Copyright © 1997-2013 Curt Hill Turtles The beginning of media computation

Copyright © 1997-2013 Curt Hill

TurtlesThe beginning of media

computation.

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


A Turtle in a World


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


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


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


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


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


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


Declaration

• Objects are declared just like primitivesClass_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


Two ways to initialize

• Assign from an existing objectTurtle george = sam;where sam is an existing turtle

• Use the new keywordTurtle bob=new Turtle(myWorld);The Turtle(myWorld) part is a constructormyWorld is the world that the turtle will be in


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


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– It is easier to just modify the JRE


JRE Modification• Download TurtleGraphics.jar from class

web site– Remember where you put it

• Select Window and then Preferences• Choose preferences• Open Java and Installed JREs in

preferences• Edit the JRE• Click Add External JARs• Find the downloaded TurtleGraphics.jar


Lab Computers• Some of you may be running

Eclipse in a computer lab• Sometimes these are periodically

cleaned– Every night– During a reboot

• In such cases you should put the TurtleGraphics.jar file on your space:– Flash drive– Your space on another drive


Downloading


Window Preferences


Preferences


Open Java in Preferences


Choose Installed JREs


Select and Edit


After Edit Button


Add External Jar


Finishing up

• Once TurtleGraphics.jar has been selected select the OK or Finish button until back at the main Eclipse window

• This modifies the JRE and we should not need to do it a second time

• One more thing may be needed, the forbidden access error needs to be disabled


Java Errors


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


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 constructorCopyright © 1997-2013 Curt Hill

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


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


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:


Code Completion


Forward chosen


The new code


Fill in the value


Getting Started

• Create a new project• Create a new class• Add import• Add five lines of code within main


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); t.forward(100);}}

Run It

• After the program is complete run it

• A image similar to the next screen should occur


The Run


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


Try It

• Now we have a glorified etch-a-sketch– 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


Documents

Copyright © 1997-2013 Curt Hill Turtles The beginning of media computation