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Objects

Objects. The Heart of the Matter - The "Black Box". Object-oriented software is all about objects . An object is a "black box" which receives and sends messages (i.e., function calls).

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Objects

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  1. Objects

  2. The Heart of the Matter - The "Black Box" • Object-oriented software is all about objects. • An object is a "black box" which receives and sends messages (i.e., function calls). • A black box actually contains methods (functions) and data (variables) which the methods operate on. • In o-o (object-oriented) programming, methods and data are merged into a single indivisible thing -- an object.

  3. Messages • Why the "black box" metaphor for an object? • A primary rule of object-oriented programming is that: as the user of an object, you should never need to peek inside the box! • All communication to it is done via method calls. • So you shouldn't have to know anything about what is in the black box in order to use it. • Methods are functions.

  4. Encapsulation • As long as you deal with objects as black boxes via their methods (i.e., calling their functions), the software is guaranteed to work. • Providing access to an object only through its methods, while keeping the details of the code and the data (i.e., the variables) secret is called information hiding. • An equivalent term is encapsulation.

  5. Classes • How are objects defined? • An object is defined via its class, which determines everything about an object. • Objects are individual instances of a class. • For example, you may create an object call Spot from class Dog. The Dog class defines what it is to be a Dog object. • You can make more than one object of the Dog class, and call them Spot, Fido, Rover, etc.

  6. “Apple” example from your text Variables (fields) methods

  7. A Representation Highlighting Encapsulation

  8. Defining a Class: the Spot Class • Defining a class is like creating your own data type. • String, PImage and PFont are all data types defined by classes. • The Spot class will produce a white “dot” along with any needed behaviors. • What data do we need to define a “spot” • x, y position • diameter

  9. A Spot Without a Class int x = 33; int y = 50; int diameter = 30; void setup() { size(100, 100); smooth(); noStroke(); } void draw() { background(0); ellipse(x, y, diameter, diameter); }

  10. A Class Without Methods Spot sp; // Declare the object void setup() { size(100, 100); smooth(); noStroke(); sp = new Spot(); // Construct the object sp.x = 33; // Assign 33 to the x field sp.y = 50; // Assign 50 to the y field sp.diameter = 30; // Assign 30 to the diameter field } void draw() { background(0); ellipse(sp.x, sp.y, sp.diameter, sp.diameter); } class Spot { // the Spot class float x, y; // The x- and y-coordinate float diameter; // Diameter of the circle }

  11. A display() Method Spot sp; // Declare the object void setup() { size(100, 100); smooth(); noStroke(); sp = new Spot(); // Construct the object sp.x = 33; sp.y = 50; sp.diameter = 30; } void draw() { background(0); sp.display(); // calling the display() method } class Spot { float x, y, diameter; void display() { // ç the display method ellipse(x, y, diameter, diameter); } }

  12. Adding a constructor Spot sp; // Declare the object void setup() { size(100, 100); smooth(); noStroke(); sp = new Spot(33, 50, 30); // Construct the object } void draw() { background(0); sp.display(); } class Spot { // Class definition float x, y, diameter; Spot(float xpos, float ypos, float dia) {// constructor method x = xpos; // Assign 33 to x y = ypos; // Assign 50 to y diameter = dia; // Assign 30 to diameter } void display() { ellipse(x, y, diameter, diameter); } }

  13. Encapsulation revisited • The Spot class was not really encapsulated until we add the constructor. • Without the constructor, the user of the class had to know how the data was stored to use the class

  14. The move() Behavior or Method class Spot { float x, y; // X-coordinate, y-coordinate float diameter; // Diameter of the circle float speed; // Distance moved each frame int direction = 1; // Motion Direction(1 is down, -1 is up) Spot(float xpos, float ypos, float dia, float sp) { // Constructor x = xpos; y = ypos; diameter = dia; speed = sp; } void move() { y += (speed * direction); if ((y > (height - diameter / 2)) || (y < diameter / 2)) { direction *= -1; } } void display() { ellipse(x, y, diameter, diameter); } }

  15. Many Spots Spot sp1, sp2, sp3; // Declare the objects void setup() { size(100, 100); smooth(); noStroke(); sp1 = new Spot(20, 50, 40, 0.5); // Construct sp1 sp2 = new Spot(50, 50, 10, 2.0); // Construct sp2 sp3 = new Spot(80, 50, 30, 1.5); // Construct sp3 } void draw() { fill(0, 15); rect(0, 0, width, height); fill(255); sp1.move(); sp2.move(); sp3.move(); sp1.display(); sp2.display(); sp3.display(); }

  16. Spot Arrays • “main” program • Spot class • Try making a Square class that is similar to the Spot class. • Create 2 Square objects and cause them to display.

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