1. The beginning of media computation Followed by a demo
Turtles
The beginning of media computation
Followed by a demo
Eclipse version
Copyright © Curt Hill

2. What is a turtle? It will show up as a icon in a window
We direct the turtle to move forward or backward as well as turn left or right
Among other things
When it moves it may leave a line as a trail
It is a line drawing tool
Copyright © Curt Hill

3. A Turtle in a World
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4. Two Objects There are two objects that are needed for this
A world
A turtle
The world provides the window and background
The sandbox for one or more turtles
The turtle
Our drawing object
Copyright © Curt Hill

5. What is an object? In Java there are two kinds of variables
Primitives and objects
A primitive represents one simple value
An int containing 4
The character ‘A’
An object bundles one or more values into one
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6. Objects Objects have properties and methods
A property is a variable that may be a variable or another object
A method is a means to make the object do something
Obtain or change property values
A constructor is a kind of method that initializes the object
Copyright © Curt Hill

7. Primitive Life Cycle We declare We assign values to them
They are only known in the method that they are used within
Such as the main method
Objects have a slightly different life cycle
Copyright © Curt Hill

8. Object Life Cycle Declare the handle
Allocate (or instantiate) using new or assignment
Declaration makes primitives exist but objects need another step
Assign and use the values
Copyright © Curt Hill

9. Terminology An object is a kind of variable
A class is a type of object
Example: String s = “S”; String is the class s is the object “S” is the value
Copyright © Curt Hill

10. Declaration
Objects are declared just like primitives Class_Name Variable_name ;
Example: Turtle george;
george is now an object handle
Cannot be used until allocated
An import statement may be needed to bring the class name within the scope
Copyright © Curt Hill

11. Two ways to initialize
Assign from an existing object Turtle george = sam; where sam is an existing turtle
Use the new keyword Turtle bob=new Turtle(myWorld); The Turtle(myWorld) part is a constructor myWorld is the world that the turtle will be in
Copyright © Curt Hill

12. Constructors Recall that a constructor is a special method of a class
It has the same name as the class
It is often overloaded by different sets of parameters
One of the constructors is always executed for a newly created object
It initializes the new object
Copyright © Curt Hill

13. Preparation Eclipse knows about the Java Runtime Environment (JRE)
This includes all Java standard objects such as Scanner
It does not know about Turtles because they are not standard Java items
Eclipse will allow new items to be added to a single project or to the JRE
In the past JRE modification worked best
Copyright © Curt Hill

14. Adding to Project Download TurtleGraphics.jar from class web site
Remember where you put it!
From within Eclipse:
Select Project and then Properties
(You may also right click project in Package Explorer)
Choose Java Build Path
Click on Classpath
Click Add External JARs
Find the downloaded TurtleGraphics.jar
Finish with Apply and Close
Copyright © Curt Hill

15. Downloading
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16. Right Click Project
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17. Click Java Build Path
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18. Click Classpath
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19. Click Add External Jar Find Turtlegraphics.jar
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20. Then click Apply and Close
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21. Finishing up
Once TurtleGraphics.jar has been selected select the OK or Finish button until back at the main Eclipse window
This modifies the project and we will need to do this for every new project
Unless I find a better way
One more thing may be needed, the forbidden access error needs to be disabled
Copyright © Curt Hill

22. TurtleGraphics Problems
This library is quite old
Sometimes it uses deprecated calls and objects
A deprecated item is slated for removal
The compile has options on whether to give errors or warnings on these
We need to disable these
This is done with the menu Windows Preferences
Copyright © Curt Hill

23. Java Errors
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24. One More Thing
In the Project creation dialogs it will ask if you want to Create a Module
Always click Don’t Create
Creation of this Module seems to interfere with using turtlegraphics in a class
Copyright © Curt Hill

25. Don’t Create
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26. Import Statement
We have told Eclipse that we need TurtleGraphics to be part of the JRE
There are a large number of things in the JRE that require an import to be used
The import for this is: import turtlegraphics.*;
This follows the package statement
Copyright © Curt Hill

27. Creating a World Each World will become a screen
Turtles must be created within a world
Create the world: World myWorld = new World();
World is class name
myWorld is variable name and could be any name, but will be used with Turtle constructor
World() is the constructor
Copyright © Curt Hill

28. Creating Turtles Once myWorld has been made we can make turtles
Create a turtle: Turtle sam = new Turtle(myWorld);
Turtle is a class name
sam is the name of one turtle
myWorld is the name of the previously created world
We may create as many as we like
Copyright © Curt Hill

29. Turtle Methods Making a turtle to exist is no big deal
We would like the turtle to do something
This takes methods
For now just two
forward(200);
Moves forward 200 pixels
turn(90)
Turn right 90 degrees
Left uses a negative
Copyright © Curt Hill

30. Eclipse Again
One of the very many advantages of Eclipse over DrJava is the code completion option
When we type in the name of a variable, that is a class, and then a dot
Eclipse suggests some methods that could be used
See the following screen:
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31. Code Completion
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32. Forward chosen
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33. The new code
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34. Fill in the value
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35. Getting Started Create a new project Create a new class Add import
Add five lines of code within main
Copyright © Curt Hill

36. Whole but Short Program
package turtle;
import turtlegraphics.*;
public class FirstTurtle {
public static void main(String[] args) {
World w = new World();
Turtle t = new Turtle(w);
t.forward(100);
t.turn(90); }
Copyright © Curt Hill

37. Run It After the program is complete run it
A image similar to the next screen should occur
Copyright © Curt Hill

38. The Run
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39. One last thought
Do not name your project or class or package the same thing as any class you will use
In particular do not use:
Turtle
World
Color
Picture
Among others
If you do you may not be able to use any of these objects
Copyright © Curt Hill

40. Try It Now we have a glorified etch-a-sketch Create a new program
Soon we will have much more
Create a new program
Put in the commands to draw a square
4 forwards
4 turns of 90
I will and you can follow me in this
Copyright © Curt Hill