Computer Science II Session 02

1
Computer Science IISession 02

• Cyber-Pets
• Vocabulary
• Interpreting the Magic Line

2
The anatomy of an OO program

• In Java, the main thing you do is write class
  definitions. (R. Morelli)
• A class definition encapsulates two (three
  actually) things
• Data (instance variables) (identity and state)
• Actions (constructors and methods). (behavior)

3
Lets look at a simple example

• CyberPets are simple creatures that do one of two
  things eat and sleep.
• CODE original/CyberPet.java

4
The anatomy of an OO program

• But a class on its own is nearly worthless.
• The purpose of a class is to serve as a template
  for creating individual objects, or instances, of
  the class.

5
The anatomy of an OO program

• An object is an instance of a class
• CyberPet chandler new CyberPet()
• While a class describes a set of objects that
  behave similarly, each instance of a class is
  distinct and has its own state.
• CyberPet joey new CyberPet()

6
The anatomy of an OO program

• Objects interact by sending each other messages.
• CODE original/CyberPetWorld.java

7
The anatomy of an OO program

• The world is a class that contains the main()
  function.
• main() is the big bang that creates one or more
  objects.
• public class CyberPetWorld
• public static void main(String args)
• CyberPet chandler new CyberPet() CyberP
  et joey new CyberPet()

8
Interpreting this magic line

• What exactly do we mean when we say?
• public static void main (String args)

9
Interpreting this magic line

• What exactly do we mean when we say?
• public static void main (String args)
• An access modifier.
• Means that anyone who knows the name of the
  class/method/variable can access to it (more on
  this later)

10
Interpreting this magic line

• What exactly do we mean when we say?
• public static void main (String args)
• Lifetime modifier
• Means that the named entity is a CLASS
  method/variable (more on this in a couple of
  days, but the quick idea is that the method main
  belongs to the class

11
Interpreting this magic line

• What exactly do we mean when we say?
• public static void main (String args)
• Return type
• Means that the method will not be returning any
  data

12
Interpreting this magic line

• What exactly do we mean when we say?
• public static void main (String args)
• Method header
• Contains the name of the method and the required
  parameter (an array of strings referenced by the
  name args) (More on that in a couple of days!)

13
CyberPet is good, but is has some weaknesses

• We would like to be able to know what an object
  is doing.
• What would we need to do in order to enable this
  functionality?
• We cant just check the instance variables,
  because they have private access.

14
Access Modifiers

• A Java programmer can control access to a class
  and its parts (both data and actions) through the
  use of Access Modifiers.
• Any item that is declared public can be accessed
  by any piece of code that knows the name of the
  item.
• Any item that is declared private can be accessed
  only be code within the same class.

15
So when do we use what?

• Because of this, we normally followed the
  following conventions --
• Classes are usually declared public, because we
  want to create instances of the class that can
  help us solve a problem.
• Methods are sometimes declared public, because
  they correspond to messages that other objects
  can send to the instance.
• Instance variables are private, because we want
  the instance to have exclusive control over
  changes to their values.

16
Access Modifiers

• But what if want to do things a little
  differently?  Could we have --
• a private class?
• Sure.  You might want to declare a private class
  because you dont want just anyone to create
  instances of it. For example, I might create a
  Car class for clients to use. The Car might
  contain instances of a wheel class, but clients
  should not be able to create their own instances.
  
• a private method?
• You might want to declare a private method,
  because it is not a part of the objects public
  interface. Perhaps it is simply a helper to one
  of the public methods. We may want to decompose
  the fixWheel method in Car into several helper
  methods.
• a public instance variable?
• You might want to declare a public instance
  variable simply for your convenience as a
  programmer. BUT..

17
CyberPet is good, but is has some weaknesses

• We would like to be able to know what an object
  is doing.
• What would we need to do in order to enable this
  functionality?
• We cant just check, because that information is
  private.
• Therefore, we need to add an accessor method.

18
Accessor Method

• A public method which provides one instance with
  information about a second instance.

19
Accessor Method

• Sometimes these provide direct access to the
  value of a variable
• public boolean getIsEating()
• return isEating
• public boolean getIsSleeping()
• return isSleeping

20
Accessor Method

• But in many cases it will be better to remember
  the old adage
• Ask not what you can do for your class
• but what your class can do for you.
• (my apologies to JFK).

21
Accessor Method

• public void printStatus()
• if (isEating)
• System.out.println(I am eating)
• else
• System.out.println(I am sleeping)
• CODE status/CyberPet.java