Lec 12 Writing an Instantiable Class

Agenda

• Review objects and classes• Making our own class to create objects

– Our first "Instantiable" class– with it's own Javadoc!

• Constructor method• Parameter passing into methods• Returning values from methods

We have been learning how to represent data in Java

• Primitives– int, char, double, boolean– only hold simple pieces of data

• a temperature, a choice, how many layers of lettuce

• Objects (of existing Instantiable Classes)– Island, Particle, GolfAnimator, String, Scanner

• more complicated "things"• data and methods (actions) combined• Class as a blueprint that lets us make many "instances"

Remember: 3 Different Kinds of Classes1. Application Class – what we've been doing

– Has public static void main ( String [] args)– all of our programs to this point

2. Classes with only static utility methods – like Math class, NumericalStuff, StringUtil, Coins– to invoke a method:

• nameOfClass.method(parameters);• double flip = Math.random(); // picks a random number

// and stores in flip• Coins.dispenseQuarter(); // prints a quarter on console

Instantiable Class3. Instantiable Class – used to make objects from

– String, Scanner, Point, Island, GolfAnimator, Particle– to invoke a method

• nameOfObject.method(parameters);– Island desert = new Island(5); // desert is name of new Island

– desert.moveNorth(); // move N on Island desert

– Island hawaii = new Island(1000); // hawaii is new Island– hawaii.moveEast(); // move E on Island hawaii

Review: Classes and Objects

Object (reference) variables

How to represent a Balloon in Java?

• Data to define it:sizecolor

• Methods to modify it:can make a new Ballooncan inflate itcan get the size, colorcan change the colorcan pop it!

size

Color.YELLOW

Structure of an Instantiable Class

class Balloonsizecolor

BallooninflategetSizegetColorsetColorpop

Class NameInstance Varbls

Methods

Constructor methodsame name as class

Balloon Demonstration• We will now create our own Balloon class• If you want you can just download these into new

class files and run– Balloon Class– BalloonApp Class

• We will also learn the following– parameter passing into a method– returning a value– creating our own JavaDoc for our balloon class

• Also, here is the Particle class from our previous labs

Using a class -- objects• A class is a blueprint for making objects• When you declare a variable of your new class it’s

called an object reference variable:Balloon hotAir;hotAir = new Balloon(5,Color.RED);

Balloon bal=new Balloon(2,Color.BLUE); Balloon weather= new Balloon(200,Color.WHITE);

hotAir

balweather

You can use the public methods defined for the class

Balloon bal= new Balloon(5,Color.BLUE);

bal.inflate();

bal.pop();

Parameter Passing• When we "call" a method: (in BalloonApp class)

• We jump into the method with the arguments(Balloon class)

• When a method is invoked or "called":

• We jump into the method (with any params)

• When a value is "returned" from a method– we exit method body– return value replaces the "caller"

Return statements

Why Classes and Objects ?

• It may seem overwhelming or unnecessary at first

• As the complexity/size of your code increases, classes will help you modularize your code

• Helps you visualize and solve problems better

• Brings more order into your code

Dairy.java

• make Lab12DairyApp folder• Create new class, Dairy.java fill in definition of

– Constructor– getNumCows– addCow

• Create a new class DairyApp.java– test your new Dairy class by constructing a dairy

farm, adding cows and printing the number of cows