PowerPoint Presentation

Introduction toProgrammingIntroduction toProgramming

Writing Java Beginning JavaProgramsWriting Java Beginning JavaPrograms

Review of Lab 1Review of Lab 1

►Built-in Types►Built-in Types

Integer typesInteger types

►No decimal point►No decimal point

►integer math-op integer yields an integer►integer math-op integer yields an integer

►byte, short, int, long►byte, short, int, long

Real typesReal types

►Have decimal point►Have decimal point

►real math-op real yields a real►real math-op real yields a real

►float, double►float, double

Review of lab 1 ContinuedReview of lab 1 Continued

►Java Variables►Java Variables

AttributesAttributes

►Name►Name

►Type►Type

►Value►Value

Specifies where data is stored in memorySpecifies where data is stored in memory

Java requires that a variable be declared before it canbe used.Java requires that a variable be declared before it canbe used.

Declaration Form:Declaration Form:

►Type variable-name;►Type variable-name;

►Can only declare once!►Can only declare once!

Review of Lab 1Review of Lab 1

►What is wrong?►What is wrong?

int firstNum = 0;int firstNum = 0;

int firstNum = firstNum + 1;int firstNum = firstNum + 1;

►What does the = operator in Java mean?►What does the = operator in Java mean?

►What is the type of the result of►What is the type of the result of

double total, price;double total, price;

int amt;int amt;

total = price * amt;total = price * amt;

►What is wrong with►What is wrong with

amt = total/price;amt = total/price;

Review of Lab 1 ContinuedReview of Lab 1 Continued

►Variable Naming Conventions►Variable Naming Conventions

Start with letter or _ follow with any number ofletters, digits, or _Start with letter or _ follow with any number ofletters, digits, or _

Names are case sensitiveNames are case sensitive

Convention – variable names start with lowercase letter.Convention – variable names start with lowercase letter.

Names should be meaningful.Names should be meaningful.

►sales►sales

►noItems►noItems

►qtyOnHand►qtyOnHand

Camel notation

The Semicolon RuleThe Semicolon Rule

►Always end a Java statement with asemicolon!►Always end a Java statement with asemicolon!

Be careful! The interaction pane does notrequire a semicolon after each statement.Be careful! The interaction pane does notrequire a semicolon after each statement.

In a program typed into the definition pane,Java does.In a program typed into the definition pane,Java does.

In the interaction pane, typing a semicolon aftera statement does not show the value of theresult. Leaving the semicolon off does.In the interaction pane, typing a semicolon aftera statement does not show the value of theresult. Leaving the semicolon off does.

The Semicolon RuleThe Semicolon Rule

►Always end a Java statement with asemicolon!►Always end a Java statement with asemicolon!

The code pad in BlueJ requires a semicolonafter each Java statement.The code pad in BlueJ requires a semicolonafter each Java statement.

If you forget the semicolon, you will get anerror in code pad reminding you that asemicolon is required.If you forget the semicolon, you will get anerror in code pad reminding you that asemicolon is required.

Object VariablesObject Variables

►Java is an Object-oriented programming language.►Java is an Object-oriented programming language.

►We work mostly with objects.►We work mostly with objects.

►However, built-in numeric types are not objects.ie. int, float, double, …►However, built-in numeric types are not objects.ie. int, float, double, …

►Strings are objects.►Strings are objects.

►String myName;►String myName;

myName is a reference to a string objectmyName is a reference to a string object

Objects must be created before they are assigned.Objects must be created before they are assigned.

myName has the initial value of null (it does notreference a string object)myName has the initial value of null (it does notreference a string object)

Much more on this later.Much more on this later.

Classes and ObjectsClasses and Objects

►Classes are templates that define what objectslook like and how they behave.►Classes are templates that define what objectslook like and how they behave.

►An object is an instantiations of a class►An object is an instantiations of a class

►Class names in Java should start with a capitalletter.►Class names in Java should start with a capitalletter.

►Turtle is a class►Turtle is a class

It defines object methods that let you use a Turtleobject.It defines object methods that let you use a Turtleobject.

//create a turtle object from the Turtle class//create a turtle object from the Turtle class

Turtle myTurtle = new Turtle(myWorld); Turtle myTurtle = new Turtle(myWorld);

Class name definesthe type

Makes anew object

Constructor:Method thatinitializes anew obj.

Object (Instance) MethodsObject (Instance) Methods

►The behavior of an object is defined by themethods that belong to the method.►The behavior of an object is defined by themethods that belong to the method.

►Each instance of an object has it own copyof all the methods defined in the object’sclass that are not Class Methods.►Each instance of an object has it own copyof all the methods defined in the object’sclass that are not Class Methods.

►Turtle turtle1 = new Turtle(myWorld);►Turtle turtle1 = new Turtle(myWorld);

Turtle turtle2 = new Turtle(myWorld);Turtle turtle2 = new Turtle(myWorld);

turtle1.turnRight(); turtle2.turnRight();turtle1.turnRight(); turtle2.turnRight();

Note: both turtles have a turnRight method

Class MethodsClass Methods

►Some classes have methods that belong to theclass and not an instance of the class.►Some classes have methods that belong to theclass and not an instance of the class.

►java.Math►java.Math

Class that defines math functions to useClass that defines math functions to use

Would not want multiple math objects that eachimplement an abs method.Would not want multiple math objects that eachimplement an abs method.

Class methods are methods that belong to only theclass and hence there is only one copy of themethod for all objects of the class.Class methods are methods that belong to only theclass and hence there is only one copy of themethod for all objects of the class.

int myVal = Math.abs(-12);int myVal = Math.abs(-12);

A Basic Java ProgramA Basic Java Program

// if you have to add import statements put// if you have to add import statements put

//them here//them here

public class CS1Test {public class CS1Test {

public static void main(String [] args) { public static void main(String [] args) {

// put you Java program statements here // put you Java program statements here

} }

}}

►You will see what this is for in a minute.►You will see what this is for in a minute.

Working With TurtlesExample of using classesWorking With TurtlesExample of using classes

►Get a world for our turtle to live in►Get a world for our turtle to live in

World csExWorld = new World();World csExWorld = new World();

►Put a turtle in our world►Put a turtle in our world

Turtle ourTurtle = new Turtle(csExWorld);Turtle ourTurtle = new Turtle(csExWorld);

►Check the state of our turtle►Check the state of our turtle

System.out.println(ourTurtle);System.out.println(ourTurtle);

System.out.println(ourTurtle.toString());System.out.println(ourTurtle.toString());

Parameterthatspecifiesthe worldto put theturtle in.

Talking to a TurtleTalking to a Turtle

►When we want an object to do somethingwe send it a message.►When we want an object to do somethingwe send it a message.

►Make the turtle wander around►Make the turtle wander around

ourTurtle.forward(20);ourTurtle.forward(20);

ourTurtle.turnLeft();ourTurtle.turnLeft();

ourTurtle.forward(40);ourTurtle.forward(40);

ourTurtle.turn(45);ourTurtle.turn(45);

ourTurtle.forward(65);ourTurtle.forward(65);

How Do I Know What Turtles CanDo?How Do I Know What Turtles CanDo?

►The set of methods that define what a classcan do is called an API (Application ProgramInterface).►The set of methods that define what a classcan do is called an API (Application ProgramInterface).

►Java defines a special format called JavaDocfor presenting APIs.►Java defines a special format called JavaDocfor presenting APIs.

►You can find a link to the doc on thedesktop of the lab machines and atC:\JavaDocumentation\Intro2CS\doc\index.html.►You can find a link to the doc on thedesktop of the lab machines and atC:\JavaDocumentation\Intro2CS\doc\index.html.

Problem: Draw a squareProblem: Draw a square

►Describe how the turtle will travel in a square.►Describe how the turtle will travel in a square.

►Program Code:►Program Code:

//draw side one//draw side one

ourTurtle.forward(100);ourTurtle.forward(100);

// turn 90 deg counterclockwise// turn 90 deg counterclockwise

ourTurtle.turnLeft();ourTurtle.turnLeft();

// draw side two// draw side two

ourTurtle.forward(100);ourTurtle.forward(100);

// turn 90 deg counterclockwise// turn 90 deg counterclockwise

ourTurtle.turnLeft();ourTurtle.turnLeft();

// draw side three// draw side three

ourTurtle.forward(100);ourTurtle.forward(100);

//turn 90 degrees counterclockwise//turn 90 degrees counterclockwise

ourTurtle.turnLeft();ourTurtle.turnLeft();

// draw side four// draw side four

ourTurtle.forward(100);ourTurtle.forward(100);

Reusing CodeReusing Code

►What if we wanted to draw a secondsquare?►What if we wanted to draw a secondsquare?

We would have to retype the code a secondtime.We would have to retype the code a secondtime.

Not good! Is not there a better way?Not good! Is not there a better way?

►Define a method that draws a square.►Define a method that draws a square.

►We can invoke it every time we need todraw a square.►We can invoke it every time we need todraw a square.

The drawSquare MethodThe drawSquare Method

public void drawSquare() {public void drawSquare() {

//draw side one//draw side one

this.forward(100);this.forward(100);

// turn 90 deg counterclockwise// turn 90 deg counterclockwise

this.turnLeft();this.turnLeft();

// draw side two// draw side two

this.forward(100);this.forward(100);

// turn 90 deg counterclockwise// turn 90 deg counterclockwise

this.turnLeft();this.turnLeft();

// draw side three// draw side three

this.forward(100);this.forward(100);

//turn 90 degrees counterclockwise//turn 90 degrees counterclockwise

this.turnLeft();this.turnLeft();

// draw side four// draw side four

this.forward(100);this.forward(100);

}}

}

Body of the method

Parenthesesrequired

Opening and Closing BraceRequired

1.Open the Turtle.java filefound in C:\intro-prog-java\bookClasses

2.Put the method definition atthe bottom of the classwhere the documentationdirects.

3.Create a World, a Turtle,and draw a square.

How do we draw two squares?How do we draw two squares?

►Call drawSquare() twice.►Call drawSquare() twice.

►What happened?►What happened?

►Cannot call drawSquare again it will draw over topthe square we have drawn.►Cannot call drawSquare again it will draw over topthe square we have drawn.

►Need to move the Turtle before we draw again.►Need to move the Turtle before we draw again.

►How? Look at documentation►How? Look at documentation

Pen upPen up

Move to new positionMove to new position

Pen downPen down

ParametersParameters

►How do we draw squares of different sizes?►How do we draw squares of different sizes?

►Observation: Each time we draw a square weneed to change the 100 to the size we want.►Observation: Each time we draw a square weneed to change the 100 to the size we want.

►This could generate a lot of almost identicalmethods.►This could generate a lot of almost identicalmethods.

►Use a parameter to specify the width.►Use a parameter to specify the width.

public void drawSquare(int width) { … }public void drawSquare(int width) { … }

Change 100 to widthChange 100 to width

►Call as:►Call as:

ourTurtle.drawSquare(50);ourTurtle.drawSquare(50);

Draw the squares in Different ColorsDraw the squares in Different Colors

►Back to the documentation►Back to the documentation

public void setColor(java.awt.Color color)public void setColor(java.awt.Color color)

Need a Color object found in java.awt packageNeed a Color object found in java.awt package

Must import this package to use itMust import this package to use it

Find information in Java 1.5 API DocumentationFind information in Java 1.5 API Documentation

►Invoke►Invoke

ourTurtle.setColor(Color.BLUE);ourTurtle.setColor(Color.BLUE);

Draw a blue square.Draw a blue square.

How do you …?How do you …?

►Save the program?►Save the program?

Put it in a CSTurtleTest classPut it in a CSTurtleTest class

►Hide the turtle?►Hide the turtle?

►Draw the squares at an angle to thewindow?►Draw the squares at an angle to thewindow?