Object-Oriented Programming

Object-oriented programming  First goal: Define and describe the objects of the world  Noun-oriented  Focus on the domain of the program  The object-oriented analyst asks herself: “The program I’m trying to write relates to the real world in some way. What are the things in the real world that this program relates to?”  Example: Imagine you’re building an O-O Banner  What are the objects?  Students, transcripts, classes, catalog, major-requirements, grades, rooms…

Objects  Objects as models of real world entities  Objects as Cells  Independent, indivisible, interacting—in standard ways  Scales well  Complexity: Distributed responsibility  Robustness: Independent  Supporting growth: Same mechanism everywhere  Reuse: Provide services, just like in real world

Defining an object  Objects know things.  Data that is internal to the object.  We often call those instance variables.  Objects can do things.  Behavior that is internal to the object.  We call functions that are specific to an object methods.  But you knew that one already.  We access both of these using dot notation  object.variable  object.method()

Classes  Objects are instances of classes in many object-oriented languages.  Including Smalltalk, Java, JavaScript, and Python.  A class defines the data and behavior of an object.  A class defines what all instances of that class know and can do.

Example to motivate objects: SlideShow  Let’s build a program to show a slide show.  It shows a picture.  Then plays a corresponding sound.  We’ll use the introduced-but-never-used blockingPlay() to make the execution wait until the sound is done.

Slideshow def playslideshow(): pic = makePicture(getMediaPath("barbara.jpg")) snd = makeSound(getMediaPath("bassoon-c4.wav")) show(pic) blockingPlay(snd) pic = makePicture(getMediaPath("beach.jpg")) snd = makeSound(getMediaPath("bassoon-e4.wav")) show(pic) blockingPlay(snd) pic = makePicture(getMediaPath("santa.jpg")) snd = makeSound(getMediaPath("bassoon-g4.wav")) show(pic) blockingPlay(snd) pic = makePicture(getMediaPath("jungle2.jpg")) snd = makeSound(getMediaPath("bassoon-c4.wav")) show(pic) blockingPlay(snd)

What’s wrong with this?  From Procedural Abstraction:  We have duplicated code.  We should get rid of it.  From Object-Oriented Programming:  We have an object: A slide.

The Slide Object  What does a slide know?  It has a picture.  It has a sound  What can a slide do?  Show itself.  Show its picture.  (Blocking) Play its sound.

We’ve been doing this already, of course.  You’ve been using objects already, everywhere.  Pictures, sounds, samples, colors—these are all objects.  We’ve been doing aggregation.  We’ve worked with or talked about lists of pictures, sounds, pixels, and samples  The functions that we’ve been providing merely cover up the underlying objects.

Using picture as an object >>> pic=makePicture(getMediaPath("barbara.jpg")) >>> pic.show()

What are Turtles?  Turtles are objects.  In JES, a turtle is represented as a small image of a turtle composed of a shell and a body.

What are Worlds?  Worlds are where turtles are displayed.  Worlds have height and width similar to images.  One world can hold multiple turtles.  A world = window where the turtle is

Making Turtles and Worlds  To make a turtle you must make a world first.  Make a world using the makeWorld(width, height) function ex. world = makeWorld(200, 200)  Once you have a world use the makeTurtle(world) function ex. turtle = makeTurtle(world)

Start with a world(s)  makeWorld() will create a default sized window 640 x 480 pixels  makeWorld(width, height) can be used to get the size window that you want  give your window a name: world = makeWorld(200, 300) or moshPit = makeWorld(400, 400)  You can actually have more than one window being active! footballField = makeWorld(360, 160) danceFloor = makeWorld(200, 200) quarterback = makeTurtle(footballField) fullback = makeTurtle(footballField) dancer1 = makeTurtle(danceFloor) dancer2 = makeTurtle(danceFloor)

What Can You Do With Turtles?  Once you have a turtle you can tell it to do things or change its appearance by using methods.  Methods require dot notation similar to modules. Here is an example of using methods to turn and move a turtle.

Moving Turtles  Methods for moving turtles:  forward(), moves a turtle forward by 100 pixels.  forward(x), moves a turtle forward by x pixels  backward(), moves a turtle backward 100 pixels.  backward(x), moves a turtle backward by x pixels.  moveTo(x, y), moves a turtle to the x and y position in the world.

Turning Turtles  Methods for turning/rotating turtles:  turnRight(), rotates a turtle 90 degrees to the right.  turnLeft(), rotates a turtle 90 degrees to the left.  turn(d), rotates a turtle by d number of degrees to the right. A negative number will rotate the turtle to the left.  turnToFace(turtle), rotates a turtle face another turtle.  turnToFace(x,y), rotates a turtle to an x and y position on the world.

Modifying Turtles  Methods for changing the appearance of turtles:  setColor(c), changes a turtle's shell and body color to color c.  setShellColor(c), changes a turtle's shell color to color c.  setBodyColor(c), changes a turtle's body color to color c.  setWidth(width), changes the width of a turtle.  setHeight(height), changes the height of a turtle.

Turtle Visibility  If you do not want to see a turtle anymore you can hide it.  Methods for changes the visibility settings of turtles:  hide(), stops displaying a turtle on the world. (Just makes it invisible – it is still there.)  show(), begins displaying a turtle on the world if that turtle is hidden.

The Pen  Turtles normally leave a trail when they move. You can control aspects of turtle's trail. Methods to control a turtle's pen:  penUp(), tells a turtle to stop making a trail.  penDown(), tells a turtle to begin making a trail.  setPenColor(c), changes a turtle's trail color to color c.  setPenWidth(w), changes a turtle's trail width to w.

Dropping Images  Turtles can drop images onto the world. This can be useful if you your want your world to have a background image.  First you will have to make an image using the makePicture(file) function. ex. image = makePicture(“fluffy.jpg”)  Then use the method drop(image) to drop the image onto the world.  The top left corner of the image will be the location of the turtle.

Turtle Choreography  A simple way to create an animation using turtles is by using for loops and the time module.  First import the time module: ex. import time  Create a for loop that tells a turtle to repeat an action, and end the for loop with a time.sleep() function.  time.sleep(s), creates a delay s seconds long before continuing.

See list of commands See commands from JES. Click "JES Functions"->"Turtles" We'll be using Object-oriented style when calling the turtle functions: fred.forward(95) instead of forward(fred, 95)

Worlds know what turtles are in them  Each time we add a turtle, the world keeps track  We can ask a world to tells what turtles are in it: getTurtleList(world)  It returns an array (linear collection) of turtles objects!  This is similar to getPixels(pic), which returns a linear collection of pixel objects

Recall what we know about operating on lists  We can access an element in the list using [ ]  We can iterate (loop) over a list of turtles just like we loop over a list of numbers  Recall how the for loop works!  Runs once for every element in the list  First time around stores the first element in the loop variable  Second time around, stores the second element in the loop variable  This continues until we get to the end of the list

Turtles know where they are in the world  Get a turtle’s position very easily… just ask it!  getXPos()  getYPos()