A simple (and generally useless) class

1
A simple (and generally useless) class

• The Laugher class
• Download Laugher.zip demo
• Three versions
• Laugher1
• Laugher2
• Laugher3

2
Laugher1 Constructors and Methods

• Basic class definition
• Constructor
• Single method laugh()
• No instance variables
• In the method
• Identify the prototype
• Identify the body
• Identify the label
• Identify the return type
• What does the constructor do?
• Guarantees that the object starts its life with a
  sensible set of values in its instance variables
• In this case, no instance variables, ctor does
  nothing

3
Laugher2 Parameters and Overloading

• There are two methods with the same name laugh()
  -gt overloading
• What is different about them?
• Label?
• Return type?
• Parameter list?
• What difference does the parameter make?

4
Laugher3 Instance Variables

• A String variable inside the class but outside
  any methods
• Every method and ctor has access to it
• Maintains the state of the object (what is the
  current defaultSyllable?)
• Made private so that only class methods have
  access to it

5
Defining Classes

• Class definition consists of
• Method definitions
• Declarations of instance variables
• We will see these today
• Method definition consists of
• Prototype of the method
• Method body (sequence of Java statements)
• Prototype consists of
• Return type
• Name of the methods
• Parameter list
• A constructor is the same as a method
• Except it lacks a return type

6
Public vs Private

• Methods and instance variables are labeled public
  or private
• To permit (public) or prevent (private) access
  from code outside of the class
• What is labeled private?
• Instance variables
• Helper methods purpose to help implement other
  methods
• What is labeled public?
• Methods that are part of the interface
• In other words, that define the behavior of the
  class

7
Kinds of variables

• Parameters
• Variables declared within a method prototype
• Initialized with the values given by the
  arguments of the invoking message
• Local variables
• Variables declared within the body of a method
• Must be initialized explicitly by the program
• Both local variables and parameters are
• Created when the method is invoked
• Destroyed when the method returns
• Local to the method that invokes them
• Cannot be seen by other methods or parts of the
  program
• Different methods can have local variables and
  parameters of the same name

8
Kinds of variables

• Instance variables
• Declared outside of method bodies
• Created when the object is created
• Destroyed when the object is destroyed
• Used to preserve information between method
  invocations
• Each method has access to an instance variable
• Values of the instance variables determine the
  state of the object

9
Static methods

• Methods not associated with any particular object
• They are associated with the class as a whole
• main() is a static method

10
Designing a class Process

• Decide on the behavior the class will provide
• Starting to think about methods
• Determine the way the class will be used
• Interface
• Think about the prototypes of the methods
• Write a sample program using the class
• Does the design make sense so far?
• Write a skeleton of the class definition
• Class definition shell
• Prototypes and empty method bodies

11
InteractiveIO Class

• A class to be used to get information from the
  user and display information to the monitor
• Determine the behavior
• Write a message to the monitor
• Prompt for and read a string from the keyboard

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InteractiveIO Class

• Interface and Prototypes
• Declare an InteractiveIO reference
• InteractiveIO interIO
• Instantiate (create) a new InteractiveIO object
• interIO new InteractiveIO()
• public interactiveIO()
• Send the object a message to write a String to
  the monitor
• interIO.write(Send this message to monitor.)
• public void write(String str)
• Prompt for and read a String from the keyboard
• String str
• str interIO.promptAndRead(Enter a name )
• public String promptAndRead(String str)

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InteractiveIO Class

• Sample Program
• import java.io.
• class tryInteractiveIO
• public static void main(String arg)
• throws Exception
• InteractiveIO interIO
• String line
• interIO new InteractiveIO()
• line interIO.promptAndRead(Enter a word
  )
• interIO.write(line)

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InteractiveIO Class

• Class definition skeleton
• class InteractiveIO
• // instance variables if we need them
• public InteractiveIO // ctor
• // statements
• // write str to monitor
• public void write(String str)
• // statements
• // write str to monitor, read String from
  keyboard,
• // return reference to it
• public String promptAndRead(String str)
• // statements

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Implementing InteractiveIO

• Writing the method bodies
• Start working an any methods
• You can stop working on one method to go work on
  another
• Download InteractiveIO.zip demo
• Look at improvements to InteractiveIO found in
  class InteractiveIO2

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Name class

• As an exercise, we will design and implement a
  Name class
• Download Name.zip demo
• Include methods to print a name and allow
  interactive input of a name
• To print we need the target passed as parameter
• To read, we will use a static method

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Static methods

• Defined in the same way as other methods, except
  for the keyword static as a label
• This method belongs to the class, not to a
  particular instance of it
• Must be invoked independently of any instance of
  the class
• Use the class name as the receiver of the message
• Name.read(brKey)
• May not access any instance variables
• which belong to particular instances of a class