1. Introduction to Object-Oriented Programming in Alice and Java
Barb Ericson
Georgia Institute of Technology
Aug 2006
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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
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3. 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
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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
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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?
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Introduction to Alice
Free development environment
From Carnegie Mellon University
Available from
Used to create
3D movies
3D games
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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
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Turning On Java Syntax
Click in Edit and then Preferences
Change the display my program: from Alice Style to Java Style in Color
Quit Alice and start it up again
It will keep this setting until you change it again
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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
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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
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Create Your Own Movie
Click on File
Then New World
Click on a template
Background and ground type
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12. 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
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13. 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
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14. 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
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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
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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
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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
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18. 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
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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)
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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?
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21. 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
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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
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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
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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
Seymour was also involved in the invention of the Logo programming languages.
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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
This set of turtle clip art was created by Robert Savannah, an artist working for the U. S. Fish and Wildlife Service. You can find it at
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26. Open Preferences in DrJava
Open the preferences window to add the book classes to the classpath. The classpath tells Java where to look for
classes.
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27. Adding Book Classes to Classpath
Add bookClasses
directory
Be sure to add the bookClasses directory and not bookClasses\bookClasses. Also you often need to close DrJava and start it again in order to get the new classpath.
Click on Add
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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
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29. 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
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30. 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();
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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
The object is created and is in computer memory but we don’t have a way to find where it is.
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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
In programming this is called declaring a variable
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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();
A reference to an object is a way to calculate the address of the object in memory. It is not the address of the object. References are not pointers.
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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();
null
earth
World
Object 1
earth
earth
World
Object 2
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35. 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
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36. 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
> earth = new World();
The first example would try to name two different boxes with the same name. But, then how would you know which one is the right one when someone refers to that name?
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37. 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
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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.NoSuchMethodException: Turtle constructor
Turtle tommy = new Turtle(earth1);
A turtle must be created in a world or you will get an error java.lang.NoSuchMethodException: Turtle constructor.
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39. 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
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40. 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
Turtles cycle through 4 different colors. The 5th turtle you create will have the same color as the first turtle.
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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);
Passing the amount to move forward by is also called passing a parameter.
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Turning a Turtle
Turtles can turn
Right
jen.turnRight();
jen.forward();
Left
shar.turnLeft();
shar.forward(50);
Turtles turn by 90 degrees by default.
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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);
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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);
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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
You can do these in the interactions pane.
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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);
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Moving to a Location
639 X
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 Y
479
There are 640 pixels in the width (0 to 639) and 480 in the height (0 to 479).
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Challenge
Create a World object
Don’t forget to declare a variable to hold a reference to it
Create a turtle object
Use the turtle to draw a
Rectangle (but, not a square)
Diamond
Hexagon
Use the up arrow to reuse previous commands
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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);
You will need to reset the interactions pane before you try this. Click on the “Reset” button at the top of the window.
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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
Other colors are java.awt.Color.BLACK, java.awt.Color.BLUE, java.awt.Color.GREEN, java.awt.Color.GRAY, java.awt.Color.YELLOW, java.awt.Color.ORANGE, java.awt.Color.CYAN, java.awt.Color.MAGENTA, java.awt.Color.WHITE
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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);
When you change the pen color all the lines drawn by that turtle will change to the new color.
When you change the turtle color you are really changing the body color. The shell color will be a darker shade, if there is no specified shell color.
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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);
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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
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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
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Example Method
public void drawSquare() {
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
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56. Adding a Method to a Class
1. Open file Turtle.java
3. Compile open files
2. Type the method before the last } // end
To add a method to a class open the file that defines the class. Then add the method before the last curly brace in the file. Then compile the file by clicking on the Compile All button. Then you are ready to try out your method in the interactions pane.
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Compile Errors
Case matters in Java! turnright isn’t the same as turnRight
You can click on each error. The definitions pane will go to that error and highlight the code that is in error.
Common errors are wrong case, wrong name, wrong parameters, missing ending semicolon, and that the method is inside the class (before the last } in the file).
After you fix any errors you will need to compile again. Keep compiling and fixing errors until there are no more errors.
Clicking on the error takes you to the code and highlights it.
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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);
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59. 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
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60. Better Method to Draw a Square
A method to draw a square
public void drawSquare() {
int width = 30;
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
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61. 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);
Or use the saved script if you saved the last interactions history
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Passing a Parameter
public void drawSquare(int 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
Adding parameters to methods makes them more reusable.
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63. 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);
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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)
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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
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66. Creating a Method in Alice
You added a method to the Turtle class to teach Turtle objects how to do draw a square
You can create methods in Alice as well
Let’s teach our bunny how to hop
Click on bunny in the objects window
Then click on the create new method button
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Naming the Method
We name Java methods starting with a lowercase letter
So name this method hop
Now drag in the commands to make the bunny hop
The bunny should go up and forward and then go down and forward
Use doTogether to do two things at the same time
Use doInOrder to do things on after the other
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Changing the Amount
You can change the amount to go forward by
Click on the downward arrow next to the current amount
Pick one of the values or use other to enter a number
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Try Out the Method
Click on the world.my first method tab
This is the method that runs when you click on play
Drag in the hop method
Click play to try it
And see what it does
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Having bunny2 hop
If you click on bunny2 it won’t have the hop method
It was created before you added this method to the Bunny class
So delete bunny2
Click right on it and select delete
Click on the add objects button to get the mouse controls back
And make a copy of bunny
And position it with the mouse controls
Then click the done button
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Adding bunny2 hopping
Click on bunny2 in the objects window
Click the method tab
See that bunny2 now has a hop method as well
Drag the hop method to the world.my first method
There is an implicit doInOrder around these
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72. Adding parameters to Alice Methods
To add a parameter to a method in Alice
Click on the method tab
Click on the create new parameter button
Name the parameter and pick the type
Change the code to use the parameter
Click down arrow and pick expressions and then the parameter
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73. Trying a Method with a Parameter
Specify the value for the parameter when the method is called
You can’t use this new method with bunny2
Since it was created before you changed the method
So delete bunny2 and create a new copy of bunny
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74. Creating Subclasses in Alice
When you create a new method in Alice or modify an existing method
You are in affect creating a new subclass that has that method or that version of the method
A subclass inherits the fields and methods from the parent class but can add new fields and methods or change inherited ones
You should name and save this subclass
So you can reuse it
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75. Saving a Subclass in Alice
Rename the variable that refers to the object
Right click on it in the Object tree and select rename
Save the class
Right click on the object in the Object tree and select save object
It will save the class definition which starts with an upper case letter
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Use Subclass in Alice
Select File and then Import
And select the class file you created
This will automatically add an object of the new class to your world
You may need to move the new object to see it
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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
You can pass parameters to methods
To make them more flexible and reusable
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