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<h1>Objects</h1>

<p class="license">This tutorial is for Processing version 1.1+.

If you see any errors or have comments, please <a href=

"https://github.com/jdf/processing-py-site/issues?state=open">

let us know</a>. This tutorial is from the book, <a href=

"http://www.processing.org/learning/books/#shiffman">Learning

Processing</a>, by Daniel Shiffman, published by Morgan Kaufmann

Publishers, Copyright &copy; 2008 Elsevier Inc. All rights

reserved.</p>

<p>&nbsp;</p>

<h3>I'm Down with OOP</h3>Before we begin examining the details

of how object-oriented programming (OOP) works in Processing,

let's embark on a short conceptual discussion of "objects"

themselves. Imagine you were not programming in Processing, but

were instead writing out a program for your day, a list of

instructions, if you will. It might start out something like:

<ul>

<li>Wake up.</li>

<li>Drink coffee (or tea).</li>

<li>Eat breakfast: cereal, blueberries, and soy milk.</li>

<li>Ride the subway.</li>

</ul>What is involved here? Specifically, what things are

involved? First, although it may not be immediately apparent from

how we wrote the above instructions, the main thing is you , a

human being, a person. You exhibit certain properties. You look a

certain way; perhaps you have brown hair, wear glasses, and

appear slightly nerdy. You also have the ability to do stuff ,

such as wake up (presumably you can also sleep), eat, or ride the

subway. An object is just like you, a thing that has properties

and can do stuff.<br>

<br>

So how does this relate to programming? The properties of an

object are variables; and the things an object can do are

functions. Object-oriented programming is the marriage of all of

the programming fundamentals: data and functionality.<br>

<br>

Let's map out the data and functions for a very simple human

object:<br>

<br>

<strong>Human data</strong>

<ul>

<li>Height.</li>

<li>Weight.</li>

<li>Gender.</li>

<li>Eye color.</li>

<li>Hair color.</li>

</ul><strong>Human functions</strong>

<ul>

<li>Sleep.</li>

<li>Wake up.</li>

<li>Eat.</li>

<li>Ride some form of transportation.</li>

</ul>Now, before we get too much further, we need to embark on a

brief metaphysical digression. The above structure is not a human

being itself; it simply describes the idea, or the concept,

behind a human being. It describes what it is to be human. To be

human is to have height, hair, to sleep, to eat, and so on. This

is a crucial distinction for programming objects. This human

being template is known as a <em>class</em>. A <em>class</em>is

different from an <em>object</em>. You are an object. I am an

object. That guy on the subway is an object. Albert Einstein is

an object. We are all people, real world <em>instances</em>of the

idea of a human being.<br>

<br>

Think of a cookie cutter. A cookie cutter makes cookies, but it

is not a cookie itself. The cookie cutter is the <em>class</em>,

the cookies are the <em>objects</em>.

<h3>Using an Object</h3>Before we look at the actual writing of a

class itself, let's briefly look at how using objects in our main

program (i.e., <strong>setup()</strong>and

<strong>draw()</strong>) makes the world a better place.<br>

<br>

Consider the pseudo-code for a simple sketch that moves a

rectangle horizontally across the window (we'll think of this

rectangle as a "car").<br>

<br>

<strong>Data (Global Variables)</strong>:

<ul>

<li>Car color.</li>

<li>Car x location.</li>

<li>Car y location.</li>

<li>Car x speed.</li>

</ul><strong>Setup</strong>:

<ul>

<li>Initialize car color.</li>

<li>Initialize car location to starting point.</li>

<li>Initialize car speed.</li>

</ul><strong>Draw</strong>:

<ul>

<li>Fill background.</li>

<li>Display car at location with color.</li>

<li>Increment car's location by speed.</li>

</ul>To implement the above pseudo-code, we would define global

variables at the top of the program, initialized them in setup(),

and call functions to move and display the car in draw().

Something like:

<pre>

c = color(0)

x = 0.0

y = 100.0

speed = 1.0

def setup():

size(200, 200)

def draw():

background(255)

move()

display()

def move():

global x

x = x + speed

if x &gt; width:

x = 0

def display():

fill(c)

rect(x, y, 30, 10)

Object-oriented programming allows us to take all of the

variables and functions out of the main program and store them

inside a car object. A car object will know about its data -

<em>color</em>, <em>location</em>, <em>speed</em>. The object

will also know about the <em>stuff it can do</em>, the methods

(functions inside an object) - the car can <em>drive</em>and it

can be <em>displayed</em>.<br>

<br>

Using object-oriented design, the pseudocode improves to look

something like this:<br>

<br>

<strong>Data (Global Variables)</strong>:

<ul>

<li>Car object.</li>

</ul><strong>Setup</strong>:

<ul>

<li>Initialize car object.</li>

</ul><strong>Draw</strong>:

<ul>

<li>Fill background.</li>

<li>Display car object.</li>

<li>Drive car object.</li>

</ul>Notice we removed all of the global variables from the first

example. Instead of having separate variables for car color, car

location, and car speed, we now have only one variable, a Car

variable! And instead of initializing those three variables, we

initialize one thing, the Car object. Where did those variables

go? They still exist, only now they live inside of the Car object

(and will be defined in the Car class, which we will get to in a

moment).<br>

<br>

Moving beyond pseudocode, the actual body of the sketch might

look like:

<pre>

myCar = Car()

def draw():

background(255)

myCar.drive()

myCar.display()

We are going to get into the details regarding the above code in

a moment, but before we do so, let's take a look at how the Car

class itself is written.

<h3>Writing the Cookie Cutter</h3>The simple Car example above

demonstrates how the use of objects in Processing makes for

clean, readable code. The hard work goes into writing the object

template, that is the class itself. When you are first learning

about object-oriented programming, it is often a useful exercise

to take a program written without objects and, not changing the

functionality at all, rewrite it using objects. We will do

exactly this with the car example, recreating exactly the same

look and behavior in an object-oriented manner.<br>

<br>

All classes must include four elements: name, data

initialization, and methods. (Technically, the only actual

required element is the class name, but the point of doing

object-oriented programming is to include all of these.)<br>

<br>

Here is how we can take the elements from a simple

non-object-oriented sketch and place them into a Car class, from

which we will then be able to make Car objects.<br>

<br>

<table width="656">

<tr>

<td></td>

</tr>

<tr>

<td class="obj_pre">

<pre>

# Simple non-OOP car

</td>

<td>&rarr;</td>

<td class="obj_post">

<pre>

class Car(object):

</td>

<td>&rarr;</td>

<td class="obj_desc">The class name</td>

</tr>

<tr>

<td class="obj_pre obj_yellow">

<pre>

c = color(255)

xpos = 100

ypos = 100

xspeed = 1

</td>

<td>&rarr;</td>

<td class="obj_post obj_yellow">

<pre>

def __init__(self):

self.c = color(255)

self.xpos = 100

self.ypos = 100

self.xspeed = 1

</td>

<td>&rarr;</td>

<td class="obj_desc">Initializing data</td>

</tr>

<tr>

<td class="obj_pre">

<pre>

def setup():

size(200, 200)

def draw():

background(0)

display()

drive()

</td>

<td></td>

<td class="obj_post"></td>

<td></td>

<td class="obj_desc"></td>

</tr>

<tr>

<td class="obj_pre obj_yellow">

<pre>

def display():

rectMode(CENTER)

fill(c)

rect(xpos, ypos, 20, 10)

def drive():

xpos = xpos + xspeed

if xpos &gt; width:

xpos = 0

</td>

<td>&rarr;</td>

<td class="obj_post obj_yellow">

<pre>

def display(self):

rectMode(CENTER)

fill(self.c)

rect(self.xpos, self.ypos,

20, 10)

def drive(self):

self.xpos += self.xspeed

if self.xpos &gt; width:

self.xpos = 0

</td>

<td>&rarr;</td>

<td class="obj_desc">Functionality</td>

</tr>

</table><br>

<br>

<strong>Class Name</strong>: The name is specified by <tt>class

WhateverNameYouChoose(object)</tt>. We then enclose all of the

code for the class in an indented block. Class names are

traditionally capitalized (to distinguish them from variable

names, which traditionally are lowercase).<br>

<br>

<strong>Data initialization</strong>: Python will automatically

call a method named <tt>__init__</tt> whenever you create an

object from a class. You should define this method in your class

and initialize its data here. When you're writing code inside of

a method, there's a special word <tt>self</tt> that allows you to

set variables that are defined on each individual instance of a

class. Variables defined this way are often referred to as

"instance variables." (The <tt>__init__</tt> method is analogous

to "constructors" in other object-oriented languages, like Java

and C++.)<br>

<br>

<strong>Functionality</strong>: We can add functionality to our

object by writing methods. The first parameter of every method

(including <tt>__init__</tt>) should be <tt>self</tt>. Python

automatically passes this parameter to the method when you call

it; it's what allows you to access each object's instance

variables inside of the method.<br>

<br>

Note that the code for a class exists as its own block and can be

placed anywhere outside of <strong>setup()</strong> and

<strong>draw()</strong>, as long as it's defined before you use

it.

<h3>Using an Object: The Details</h3>Earlier, we took a quick

peek at how an object can greatly simplify the main parts of a

Processing sketch (i.e. <strong>setup()</strong> and

<strong>draw()</strong>).

<pre>

<strong># Step 1. Instantiate the object.</strong>

myCar = Car()

def setup():

size(200, 200)

def draw():

background(255)

<strong># Step 2. Call methods on the object.</strong>

myCar.drive()

myCar.display()

</pre>Let's look at the details behind the above steps outlining

how to use an object in your sketch.<br>

<br>

<strong>Step 1. Instantiating an object variable.</strong><br>

<br>

In order to initialize a variable (i.e., give it a starting

value), we use an assignment operation - variable equals

something. With other Python data types, it looks like this:

<pre>

# Variable Initialization

var = 10 # var equals 10

Initializing an object is a bit more complex. Instead of simply

assigning it a value, like with an integer or floating point

number, we have to <i>instantiate</i> the object. We do this by

calling the name of the class as though it were a function.

<pre>

# Object instantiation

myCar = Car()

In the above example, "myCar" is the object variable name and "="

indicates we are setting it equal to something, that something

being a new instance of a Car object. What we are really doing

here is initializing a Car object. When you initialize a

primitive variable, such as an integer, you just set it equal to

a number. But an object may contain multiple pieces of data.

Recalling the Car class, we see that this line of code calls the

<em>__init__ method</em>, a special method that initializes all

of the object's variables and makes sure the Car object is ready

to go.<br>

<br>

<strong>Step 2. Using an object</strong><br>

<br>

Once we have successfully instantiated an object, we can use it.

Using an object involves calling methods that are built into that

object. A human object can eat, a car can drive, a dog can bark.

Calling a function inside of an object is accomplished via dot

syntax: variableName.objectFunction(Function Arguments)<br>

<br>

In the case of the car, none of the available functions has an

argument so it looks like:

<pre>

# Functions are called with the "dot syntax".

myCar.drive()

myCar.display()

<h3>Arguments to the __init__ method</h3>In the above examples,

the car object was initialized like so:

<pre>

myCar = Car()

This was a useful simplification while we learned the basics of

OOP. Nonetheless, there is a rather serious problem with the

above code. What if we wanted to write a program with two car

objects?

<pre>

# Creating two car objects

myCar1 = Car()

myCar2 = Car()

This accomplishes our goal; the code will produce two car

objects, one stored in the variable myCar1 and one in myCar2.

However, if you study the Car class, you will notice that these

two cars will be identical: each one will be colored white, start

in the middle of the screen, and have a speed of 1. In English,

the above reads:<br>

<br>

<strong><em>Make a new car.</em></strong><br>

<br>

We want to instead say:<br>

<br>

<strong><em>Make a new red car, at location (0,10) with a speed

of 1.</em></strong><br>

<br>

So that we could also say:<br>

<br>

<strong><em>Make a new blue car, at location (0,100) with a speed

of 2.</em></strong><br>

<br>

We can do this by placing arguments inside of the parentheses:

<pre>

myCar = Car(color(255,0,0), 0, 100, 2)

The <tt>__init__</tt> method must be rewritten to incorporate

these arguments:

<pre>

def __init__(self, c, xpos, ypos, xspeed):

self.c = c

self.xpos = xpos

self.ypos = ypos

self.xspeed = xspeed

In my experience, the use of <tt>__init__</tt> method arguments

to initialize object variables can be somewhat bewildering.

Please do not blame yourself. The code is strange-looking and can

seem awfully redundant: "For every single variable I want, I have

to add an argument to the <tt>__init__</tt> method?"<br>

<br>

Nevertheless, this is quite an important skill to learn, and,

ultimately, is one of the things that makes object-oriented

programming powerful. But for now, it may feel painful. Let's

looks at how parameter works in this context.<br>

<br>

<img src="imgs/init_arguments.png"><br>

<br>

Parameters are local variables used inside the body of a function

that get filled with values when the function is called. In the

examples, they have <em>one purpose only</em>, which is to

initialize the variables inside of an object. These are the

variables that count, the car's actual color, the car's actual

<em>x</em> location, and so on. The <tt>__init__</tt> methods

arguments are just <em>temporary</em>, and exist solely to pass a

value from where the object is made into the object itself. You

can name these function parameters whatever you want, of

course&mdash;they don't have to have the same names as the

instance variables. However, it is advisable to choose a name

that makes sense to you, and also to stay consistent.<br>

<br>

We can now take a look at the same sketch with multiple object

instances, each with unique properties.

<pre>

# Even though there are multiple objects, we still only need one class.

# No matter how many cookies we make, only one cookie cutter is needed.

class Car(object):

# The Constructor is defined with arguments.

def __init__(self, c, xpos, ypos, xspeed):

self.c = c

self.xpos = xpos

self.ypos = ypos

self.xspeed = xspeed

def display(self):

stroke(0)

fill(self.c)

rectMode(CENTER)

rect(self.xpos, self.ypos, 20, 10);

def drive(self):

self.xpos = self.xpos + self.xspeed;

if self.xpos > width:

self.xpos = 0

myCar1 = Car(color(255, 0, 0), 0, 100, 2)

myCar2 = Car(color(0, 255, 255), 0, 10, 1)

def setup():

size(200,200)

def draw():

background(255)

myCar1.drive()

myCar1.display()

myCar2.drive()

myCar2.display()

<h3>Objects are data types too!</h3>Assuming this is your first

experience with object-oriented programming, it's important to

take it easy. The examples here just one class and make, at most,

two or three objects from that class. Nevertheless, there are no

actual limitations. A Processing sketch can include as many

classes as you feel like writing.<br>

<br>

If you were programming the Space Invaders game, for example, you

might create a <em>Spaceship</em> class, an <em>Enemy</em> class,

and a <em>Bullet</em> class, using an object for each entity in

your game.<br>

<br>

In addition, although not primitive, classes are data types just

like integers and floats. And since classes are made up of data,

an object can therefore contain other objects! For example, let's

assume you had just finished programming a <em>Fork</em> and

<em>Spoon</em> class. Moving on to a <em>PlaceSetting</em> class,

you would likely include variables for both a <em>Fork</em>

object and a <em>Spoon</em> object inside that class itself. This

is perfectly reasonable and quite common in object-oriented

programming.

<pre>

class PlaceSetting(object):

def __init__(self):

fork = Fork()

spoon = Spoon()

Objects, just like any data type, can also be passed in as

arguments to a function. In the Space Invaders game example, if

the spaceship shoots the bullet at the enemy, we would probably

want to write a function inside the Enemy class to determine if

the Enemy had been hit by the bullet.

<pre>

def hit(self, bullet):

bulletX = bullet.getX()

bulletY = bullet.getY()

# Code to determine if

# the bullet struck the enemy

<p>&nbsp;</p>

<p class="license">This tutorial is for Processing version 1.1+.

If you see any errors or have comments, please <a href=

"https://github.com/jdf/processing-py-site/issues?state=open">

let us know</a>. This tutorial is from the book, <a href=

"http://www.processing.org/learning/books/#shiffman">Learning

Processing</a>, by Daniel Shiffman, published by Morgan Kaufmann

Publishers, Copyright &copy; 2008 Elsevier Inc. All rights

reserved.</p>