used a “ for ” loop to access each element in the array.  e size of that array was

ﬁ xed—we were limited to having N and only N elements.

 ere are alternatives. One option is to use a very large array and use a variable to track how much of the

array to use at any given time (see Chapter 10’s rain catcher example). Processing also oﬀ ers expand( ) ,

contract( ), subset( ) , splice( ) and other methods for resizing arrays. It would be great, however, to have a

class that implements a ﬂ exibly sized array, allowing items to be added or removed from the beginning,

middle, and end of that array.

 is is exactly what the Java class ArrayList, found in the java.util package, does.

 e reference page is here: http://java.sun.com/j2se/1.4.2/docs/api/java/util/ArrayList.html .

Using an ArrayList is conceptually similar to a standard array, but the syntax is diﬀ erent. Here is some

code (that assumes the existence of a class “ Particle ” ) demonstrating identical results, ﬁ rst with an array,

and second with an ArrayList. All of the methods used in this example are documented on the JavaDoc

reference page.

// THE STANDARD ARRAY WAY

int MAX = 10;

//declaring the array

Particle[] parray = new Particle[MAX];

// Initialize the array in setup

void setup() {

for (int i = 0; i < parray.length; i + + ) {

parray[i] = new Particle();

}

// Loop through the array to call methods in draw

void draw() {

for (int i = 0; i < parray.length; i + + ) {

Calling a function found at http://java.sun.com/

j2se/1.4.2/docs/api/java/util/Random.html

The Standard Array Way

This is what we have been

doing all along, accessing

elements of the array via

an index and brackets—[].

Java 429

Particle p = parray[i];

p.run();

p.display();

}

// THE NEWFANGLED ARRAYLIST WAY

int MAX = 10;

// Declaring and creating the ArrayList instance

ArrayList plist = new ArrayList();

void setup() {

for (int i = 0; i < MAX; i + + ){

plist.add(new Particle());

}

void draw() {

for (int i = 0; i < plist.size(); i + + ){

Particle p = (Particle) plist.get(i);

p.run();

p.display();

}

In this last “ for ” loop, we have to cast the object as we get it out of the ArrayList. An ArrayList does not

keep track of the type of objects stored inside—even though it may seem redundant, it is our job to remind

the program by placing the class type (i.e., Particle) in parentheses.  is process is known as casting .

Of course, the above example is somewhat silly, because it does not take advantage of the ArrayList’s

resizability, using a ﬁ xed size of 10. Following is a better example that adds one new Particle to the

ArrayList with each cycle through draw( ) . We also make sure the ArrayList never gets larger than 100

particles. See Figure 23.3 .

The New ArrayList Way

Elements of an ArrayList are added and accessed

with functions (rather than brackets)—add() and

get(). All of these functions are documented in the

Java reference: http://java.sun.com/j2se/1.4.2/docs/

api/java/util/ArrayList.html.

The term “particle system” was coined in 1983 by William T. Reeves as he developed the “Genesis”

effect for the end of the movie, Star Trek II: The Wrath of Khan.

A particle system is typically a collection of independent objects, usually represented by a simple shape

or dot. It can be used to model types of natural phenomena, such as explosions, ﬁ re, smoke, sparks,

waterfalls, clouds, fog, petals, grass, bubbles, and so on.

Example 23-2: Simple particle system with ArrayList

ArrayList particles;

void setup() {

size(200,200);

particles = new ArrayList();

smooth();

}

An object is added to an ArrayList with add().

The size of the ArrayList is returned by size().

An object is accessed from the ArrayList with get(). You must “cast”

whatever comes out of the ArrayList with a type, that is (Particle).

Casting is discussed in Chapter 4.

ﬁ g. 23.3

430 Learning Processing

void draw() {

particles.add(new Particle());

background(255);

// Iterate through our ArrayList and get each Particle

for (int i = 0; i < particles.size(); i + + ) {

Particle p = (Particle) particles.get(i);

p.run();

p.gravity();

p.display();

}

// Remove the first particle when the list gets over 100.

if (particles.size() > 100) {

particles.remove(0);

}

// A simple Particle class

class Particle {

float x;

float y;

float xspeed;

float yspeed;

Particle() {

x = mouseX;

y = mouseY;

xspeed = random(–1,1);

yspeed = random(–2,0); }

void run() {

x = x + xspeed;

y = y + yspeed;

}

void gravity() {

yspeed + = 0.1;

}

void display() {

stroke(0);

fill(0,75);

ellipse(x,y,10,10);

}

A new Particle object is added to the

ArrayList every cycle through draw().

The ArrayList keeps track of how many

elements it is storing and we iterate

through them all.

If the ArrayList has more than 100 elements

in it, we delete the ﬁ rst element, using

remove().

Java 431

Particle

A

Particle

B

Particle

C

Particle

D

Particle

E

C is removed....

ArrayList particles

012

size = 5 size = 4

34

Particle

A

Particle

B

Particle

D

Particle

E

0123

ﬁ g. 23.4

Hint: Add a function that returns a boolean in the Particle class.

_______ finished() {

if (_______) {

return _______;

} else {

return false;

}

Hint: In order for this to work properly, you must iterate through elements of the ArrayList

backward! Why? Because when an element is removed, all subsequent elements are shifted to the

left (see Figure 23.4 ).

for (int i = _______; i _______; i_______) {

Particle p = (Particle) particles.get(i);

p.run();

p.gravity();

p.render();

if (_______) {

_______;

}

Exercise 23-2 : Rewrite Example 23-2 so that particles are removed from the list whenever

they leave the window (i.e., their y location is greater than height).

Shiffman Daniel. Learning processing

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