Programming with Objects

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Programming with Objects

• Intro to object-orientation and how to use
  objects in Java
• James Brucker

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The Realities of Software

• Useful, real-world software
• 1. Change
• 2. Complexity

3
The Problem

• "Programming is fun, but developing quality
  software is hard. In between the nice ideas, the
  requirements or the "vision", and a working
  software product, there is much more than
  programming.
• Analysis and design, defining how to solve the
  problem, what to program, capturing this design
  ..., review, implement, and evolve is what lies
  in the core of this book."
• Forward to Larman, Applied UML and Patterns, 3E.
  Forward by Philipe Kruchten (IBM Rational)

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Intrinsic Complexity of Software

• About the future of software development...
• "There is no single development, in either
  technology or in management technique, that by
  itself promises even one order of magnitude
  improvement in productivity, in reliability, in
  simplicity."
• "No Silver Bullet" by Frederick Brooks.
  Computer, 1987
• See also page 5-6 for Brooks comments about
  object-oriented approach.

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Millers Law

• At any one time, a person can concentrate on at
  most 7 ? 2 chunks (units of information)
• To handle larger amounts of information, use
  stepwise refinement
• Concentrate on the aspects that are currently the
  most important
• Postpone aspects that are currently less critical
• Every aspect is eventually handled, but in order
  of current importance
• This is an incremental process

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Benefit of Object-Orientation (1)

• Encapsulate complexity
• divide program into classes
• each class has its own responsibilities and data
• class has only one purpose
• class has simple interface
• hide implementation details

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Benefit of Object-Orientation (2)

• Encapsulate change
• each class presents only a simple public
  interface
• class hides implementation details
• as a result...
• we can localize the effect of change

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Benefit of Object-Orientation (3)

• Reuse code
• classes are reusable
• polymorphism lets us interchange parts
• inheritance lets us build new classes that reuse
  code from old classes.

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Benefit of Object-Orientation (4)

• Better abstraction
• objects make good models for things in the real
  world (problem domain)
• let us think about the problem instead of the
  code
• simplify the problem so we can think about
  problem without too many details

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Objects and Programs

• An OO program consists of objects that interact
  with each other.

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Classes

• A class is a blueprint or definition for a kind
  of object.
• A class defines the attributes and behavior of
  objects.

Cat birthday color sex sleep( ) eat( Food ) play(
) chase( Object )
attributes are characteristics of objects. In
Java variables, "fields"
behavior is what the object can do. In Java
methods
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Objects

• Objects are instances of a class.
• In Java, to create a new object use "new", e.g.
• Cat somecat new Cat( )
• create the object in memory
• invoke the constructor of Cat class to initialize
  values.
• Object creation can use values, too
• Cat kitten new Cat( Color.BLACK, "male", ... )

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3 Fundamental Characteristics of Objects

• Objects have
• state - the current condition of an object
• behavior - the actions or messages an object can
  accept
• identity - every object is distinguishable, even
  if two objects have the same state
• How to use the methods of an object
• // call a method in the same object
• turnLeft( )
• canMove( )

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Bank Account Example

• BankAccount class is the definition for bank
  accounts.

UML class diagram
class name BankAccount attributes accountNumber,
name, balance behavior getBalance( ),
credit(amount), debit(amount), getName( )
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Bank Account Object

• An object is an actual instance of the class.
• rich is a bank accoount
• Example

BankAccount rich new BankAccount("Taksin
Shinawat" ) rich.credit( 2000000000
) rich.credit( 1000000000 ) // needs money to
buy a football team... rich.debit( 500000000 )
// does he have enough to buy Hawaii? rich.getBal
ance( ) // 2,500,000,000
UML object diagram
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Object Properties

• Example
• For the "rich" object, what are...
• state?
• behavior?
• identity?

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HondaCivic Example

• The definition of a HondaCivic consists of
• specifications
• design documents
• blue prints
• list of parts
• list of qualified suppliers
• instructions for assembly
• inspection procedure and check lists
• operating instructions
• maintenance manual and procedures

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HondaCivic class

• But, the Honda Civic owner doesn't need to know
  all these details. He needs to know about a
  Honda Civic's properties (public attributes) and
  behavior.
• For example (simplified)

properties(attributes)
behavior
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Buying A Honda Civic

• You go to the Honda dealer and say...

I want aHonda Civic
Yes, sir. This way, please...
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Buying A Honda Civic (2)

• the dealer offers you the class for a Honda
  Civic...

Here you are! All the documents and blue prints
for a Honda Civic. ... that will be 1,000,000,000
Baht, please.
Construction and operation of a Honda Civic
complete documents.
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Buying A Honda Civic (3)

• but you can't drive blue prints and documents

That's not exactly what I had in mind. I want a
car I can drive...
I see... you want an instance of Honda Civic -- a
Honda Civic object.
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Buying A Honda Civic (4)
Silver, 4 door, automatic transmission, tinted
windows, ...
yourcar new HondaCivic("silver", 4door,
automatic,... )
attributes
behavior
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Review Class versus Objects
HondaCivic
Defines the properties and behavior for all
instances (objects) of this class.
Specific realization of the class
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Object Identity Example

• Primitive data types don't have identity.
• You cannot distinguish two primitives when the
  values are same.
• primitive variables represent values not objects

double x 10 double y 10 if ( x y )
System.out.print("same") // true
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Object Identity Example

• Objects do have identity.
• You can distinguish two objects even when the
  values are the same.
• object variable is a reference to an object

Double x new Double(10) Double y new
Double(10) if ( x y ) System.out.print("same")
// false
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Object Identity Example (2)

• Two Honda Civic cars can be distinguished even if
  they exactly the same features and same state
  (brand new)

!
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Second Program

• / Print an impersonal greeting message
• _at_author James Brucker
• /
• public class Greeting
• private String who
• / constructor for new objects /
• public Greeting( String name )
• who name // save the name
• public void sayHello( )
• System.out.println("Hello, "who)
• public void sayGoodbye( )
• System.out.println("Goodbye, "
• who)

This is a Javadoc comment.
Any text placed inside / .... / is ignored by
the compiler.
An attribute of this class.
Each object will have a variable named "who" that
contains data unique to each object (instance) of
this class.
An attribute is a property of an object --
information the object uses to do its job.
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Constructor for Objects

• / Print an impersonal greeting message
• _at_author James Brucker
• /
• public class Greeting
• private String who
• / constructor for new objects /
• public Greeting( String name )
• who name // save the name
• public void sayHello( )
• System.out.println("Hello, "who)
• public void sayGoodbye( )
• System.out.println("Goodbye, "
• who)

A constructor for the class.
The constructor is a method that is called when a
new object is created. A constructor usually
initializes the attributes of an object, but does
not return any value ... not even a void.
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Methods define Behavior

• / Print an impersonal greeting message
• _at_author James Brucker
• /
• public class Greeting
• private String who
• / constructor for new objects /
• public Greeting( String name )
• who name // save the name
• public void sayHello( )
• System.out.println("Hello, "who)
• public void sayGoodbye( )
• System.out.println("Goodbye, "
• who)

Methods are the actions that an object can
perform. Methods define behavior or
responsibilities of objects.
A method of the class.
Methods can access the attributes of the object
that they belong to. This method uses the who
attribute. The value of who was set by the
constructor.
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Another method

• / Print an impersonal greeting message
• _at_author James Brucker
• /
• public class Greeting
• private String who
• / constructor for new objects /
• public Greeting( String name )
• who name // save the name
• public void sayHello( )
• System.out.println("Hello, "who)
• public void sayGoodbyte( )
• System.out.println("Goodbye, "
• who)

Another method of the class.
No main method!
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Creating Greeting Objects
We'll create a separate "test" class (in its own
file) that creates and uses Greeting objects.

• / Test the Greeting class.
• /
• public class TestGreeting
• public static void main(String args)
• Greeting a new Greeting("John")
• Greeting b new Greeting("Nok")
• a.sayHello( )
• b.sayHello( )
• b.sayGoodbye( )
• a.sayGoodbye( )

Create a Greeting object.
Create another Greeting object.
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Sending a Message to an Object

• / Test the Greeting class.
• /
• public class TestGreeting
• public static void main(String args)
• Greeting a new Greeting("John")
• Greeting b new Greeting("Nok")
• a.sayHello( )
• b.sayHello( )
• b.sayGoodbye( )
• a.sayGoodbye( )

In object-oriented speaking, people say "we send
a message to an object" to ask it to do
something..
Invoke sayHello method of the objects.
Output Hello, John. Hello, Nok. Goodbye,
Nok. Goodbye, John.
Notice that each object remembers its own
attributes.
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Sending messages
TestGreeting
create
Greeting
sayHello( )
sayGoodbye( )
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3 Fundamentals of Object-Oriented Paradigm

• Encapsulation - a class includes both the data
  and operations that operate on the data. It can
  hide the implementation details.
• A user of the class only sees the public
  interface.
• Polymorphism - many classes can perform the same
  behavior and we can use them interchangably.
• Inheritance - one class can build on top of
  another class and inherit all its methods and
  attributes.

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Examples