Chapter 12 ObjectBased Programming 1

1
Chapter 12 Object-Based Programming 1
Introduction

• What is Object Oriented Programming?
• Object oriented Programming languages (OOP for
  short) include all the features of structured
  programming and add more powerful ways to
  organize algorithms and data structures.
• There are three key features of OOP languages
• Encapsulation
• Inheritance
• Polymorphism
• All of them are tied to the notion of a class
• These two lectures focus on encapsulation through
  the use of classes and objects - Object Based
  Programming.

2
Classes and Objects

• A class defines a set of variables (data members)
  that can be associated with the methods which act
  on an object with the object itself.
• An object is created through the instantiation of
  a class.
• You can look around you now and see many examples
  of real-world objects your dog, your desk, your
  television set, your bicycle.
• These real-world objects share two
  characteristics they all have state and they all
  have behavior. For example, cars have state
  (current gear, number of seats, four wheels,
  etc.) and behavior (braking, accelerating,
  slowing down, changing gears).

3
Classes

• A class can be viewed as a blueprint of an
  object.
• It is the model or pattern from which objects are
  created.
• For example, the String class is used to define
  String objects.
• Each String object contains specific characters
  (its state).
• Each String object can perform services
  (behaviors) such as toUpperCase.

4
Classes
class TestStringClass public static void
main(String s) String s1 new
String("Programming") String s2 new
String("Java") String s3 new
String("ivet-ty") System.out.println(s1.toU
pperCase()) System.out.println(s2.toUpperCas
e()) System.out.println(s3.toUpperCase())

In this example, String is class. s1, s2 and s3
are the objects (or references to objects,
precisely speaking). Each of them has different
state but same behavior.
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Classes and Objects
String (class)

• The String class can be used to define as many
  String objects as you like.
• Its reusability is very high.
• The String class is provided for us by the Java
  standard class library.
• But we can also write our own classes that define
  specific objects that we need

6
Classes

• The following diagram explains the relationship
  between class and object.

7
Classes

• A class contains data declarations and method
  declarations

int x, y char ch
Data declarations
Method declarations
8
Example

• Let's define a very simple Employee class which
  has one data member and two methods.

• class Employee
• int empnum // data member (instance
  variable)
• public int getNum() // method
• return empnum
• public void setNum(int newNum) // method
• empnum newNum

9
Example

• Thus when we create a new object we say we are
  instantiating the object. Each class exists only
  once in a program, but there can be many
  thousands of objects that are instances of that
  class.
• To instantiate an object in Java we use the new
  operator. Here's how we'd create a new employee
• Employee x new Employee()

• The instantiation can also be done in two lines
• Employee x
• x new Employee()

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Example

• The members of an object are accessed using the .
  (dot) operator, as follows
• Employee x new Employee()
• x.setNum(12651)

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Example

• We define a test driver class to test the
  Employee class

• class Employee
• private int empnum
• public int getNum()
• return empnum
• public void setNum(int newNum)
• empnum newNum
• class TestEmployee
• public static void main(String args)
• Employee emp1 new Employee()
• Employee emp2 new Employee()
• emp1.setNum(12651)
• emp2.setNum(36595)
• System.out.println("num of emp1 "
  emp1.getNum())
• System.out.println("num of emp2 "
  emp2.getNum())

Sample Run gt java TestEmployee num of emp1
12651 num of emp2 36595
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• Note when you compile the file Employee.java, you
  get two class files
• Employee.class and TestEmployee.class.
• You can save the above program (two class
  definitions) into one source file.
• But if both of them are public, they must be
  defined in separate files.
• Every Java class must extend another class
• If class does not explicitly extend another class
• class implicitly extends Object
• So class Employee extends java.lang.Object
• is same as class Employee

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Encapsulation

• As you can see from the above diagram, an
  object's variables make up the centre or nucleus
  of the object. Methods surround and hide the
  object's nucleus from other objects in the
  program.
• This is called encapsulation.

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Encapsulation

• Encapsulation is used to hide unimportant
  implementation details from other objects.
• Thus, the implementation details can change at
  any time without affecting other parts of the
  program.
• Benefits of Encapsulation
• Modularity -- each object can be written and
  maintained independently of other objects. Also,
  an object can be easily passed around in the
  system.
• Information hiding -- an object can maintain
  private information and methods that can be
  changed at any time without affecting the other
  objects that depend on it

15
A BAD Example

• The following example shows an incorrect use of
  public data member.

• class PublicDataEmployee
• public int empnum
• class PrintEmployee1
• public static void main(String args)
• PublicDataEmployee emp new
  PublicDataEmployee()
• emp.empnum 12651
• System.out.println("num of emp "
  emp.empnum)

gt java PrintEmployee1 num of emp 12651
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A GOOD Example

• The following shows how to fix the problem in the
  previous example.

• class PrivateDataEmployee
• private int empnum
• public int getNum()
• return empnum
• public void setNum(int newNum)
• empnum newNum
• class PrintEmployee2
• public static void main(String args)
• PrivateDataEmployee emp new
  PrivateDataEmployee()
• emp.setNum(12651)
• System.out.println("num of emp "
  emp.getNum())

gt java PrintEmployee2 num of emp 12651
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Why is PrivateDataEmployee better?

• PublicDataEmployee is shorter than
  PrivateDataEmployee.
• Why do we say PrivateDataEmployee is better?
• Suppose the company wants to make the employee
  number more meaningful by dividing the employee
  number into two parts, deptID and staffID, and
  the employee number is obtained by the formula
  deptID1000 staffID.
• e.g. If the IT Dept has department ID 13 and the
  staff ID of Paul Leung is 228, then his employee
  number is 13228.

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• Consider the class PublicDataEmployee, it needs
  to be changed to

class PublicDataEmployee2 public int
deptID public int staffID class
PrintEmployee3 public static void
main(String args) PublicDataEmployee2
emp new PublicDataEmployee2() emp.empnum
12651 System.out.println("num of emp "
emp.empnum)

• PrintEmployee3 cannot be compiled anymore. You
  have to change every occurrence of emp.empnum in
  all programs that use PublicDataEmployee!!!!

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• The class PrivateDataEmployee can be modified to
  meet the new requirement easily.

gtjava PrintEmployee4 num of emp 12651
class PrivateDataEmployee2 private int
deptID private int staffID public int
getNum() return deptID1000staffID
public void setNum(int newNum)
deptID newNum/1000 staffID
newNum1000 class PrintEmployee4
public static void main(String args)
PublicDataEmployee emp new PublicDataEmployee()
emp.setNum(12651)
System.out.println("num of emp "
emp.getNum())
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• We only need to change PrivateDataEmployee2 and
  all other classes that depend on it (e.g.
  PrintEmployee4) does not need to change.
• The use of private data with set and get methods
  can also protect the data from invalid
  modification.
• e.g. To prevent setting of an employee number to
  a negative value, you can modify setNum as

public void setNum(int newNum) if (newNum lt
0) System.out.println("Invalid emp.
num.") else empnum newNum
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Controlling Access to Members

• Member access modifiers
• Control access to class instance variables and
  methods
• public
• Variables and methods accessible to other classes
• private
• Variables and methods not accessible to other
  classes
• Can be accessed via
• Accessor method (get method)
• Allow clients to read private data
• Mutator method (set method)
• Allow clients to modify private data

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class PrivateDataEmployee3 private int
empnum public int getNum() return
empnum public void setNum(int newNum)
empnum newNum class
PrintEmployee5 public static void
main(String args) PrivateDataEmployee3
emp new PrivateDataEmployee3()
emp.empnum 12651
gtjavac PrivateDataEmployee3.java PrivateDataEmploy
ee3.java15 empnum has private access in
PrivateDataEmployee3 emp.empnum 12651
1 error
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Data Scope (Revisited)

• The scope of data is the area in a program in
  which that data can be used (referenced)
• Data declared at the class level can be used by
  ALL methods in that class
• Data declared within a method can ONLY be used in
  that method
• Data declared within a method is called local data

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Method Control Flow

• The called method could be part of another class
  or object

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Initializing Class Objects Constructors

• Class constructor
• Same name as class
• No return type or return statement
• Initializes instance variables of a class object
• Cannot be called directly
• Called when instantiating object of that class
• If no constructor is defined by the programmer, a
  default "do-nothing" constructor is created for
  you.
• Java does not provide a default constructor if
  the class define a constructor of its own.

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Constructor Example

• class Employee1
• private int empnum
• public Employee1(int newNum)
• empnum newNum // call setNum is better,
  why?
• public int getNum()
• return empnum
• public void setNum(int newNum)
• empnum newNum
• class PrintEmployee5
• public static void main(String args)
• Employee1 emp new Employee1(71623)
• System.out.println(emp.getNum())

gtjava PrintEmployee5 71623
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Using Overloaded Constructors

• Overloaded constructors
• Methods (in same class) may have same name
• Must have different parameter lists
• Examples in Java
• String
• JButton
• .......

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Time2.javaLines 16-19Default constructor has
no arguments Lines 23-26Overloaded constructor
has one int argument Lines 30-33Second
overloaded constructor has two int arguments

• 1 // Fig. 8.6 Time2.java
• 2 // Time2 class definition with overloaded
  constructors.
• 3
• 4
• 5 // Java core packages
• 6 import java.text.DecimalFormat
• 7
• 8 public class Time2
• 9 private int hour // 0 - 23
• 10 private int minute // 0 - 59
• 11 private int second // 0 - 59
• 12
• 13 // Time2 constructor initializes each
  instance variable
• 14 // to zero. Ensures that Time object
  starts in a
• 15 // consistent state.
• 16 public Time2()
• 17
• 18 setTime( 0, 0, 0 )
• 19

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Time2.javaLines 36-39Third overloaded
constructor has three int arguments Lines
42-45Fourth overloaded constructor has Time2
argument

• 35 // Time2 constructor hour, minute and
  second supplied
• 36 public Time2( int h, int m, int s )
• 37
• 38 setTime( h, m, s )
• 39
• 40
• 41 // Time2 constructor another Time2
  object supplied
• 42 public Time2( Time2 time )
• 43
• 44 setTime( time.hour, time.minute,
  time.second )
• 45
• 46
• 47 // Set a new time value using universal
  time. Perform
• 48 // validity checks on data. Set invalid
  values to zero.
• 49 public void setTime( int h, int m, int s
  )
• 50
• 51 hour ( ( h gt 0 h lt 24 ) ? h 0
  )
• 52 minute ( ( m gt 0 m lt 60 ) ? m
  0 )
• 53 second ( ( s gt 0 s lt 60 ) ? s
  0 )

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Time2.java

• 70
• 71 return ( (hour 12 hour 0) ?
  12 hour 12 )
• 72 "" twoDigits.format( minute )
• 73 "" twoDigits.format( second )
• 74 ( hour lt 12 ? " AM" " PM" )
• 75
• 76
• 77 // end class Time2

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TimeTest4.java Line 15Declare six references
to Time2 objects Lines 17-22Instantiate each
Time2 reference using a different constructor

• 1 // Fig. 8.7 TimeTest4.java
• 2 // Using overloaded constructors
• 3
• 4 // Java extension packages
• 5 import javax.swing.
• 6
• 7
• 8
• 9
• 10 public class TimeTest4
• 11
• 12 // test constructors of class Time2
• 13 public static void main( String args )
• 14
• 15 Time2 t1, t2, t3, t4, t5, t6
• 16
• 17 t1 new Time2() //
  000000
• 18 t2 new Time2( 2 ) //
  020000
• 19 t3 new Time2( 21, 34 ) //
  213400

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TimeTest4.java

• 34 output "\nt3 hour and minute
  specified "
• 35 "second defaulted"
• 36 "\n " t3.toUniversalString()
  
• 37 "\n " t3.toString()
• 38
• 39 output "\nt4 hour, minute, and
  second specified"
• 40 "\n " t4.toUniversalString()
  
• 41 "\n " t4.toString()
• 42
• 43 output "\nt5 all invalid values
  specified"
• 44 "\n " t5.toUniversalString()
  
• 45 "\n " t5.toString()
• 46
• 47 output "\nt6 Time2 object t4
  specified"
• 48 "\n " t6.toUniversalString()
  
• 49 "\n " t6.toString()
• 50
• 51 JOptionPane.showMessageDialog( null,
  output,
• 52 "Demonstrating Overloaded
  Constructors",

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TimeTest4.javaDifferent outputs, because each
Time2 reference was instantiated with a different
constructor
Different outputs, because each Time2 reference
was instantiated with a different constructor