Classes / Objects An 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  Age  Height  Weight  Name  Eye color  Hair color  … Functions  Sleep  Wake-Up  Eat  Ride 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: C lass before O bject

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 step  Declare 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’