Object-Oriented Programming Basics

1
Object-Oriented Programming Basics

• Reading for this week
• BG 1-4.10
• UNIX 1,2,4,9
• JAVA 0,1
• BG Beginners Guide
• UNIX Harley Hahns Student guide to Unix,
  Harley Hahn
• JAVA An Introduction to Object Oriented
  Programming with Java, Thomas Wu

2
How can one design a program?

• Top-down structured design uses algorithmic
  decomposition where each module denotes a major
  step in some overall process
• Object-oriented design divides the problem into
  a set of objects that interacts to solve the
  problem. Your programs properties and behaviors
  are modelled based upon real objects like cars,
  books, houses, etc.

3
Why OOD?

• Software is complex (too many people is doing too
  many things the mess is inevitable ? )
• One of the main goals in programming is to avoid
  the redundancy and objects can help to do this
  (inheritance)
• Objects can help increase modularity through data
  hiding (encapsulation)

4
OK, but what is object ?

• An object is a thing, either tangible or
  intangible.
• An object-oriented program consists of many
  objects.
• An object is composed of identity, state
  (attributes, data, and their current values) and
  behavior (operations) .
• Identity? State? Behavior? Too many new
  expressions Ill get a headache!!!

5
Identity, State, Behavior

• Identity is the property of an object that
  distinguishes it from all other objects.
• The failure to recognize the difference between
  the name of the object and the object itself is
  the source of many errors in object-oriented (OO)
  programming.

6
Identity, State, Behavior

• The state of an object encompasses all of the
  (static) properties of the object plus the
  current (dynamic) values of each of these
  properties
• A property is an inherent or distinctive
  characteristic, trait, quality, or feature that
  contribute to making an object uniquely that
  object
• We will use the word attribute, or data member,
  to refer to the state of an object

7
Examples of State

• Properties
• Elevators travel up or down
• Vending machines accept coins
• Clocks indicate the current time
• Values
• Current floor
• Number of coins deposited
• The number of minutes since the last hour

8
Identity, State, Behavior

• Behavior is how an object acts and reacts, in
  terms of state changes and interactions with
  other objects.
• An operation is some action that one object
  performs upon another in order to elicit a
  reaction.
• We will use the word method to describe object
  behavior in java.
• Invoking a method causes the behavior to take
  place.

9
Objects and Classes graphical representation

• A graphical representation of an object
• lt Object name gt

• A graphical representation of a class
• lt Class name gt

10
Object example
Audi 6
BMW Z3
Corvette

• Notice that all objects are of the same type.
  All objects are cars!

11
Classes

• Classes are the definitions (or blueprints) used
  to create objects. Id say descriptions of
  objects.
• To make a car the manufacturer must first have a
  design from which to build the first car. Then,
  once all the problems are worked out, the design
  is used to build all the cars of that model.

12
Objects

• An object is an instance of a class.
• If we have a class definition called Car, then we
  can think of Audi, BMW, and Corvette as each
  being an instance (object) of the class Car,
  i.e., they are each a type of car.

13
Object example
Audi 6
BMW Z3
Corvette
Car
Car
Car

• Notice that all objects are of the same type.
  All objects are cars!

14
Classes and Objects

• An object is an instance of exactly one class!!!
• Corvette can not be an instance of a car class
  and an instance of a plane class at the same
  time.
• An instance of a class, an object, belongs to
  that particular class.
• A Corvette is a car ? Corvette belongs to the
  class Car.

15
Classes

• Once a class is defined you can create as many
  instances of the class (objects from the class)
  as you would like.
• Once a blue print is completed for the 2003
  Porsche 911, Porsche will use an assembly line to
  build as many instances of the 2003 Porsche 911
  as they wish.

16
Class car and objects- graphically
Car
This line shows an instance-of relationship.
Audi 6
BMW Z3
Corvette
Car
Car
Car
17
Defining a class

• Properties are variables which describe the
  essential characteristics of an object.
• Properties of a car color, model, make, how many
  doors, transmission type, direction of movement,
  etc.
• Behaviors are methods that describe how the
  object behaves and how the properties may be
  modified.
• Behavior of a car braking, changing gears,
  opening doors, moving forwards or backwards, etc.

18
Defining a class
Behavior or Method 1 Behavior
Behavior or
or
Method 4
Method 2 Behavior or Method 3
Properties or Instance Variables
19
Instance variables

• The class definition will include parameter
  definitions (properties) that represent data
  about a particular object, instance variables.
• Example, Jessica's car may have 4 gallons of gas
  in it while Clint's car has 10 gallons.
• The amount of gas in each car may change without
  affecting the amount of gas in the any other
  cars.
• All instances (objects) of a class will have a
  set of instance variables that are specific to
  that individual object.
• The combination of the values of these instance
  variables is known as the objects state.

20
Instance variables
Car
MaxSpeed
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155
MaxSpeed 165
MaxSpeed 145
21
Class variables

• The class definitions may also include parameter
  definitions that represent data that is shared by
  all class instances (objects), called class
  variables.
• In the case of the car class, we will define a
  maximum allowed speed, by the law (variable
  LMaxSpeed). This will be the same for each
  individual car (thats why your car have those
  annoying speed limiters ?).

22
Class variables
Car
MaxSpeed LMaxSpeed155
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155
MaxSpeed 165
MaxSpeed 145
23
Class variables

• Class variables may also be used to keep track of
  things such as how many instances of a class
  exist.
• Example lets create a counter the records how
  many cars are in the garage.

24
Class variables
Car
MaxSpeed LMaxSpeed155 NumCars 3
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155
MaxSpeed 165
MaxSpeed 145
25
Constant parameters

• If it was variables instead of parameters, that
  would be oxymoron.
• If there is a parameter whose value should not
  change while your program is running, that
  parameter type is called a constant.
• The LMaxSpeed parameter that we defined for the
  Car class is a constant. The maximal speed is
  limited by the low, and it cannot be changed once
  the car has been built.
• What about Z3 and 165 mph? Well, the owner has
  just made an illegal modification !!!

26
Messages
Audi 6 turnOnHazard()

• For Objects
• The object to whom the message is being sent.
• The name of the method (behavior) that object is
  to execute.
• Any parameters (variables) needed by that method.

• For Humans
• Who the message is for.
• What we want the person to do.
• What information is needed to do it.

27
Messages and Methods

• In order to process a message, an object needs to
  have a method defined for the requested task.
• A method is a small, well-defined piece of code
  that completes a specific task.
• For our previous example, we need to define a
  method to turn on the car's hazard lights.

28
Messages and Methods
Car
MaxSpeed LMaxSpeed155 NumCars 3 turnOnHazard()
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155 turnOnHazard()
MaxSpeed 165 turnOnHazard()
MaxSpeed 145 turnOnHazard()
29
Instance methods

• Each class can have methods that are specific to
  each object, called instance methods.
• These can only affect that object's parameters,
  i.e., its instance variables.
• Example If BMW has 4 gallons of gas and someone
  puts 6 more gallons of gas in his/her car, the
  car now has 10 gallons. The amount of gas in Audi
  and Corvette is unchanged.

30
Messages and Methods
Car
MaxSpeed LMaxSpeed155 NumCars
3 turnOnHazard() addGass(amount)
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155 turnOnHazard() addGass(amount)
MaxSpeed 165 turnOnHazard() addGass(amount)
MaxSpeed 145 turnOnHazard() addGass(amount)
31
Methods

• It is also possible that you want information
  from an object in this case, you would define a
  method that sends (returns) a message back to the
  requester containing that information.
• We need to know how much gas is in our cars, so
  we will create a new method that returns the
  value of GasLevel variable for our car.

32
Messages and Methods
Car
MaxSpeed GasLevel LMaxSpeed155 NumCars 3
Audi 6
Corvette
BMW Z3
Car
Car
Car
MaxSpeed 155 GasLevel 4 turnOnHazard() addGass
(amount) getGasLevel()GasLevel
MaxSpeed 145 GasLevel 6 turnOnHazard() addGass
(amount) getGasLevel()GasLevel
MaxSpeed 165 GasLevel 10
33
Class methods

• Class methods are used to get or manipulate
  information about all objects created from the
  class.
• Typically, class methods are changing class
  variables. For example
• Each time we move the car in or out of the
  garage, we need to add/subtract one to the number
  of cars carIn( ) carOut( )
• Also, we may want to know how many cars are
  actually in the garage getNumCars( )

34
Messages and Methods
Car
carIn()
carOut()
turnOnHazard()
getNumCars()NumCars
addGass(amount)
getGasLevel()GasLevel
Audi 6
BMW Z3
Corvette
Car
Car
Car
MaxSpeed 155 GasLevel 4 turnOnHazard() addGass
(amount) getGasLevel()GasLevel
MaxSpeed 165 GasLevel 10 turnOnHazard() addGas
s(amount) getGasLevel()GasLevel
MaxSpeed 145 GasLevel 6 turnOnHazard() addGass
(amount) getGasLevel()GasLevel
35
Types of methods

• There are 4 basic types of methods
• Modifier (sometimes called a mutator)
• Changes the value associated with an attribute of
  the object (or class)
• Accessor
• Returns the value associated with an attribute of
  the object (or class)
• Constructor
• Called once when the object is created (before
  any other method will be invoked)
• Destructor
• Called when the object is destroyed

36
OOP

• When writing object-oriented programs, first one
  must define the classes (like Car).
• Then, while the program is running, the instances
  of the classes (objects) (such as Audi, BMW,
  Corvette in our example) are created.

37
OOP

• Object-oriented programming allows the programmer
  to hide the implementation details from the other
  objects and the users.
• In other words the implementation is transparent
  to the other objects or the user.
• Example Although our computers all are different
  under the hood, we dont need to know whats
  there to be able to use them.

38
OOP - benefits

• An object can be written and maintained
  separately from the rest of the program,
  modularity.
• An object has a public face that it uses to
  communicate with other objects, but other objects
  can not directly access its instance variables,
  information hiding.

39
Inheritance

• All classes in Java are organized into a class
  hierarchy.
• The highest level classes are very general and
  the lower level classes are more specific.
• The lower level classes are based upon the higher
  level classes and inherit instance variables and
  methods from those higher level class. They also
  may contain their own (new) instance variables
  and methods beyond the higher level class
  definition.

40
Inheritance

• A higher level class is called a superclass a
  lower level class is called a subclass.
• A subclass may also be a superclass
• Inheritance allows you to define certain
  behaviors once and then to reuse those behaviors
  over and over again in the subclasses. This is
  called reusability.

41
Inheritance

• Our Car class is very general.
• Let's define a new class called BMW that contains
  the parameters model, color, engine size.

42
Inheritance
Car
BMW
43
Inheritance

• Now let's define two new classes. One for the Z3
  and another for the 3 Series Sedan.
• What might be some of the differences between the
  two classes?
• Number of doors (3, 5)
• Roof (soft or hardtop)
• Therefore, we add variables NumDoors and Roof

44
Inheritance
Car
45
Views of the class

• A class can be viewed as a sort of contract that
  specifies what instances of the class can, and
  cannot do
• It is possible to distinguish between the outside
  and inside view of a class
• The interface of a class provides its outside
  view and emphasizes the abstraction
• The implementation of a class is its inside view

46
Access

• Most classes provide three levels of access to
  their members (state and behavior)
• Public
• The part of the class that is visible to all
  clients of the class
• Protected
• The part of the class that is only visible to
  subclasses of the class
• Private
• A part of the class that is not visible to any
  other classes

47
Primitive data types

• What ?!!! Primitive? What is it?
• Certain basic data types are built into Java
• Alphabetical char
• Integral int, byte, short, long
• Real float, double
• Logical boolean
• Java supports 8 primitive types (i.e. 4 basic
  types)

48
Primitive data types
Type Description Bits Range of values
boolean Boolean 1 False, true
byte integers, very small 8 -128 to 127
short integers, small ones 16 -32768 to 32767
int integer 32 -2,147,483,648 to 2,147,483,647
long integer, very big ones 64 /- 9.2 1018
float floating point number 32 /-3.402923471038
double floating point number, very accurate 64 /-1.7976931348623157010308
char character 16 Unicode character (eg \u12d1)
49
Primitive data types

• When typing integer into a program, the number
  must have an L at the end 3932354162L
• Real numbers are represented in scientific
  notation
• mantissa -1,1
• exponent (powers of two)

50
Real data type

• Characterized by precision and range
• Precision represents number of significant
  digits that is represented in the number. Depends
  upon the number of bits in the mantissa.
• Range represents the difference between the
  largest and smallest numbers that can be
  represented. Depends upon the number of bits in
  the exponent.
• float
• 24b mantissa 8b exponent
• 6-7 decimal digits of precision
• double
• 53b mantissa 11b exponent
• 15-16 decimal digits of precision

51
Round-off error

• When a double is stored to a float, part of the
  number is lost. In this situation, the seven most
  significant digits are preserved and the rest of
  the information is lost.

52
Char

• Character values may represent both printable and
  non-printable characters
• Printable a, ,
• Non-printable \n new line, \t tab,
• Unicode 16b support for every character in the
  world \u0000 to \uFFFF