Classes / ObjectsAn introduction to object-oriented programming
What is object-oriented programming?
Object-oriented programming (OOP) allows you, the programmer, to create objects
You have been using objects all along, you just weren’t aware of it …
Every time you call a rect(); or an ellipse();, you are calling an object called an ellipse. The ellipse has properties (x,y,w,h)
Why Objects?
OOP allows you to put data about something and actions it can do together into one “object”
OOP has been around since the 1960s
Most popular current languages have either adapted or were designed with OOP
C C++ C# JavaScript Java … and Processing
Human example
Attributes
AgeHeightWeightNameEye colorHair color…
Functions
SleepWake-UpEatRide something
Classes and Objects
A Class A ‘plan’ for an object: Cookie Cutter
The general idea of a thing
Has placeholders for details, but not ‘made’ yet
It declares functions that could be done to an object
An Object
An ‘instance’ of a Class: A Cookie
A ‘real’ thing that has all of the specifics
It can be ‘told’ to execute it’s functions
• You have to have a ‘plan’ before you can make an object:
• Class before Object
Plan a Car Class
Let’s plan a simple Car List data and functions
• Let’s compare how we could convert the function we wrote to show different cars with objects
• We’ll use our setup() and draw() methods
Setup for a ‘Car’ before Objects
Use global variables for ‘properties’ Color: carColor Location: carX, carY Speed: carSpeed
In setup() Set the carColor Set the starting location Set the initial speed
In draw() Fill background Display car at location with color (may use a function) Increment car’s location based on speed
Setup multiple ‘Cars’ before Objects
Use global arrays of variables for ‘properties’ Allows us to use a loop to initialize a number of Cars Declare Color array: carColor [ ] Declare carX array: carX [ ] Declare carY array: carY [ ] Declare carSpeed array: carSpeed [ ]
In setup() Set and initialize the arrays
In draw() Fill background Loop through all the cars
Display car at location with color (may use a function) Increment car’s location based on speed
Setup a ‘Car’ using Objects
One big difference with objects is that you move all of the global variables inside the Car object
Color: carColor Location: carX, carY Speed: carSpeed Car object instead!
We end up with one variable to represent the car
Instead of initializing all of those variables, we initialize the car object!
Setup a ‘Car’ using Objects
Outside of all methods (global) Declare a ‘parking place’ for a car
In setup() Make a new car object
based on the ‘Car’ plan) Sets initial values for color,
location and speed
In draw() Fill background Tell the car to ‘move’ based on
speed Tell the car to ‘display’ itself (call a
function)
The ‘Car’ class
Convert the non-OOP Car Data to a Class
Non-OOP OOP
Setup a ‘Car’ using Objects - Data
Let’s break it down step by stepDeclare the plan for the ‘Car’
Outside draw() and setup() Put variables for color, location and speed inside
Setup a ‘Car’ using Objects - Constructor
We need to write instructions to build a car
It is called the ‘Constructor’ method Move the code that was in setup() Set variables for color, location and speed inside
Setup a ‘Car’ using Objects - Functions
Move the functions that did things to the car
Move the code to inside the Car class The will be called methods of the class Move the code that was in display()and drive()
The whole ‘Car’ (Example 8.1)
class Car { // Define a class for a car color c; // Variables. float xpos; float ypos; float xspeed;
Car() { // A constructor. c = color(175); xpos = width/2; ypos = height/2; xspeed = 1; }
void display() { // Function. // The car is just a square rectMode(CENTER); stroke(0); fill(c); rect(xpos,ypos,20,10); }
void drive() { // Function. xpos = xpos + xspeed; if (xpos > width) { xpos = 0; } }}
What was that about a ‘Constructor’?
A constructor is a special method of a class Has the same name as the class “Builds’ the object
Sets initial values
It is ‘called’ with you use new:
class Car {
Car() { // Constructor c = color(175); xpos = width/2; ypos = height/2; xspeed = 1; }
void setup() { size(200,200); // Initialize Car object myCar = new Car();}
And here is the OOPized version
Is the OOP version shorter?
Is it easier to read? Not yet maybe, but soon, and for the rest of your life.
What if we want to make more cars?
Right now, all cars are exactly the same Their constructor sets up the color, location…
How could we make ‘custom’ cars? Remember parameters? What if a Car constructor took parameters?
class Car { Car(color colp, int xp, int yp, int speedp) { c = colp; xpos = xp; ypos = yp; xspeed = speedp; }
Please review parameter passing…
Make a new frog with a length of 100
Now we can make two ‘custom’ cars
Use your new ‘parameterized’ constructor!
And imagine an array of Cars!
You can use the ‘Car’ class just like any other type
Declare an array of our new Cars object:
Car [ ] parkingLot;setup() { parkingLot = new Car[10];}
But wait… what just happened? Did you create 10 cars? No, not yet. You created 10 parking stalls for cars So we still have to ‘build’ the cars and set all of the colors,
locations and speeds…
parkingLot[0] = new Car(color, x, y..);
Filling the parking lot the easy way!
Once you have the parking lot created, Car [ ] parkingLot;setup() { parkingLot = new Car[10];}
Use a for loop to make a bunch of cars! for (int i; i < 10; i++) { parkingLot[i] = new Car(color, x, y..); } Arrays and loops work wonderfully together! And they are even more fun with objects!
Summary
Objects provide a different way of designing programs
Combine data and functionality
A class is a ‘plan’ for an object An object is an ‘instance’ of a class The ‘new’keyword is used to instantiate (create) an object from a class
Use the ‘dot’ operator ‘.’ to access member data and functions of the class
Classes are normally written in separate files One class per file Put the class ‘source’ file in the same folder as the
‘main’