Writing classes

Lec 12
Writing an Instantiable Class
• 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 Classes
1. 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:
double flip = Math.random(); // picks a random number
// and stores in flip
Coins.dispenseQuarter(); // prints a quarter on console
Instantiable Class
3. Instantiable Class – used to make objects from
– String, Scanner, Point, Island, GolfAnimator, Particle
– to invoke a method
– 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:
• Methods to modify it:
can make a new Balloon
can inflate it
can get the size, color
can change the color
can pop it!
Structure of an Instantiable Class
class Balloon
Class Name
Instance Varbls
Constructor method
same 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
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);
You can use the public methods defined for
the class
Balloon bal= new Balloon(5,Color.BLUE);
Parameter Passing
• When we "call" a method: (in BalloonApp class)
• We jump into the method with the arguments
(Balloon class)
Return statements
• 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"
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
• Brings more order into your code
• 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

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