Goals

• To become familiar with the process of defining classes
• To understand the purpose and use of constructors
• To learn how to use Processing "tabs" to separate classes

OOP

• Object Oriented Programming involves using and defining classes, and manipulating objects of classes (instances)
• Recall that an object has data (variables) and functions
• E.g. A Human object:
  Human data:
  • height
  • eye colour
  • hair colour
  • strength
  Human functions:
  • sleep
  • wake up
  • eat
  • ride some form of transportation
• A Human class is a template for Human objects
  • There can be multiple instances of Humans, each with different data values
• Exercise: Consider a car as an object. What data would a car have? What functions would it have?

Using a Class - Pseudo-code

• Suppose that class Car has functions move and display
• A pseudo-code sketch that draws a moving Car:
  
  Data (global variables):
  Car object
  
  Setup:
  Initialize Car object
  
  Draw:
  Paint background
  Display Car object
  Move Car object
• Variables for Car colour, location, and speed live inside the Car object

Using a Class - Code

• Suppose that defined class Car has functions move() and display()
• A sketch that draws a moving Car:

  //EXAMPLE: what a program that uses an object looks like
  Car myCar;
  void setup() {
    myCar = new Car();
  }
  void draw() {
    background(0);
    myCar.move();
    myCar.display();
  }
        

• Variables for Car colour, location, and speed live inside the Car object

Steps in Using a Class

1. Declare an object variable. E.g.

   Car myCar;

2. Initialize an object. E.g.

   myCar = new Car();

3. Use an object. E.g.

   myCar.move();

Defining a Class

• Syntax:
  class ClassName {
    data
    constructors
    functions
  }
• Example:
  

Class Style

• The name of the class begins with a capital letter
• Variable names and function names begin with a lower case letter

Processing Tab

• Write the definition of a class in a Processing tab:
  1. Click the "Tab" button and choose "New Tab"
  2. Name the tab by the name of the class you intend to put there
  3. Write the class definition in its tab
• The code for the new tab is stored in a new .pde file in the sketch folder
• Exercise: Define class Car for this sketch in a tab: ClassCar

Constructor Arguments

• A constructor initializes a new object's data
• To enable the programmer to specify the initial data, define constructor arguments. E.g.
  

  Car(color c_, float xpos_, float ypos_, float xspeed_) {
    c = c_;
    xpos = xpos_;
    ypos = ypos_;
    xspeed = xspeed_;
  }

• Pass parameters for the constructor arguments when creating a new object. E.g.
  

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

• Sketch: TwoCars