1. Georgia Institute of Technology Barb Ericson Georgia Institute of Technology Aug 2006 Introduction to Object-Oriented Programming in Alice and Java

2. Georgia Institute of Technology Learning Goals To introduce computation as simulation To introduce Alice To talk about the history of using turtles to teach programming To show how to create objects in Alice To show how to invoke methods on objects in Alice To show how to add classes to the classpath in DrJava To create objects in Java To invoke methods on objects in Java

3. Georgia Institute of Technology Computers as Simulators “The computer is the Proteus of machines. Its essence is its universality, its power to simulate. Because it can take on a thousand forms and serve a thousand functions, it can appeal to a thousand tastes.” Seymour Papert in Mindstorms

4. Georgia Institute of Technology Creating a Simulation Computers let us simulate things –We do this by creating models of the things we want to simulate –We need to define what types of objects we will want in our simulation and what they can do Classes define the types and create objects of that type Objects act in the simulation

5. Georgia Institute of Technology Running a Simulation How many objects does it take to get a person fed at a restaurant? –Pick people to be the customer, greeter/seater, waiter, chef, and cashier and have them role play What data does each of these people need to do their job? What do they need to know how to do? What other objects do you talk about? How about simulating going to a dentist? –Or to the movies?

6. Georgia Institute of Technology Introduction to Alice Free development environment –From Carnegie Mellon University –Available from http://www.alice.org Used to create –3D movies –3D games

7. Georgia Institute of Technology Getting Started Install Alice –Copy folder from CD Start Alice –Double Click on Icon (Alice.exe) Wait for Alice to start –Can take a few minutes

8. Georgia Institute of Technology Pick a World Click on the Examples tab –And click on a world to open it –You might need to scroll down to see this world

9. Georgia Institute of Technology Playing the Movie To play a saved movie –Click the Play button The movie or game will start to play in a separate window –Click on target to play this movie Most example worlds will just start to play

10. Georgia Institute of Technology Create Your Own Movie Click on File –Then New World Click on a template –Background and ground type

11. Georgia Institute of Technology Add Objects To the World Click the Add Objects button Select a category from the Local Gallery Select Local Gallery to get back from a category –To the list of categories

12. Georgia Institute of Technology Add Objects to the World Click on the item to select it –Click on the Class description Click Add Instance to world Button –Or drag the object from the class description and drop it into position in the world

13. Georgia Institute of Technology Position an Object in the World Once the object is in the world –You can move it by Dragging it with the mouse –Or using the mouse controls Click the Undo button to undo the last action

14. Georgia Institute of Technology Mouse Controls Use mouse control buttons (from left to right) –To move object left, right, forward, back –Up and down –Turn left or right –Turn forward or backwards –Tumble the object (free rotation) –Resize object –Copy the object

15. Georgia Institute of Technology Create More Objects You can use the copy object mouse control Or add another instance to the world –Click on the Class Click on the Add instance to world button –Or drag another object from the class description

16. Georgia Institute of Technology Objects and Classes You can make many objects of the same class –The class creates the object Each object will have a unique name in the object window Class names start with a capital letter –Object names start with a lower case letter

17. Georgia Institute of Technology Telling Objects To Do Things Click the Done button to stop adding objects Click on a reference to an object in the object window to select it –Click on the Methods tab to see what it can do

18. Georgia Institute of Technology Messages to Objects In object-oriented programming we send messages to objects –Ask them to do a method Drag a method from the left to the middle window (world - my first method)

19. Georgia Institute of Technology Who does the action? We had selected bunny in the object window –So only bunny does the action How do you think we can get bunny2 to do the same action?

20. Georgia Institute of Technology Moving Several Objects To get bunny2 to move –Select bunny2 in the objects window Or just click on it –Then select the method tab –Drag out the same methods for bunny2 –Click play to see what happens

21. Georgia Institute of Technology Deleting an Object Click on the object you want to delete –The object will be displayed with a bounding box around it Click the right mouse button –Select delete from the pop-up menu

22. Georgia Institute of Technology Using Turtles in Java We will work with Turtles in a World in Java We have to define what we mean by a Turtle to the computer –We do this by writing a Turtle class definition Turtle.java –We compile it to convert it into something the computer can understand Bytes codes for a virtual machine Turtle.class

23. Georgia Institute of Technology History of Turtles Seymour Papert at MIT in the 60s –By teaching the computer to do something the kids are thinking about thinking Develop problem solving skills Learn by constructing and debugging something –Learn by making mistakes and fixing them

24. Georgia Institute of Technology Using Turtles The Turtle Class was is part of several classes created at Georgia Tech –As part of an undergraduate class Add bookClasses to your classpath to use these classes

25. Georgia Institute of Technology Open Preferences in DrJava

26. Georgia Institute of Technology Adding Book Classes to Classpath Click on Add Add bookClasses directory

27. Georgia Institute of Technology Creating Objects To create objects we ask the object that defines the class to create it –Each object keeps a reference to the class that created it The class is like a cookie cutter –It knows how much space each object needs (shape) –Many objects can be created from the class World Object 1 World Object 2 World: Class

28. Georgia Institute of Technology Class as Object Factory A class is like a factory that creates objects of that class We ask a class to create an object by using the keyword: new ClassName We can also ask the class to initialize the object –And pass data to help initialize it

29. Georgia Institute of Technology Creating Objects in Java In Java to create an object of a class you use new Class(value, value, …); Our Turtle objects live in a World object –We must create a World object first –Try typing the following in the interactions pane: new World();

30. Georgia Institute of Technology Creating Objects If you just do –new World(); You will create a new World object and it will display –But you will not have any way to refer to it again –Once you close the window the object can be garbage collected The memory can be reused We need a way to refer to the new object –to be able to work with it again

31. Georgia Institute of Technology Naming is Important If you get a new pet one of the first things you do is name it –Gives you a way to refer to the new pet without saying Please take that dog we got yesterday for a walk. Please take Fifi for a walk. In programming we name things we want to refer to again –Gives us a way to work with them –Like the World object This is called declaring a variable

32. Georgia Institute of Technology Declaring a Variable To be able to refer to an object again we need to specify what type of thing it is and give it a name –This is also called declaring a variable –Type name; OR –Type name = new Class(value, value, …); The equal sign doesn’t mean equal –But assign the value of the variable on the left to the result of the stuff on the right –The following creates a variable named earth which refers to a World object created on the right World earth = new World();

33. Georgia Institute of Technology Declaring Variables Variables are names associated with values –If the type of the variable is null It doesn’t refer to an object yet –Variables can be reused World earth = null; earth = new World(); earth null earth World Object 1 earth World Object 2

34. Georgia Institute of Technology A Variable Associates a Name with Space A variable is like a box with a label on it –You can put something in a box –You can take something out of a box –You can even change what is in the box –The size of the box restricts what you can put in it Hat Box

35. Georgia Institute of Technology Limits on Declaring Variables You can't declare two variables with the same name! > World earth = new World(); Error: Redefinition of 'earth' You can change what an object variable refers to > World earth = new World(); > earth = new World();

36. Georgia Institute of Technology Declaring Variables and Creating Objects You can declare a variable and assign it to refer to a new object in one statement –World earth1 = new World(); –Turtle tommy = new Turtle(earth1); Declaration of variables Creating the objects

37. Georgia Institute of Technology Turtle Basics The world starts off with a size of 640 by 480 –With no turtles World earth1 = new World(); The turtle starts off facing north and in the center of the world by default –You must pass a World object when you create the Turtle object Or you will get an error: java.lang.NoSuchMethodEx ception: Turtle constructor Turtle tommy = new Turtle(earth1);

38. Georgia Institute of Technology Java Naming Conventions Notice that we capitalize the names of the classes, but not the variable names –World earth1 = new World(); –This is different than English Capitalize proper nouns (the names of things) Not the type of thing –Earth is a world. –Tommy is a turtle. In Java it is the class names that are the most important –Not the variable or method names

39. Georgia Institute of Technology Creating Several Objects You can create several World objects World mars = new World(); You can create several Turtle objects Turtle shar = new Turtle(mars); Turtle jen = new Turtle(mars); –One turtle is on top of the other

40. Georgia Institute of Technology Moving a Turtle Turtles can move forward jen.forward(); –The default is to move by 100 steps (pixels) You can also tell the turtle how far to move shar.forward(50);

41. Georgia Institute of Technology Turning a Turtle Turtles can turn –Right jen.turnRight(); jen.forward(); –Left shar.turnLeft(); shar.forward(50);

42. Georgia Institute of Technology Turning a Turtle Turtles can turn by a specified amount –A positive number turns the turtle the right jen.turn(90); jen.forward(100); –A negative number turns the turtle to the left shar.turn(-90); shar.forward(70);

43. Georgia Institute of Technology The Pen Each turtle has a pen –The default is to have the pen down to leave a trail –You can pick it up: turtle1.penUp(); turtle1.turn(-90); turtle1.forward(70); –You can put it down again: turtle1.penDown(); turtle1.forward(100);

44. Georgia Institute of Technology Drawing a Letter How would you use a turtle to draw a large letter T? Process –Create a World variable and a World object and a Turtle variable and object. –Ask the Turtle object to go forward 100 –Ask the Turtle object to pick up the pen –Ask the Turtle object to turn left –Ask the Turtle object to go forward 25 –Ask the Turtle object to turn 180 degrees –Ask the Turtle object to put down the pen –Ask the Turtle object to go forward 50

45. Georgia Institute of Technology Drawing a T World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(100); turtle1.penUp(); turtle1.turnLeft(); turtle1.forward(25); turtle1.turn(180); turtle1.penDown(); turtle1.forward(50);

46. Georgia Institute of Technology Moving to a Location A turtle can move to a particular location turtle1.penUp(); turtle1.moveTo(500,20); Coordinates are given as x and y values –X starts at 0 on the left and increases horizontally to the right –Y starts at 0 at the top of the window and increases to the bottom –A new turtle starts out at 320,240 by default X Y 639 479

47. Georgia Institute of Technology Challenge Create a World object –Don’t forget to declare a variable to hold a reference to it Create a turtle object –Don’t forget to declare a variable to hold a reference to it Use the turtle to draw a –Rectangle (but, not a square) –Diamond –Hexagon Use the up arrow to reuse previous commands

48. Georgia Institute of Technology Setting the Pen Width You can change the width of the trail the pen leaves World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.setPenWidth(5); turtle1.forward(100);

49. Georgia Institute of Technology Setting the Pen Color Use setPenColor to set the color of the pen turtle1.setPenColor(java.awt.Color.RED); There are several predefined colors –In the package java.awt A package is a group of related classes –In the class Color To use them you can use the full name –java.awt.Color.RED

50. Georgia Institute of Technology Setting Colors You can change the pen color turtle.setPenColor(java.awt.Color.RED); You can change the turtle color turtle1.setColor(java.awt.Color.BLUE); You can change the turtle’s body color turtle1.setBodyColor(java.awt.Color.CYAN); You can change the turtle’s shell color turtle1.setShellColor(java.awt.Color.RED);

51. Georgia Institute of Technology Objects can Refuse Turtles won’t move completely out of the boundaries of the world World world2 = new World(); Turtle turtle2 = new Turtle(world2); turtle2.forward(600);

52. Georgia Institute of Technology Objects send Messages Objects don’t “tell” each other what to do –They “ask” each other to do things Objects can refuse to do what they are asked –The object must protect it’s data Not let it get into an incorrect state A bank account object shouldn’t let you withdraw more money that you have in the account

53. Georgia Institute of Technology Creating a Method We can name a block of Java statements and then execute them again –By declaring a method in a class The syntax for declaring a method is –visibility returnType name(parameterList) –Visibility determines access Usually public or private The return type is the type of thing returned If nothing is returned use the keyword void –Name the method starting with a lowercase word and uppercasing the first letter of each additional word

54. Georgia Institute of Technology Example Method public void drawSquare() { this.turnRight(); this.forward(30); this.turnRight(); this.forward(30); this.turnRight(); this.forward(30); this.turnRight(); this.forward(30); } The visibility is public The keyword void means this method doesn’t return a value The method name is drawSquare There are no parameters –Notice that the parentheses are still required The keyword this means the object this method was invoked on

55. Georgia Institute of Technology Adding a Method to a Class 1. Open file Turtle.java 2. Type the method before the last } // end 3. Compile open files

56. Georgia Institute of Technology Compile Errors Clicking on the error takes you to the code and highlights it. Case matters in Java! turnright isn’t the same as turnRight

57. Georgia Institute of Technology Try the New Method Compiling resets the interactions pane –Clearing all variables But you can still use the up arrow to pull up previous statements –You will need to create a world and turtle again World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(); turtle1.turn(30); turtle1.drawSquare();

58. Georgia Institute of Technology Saving the Interactions History in DrJava You can save the interactions history into a script –And optionally edit it first before you save it –Click on Tools then on Save Interactions History And then latter load and execute the statements in the script –Click on Tools and Load Interactions History as Script Use the next button to see the next statement and click on the execute button to execute it

59. Georgia Institute of Technology Better Method to Draw a Square A method to draw a square public void drawSquare() { int width = 30; this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); } We added a local variable for the width –Only known inside the method This makes it easier to change the width of the square But, we still have to recompile to draw a different size square

60. Georgia Institute of Technology Testing the Better Method Type the following in the interactions pane World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(); turtle1.turn(30); turtle1.drawSquare(); Or use the saved script if you saved the last interactions history

61. Georgia Institute of Technology Passing a Parameter public void drawSquare(int width) { this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); this.turnRight(); this.forward(width); } Parameter lists specify the type of thing passed and a name to use to refer to the value in the method The type of this parameter is int The name is width Values are passed by making a copy of the passed value

62. Georgia Institute of Technology Testing with a Parameter Type the following in the interactions pane World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(30); turtle1.turn(30); turtle1.drawSquare(50);

63. Georgia Institute of Technology How Does That Work? When you ask turtle1 to drawSquare(30) turtle1.drawSquare(30); –It will ask the Turtle Class if it has a method drawSquare that takes an int value And start executing that method The parameter width will have the value of 30 during the executing of the method The this keyword refers to turtle1 When you ask turtle1 to drawSquare(50) turtle1.drawSquare(50); –The width will have a value of 50 –The this refers to turtle1 (the object the method was invoked on)

64. Georgia Institute of Technology Challenges Create a method for drawing a rectangle –Pass the width and height Create a method for drawing an equilateral triangle –all sides have the same length –Pass in the length Create a method for drawing a diamond Create a method for drawing a house –Using the other methods Create a method for drawing a school –Using the other methods

65. Georgia Institute of Technology Summary You can create objects from classes in Alice and Java Each object needs a unique way to reference it –In Java we call this declaring a variable You can create new methods –visibility returnType name(Type name, Type name, …) –Let’s you reuse a block of statements