 2000 Prentice Hall, Inc. All rights reserved. Chapter 26 - Java Object-Based Programming Outline 26.1Introduction 26.2Implementing a Time Abstract Data Type with a Class 26.3Class Scope 26.4Creating Packages 26.5Initializing Class Objects: Constructors 26.6Using Set and Get Methods 26.7Using the this Reference 26.8Finalizers 26.9Static Class Members

 2000 Prentice Hall, Inc. All rights reserved Introduction Object-oriented programming (OOP) –Encapsulates data (attributes) and functions (behavior) into packages called classes –Data and functions closely related Information hiding –Implementation details are hidden within the classes themselves Unit of Java programming: the class –A class is like a blueprint – reusable –Objects are instantiated (created) from the class –For example, a house is an instance of a “blueprint class” –C programmers concentrate on functions

 2000 Prentice Hall, Inc. All rights reserved Implementing a Time Abstract Data Type with a Class In our example –Define two classes, Time1 and TimeTest in separate files Only one public class per file Class definitions –Never really create definition from scratch Use extends to inherit data and methods from base class Derived class: class that inherits –Every class in Java subclass of Object Gets useful methods, discussed later –Class body Delineated by braces { } Declare instance variables and methods

 2000 Prentice Hall, Inc. All rights reserved Implementing a Time Abstract Data Type with a Class (II) Member-access modifiers –public : accessible whenever program has a reference to an object of the class –private : accessible only to member methods of that class –Member variables are usually private Methods –Access methods: public methods that read/display data public interface Clients use references to interact with objects –Utility methods: private methods that support access methods

 2000 Prentice Hall, Inc. All rights reserved Implementing a Time Abstract Data Type with a Class (II) Constructor –Special member function Same name as the class –Initializes data members of a class object –Constructors cannot return values Declarations –Once class defined, can be used as a data type –Declare objects of the class Time1 myTimeObject = new myTime1(); Declares object, initializes with constructor

 2000 Prentice Hall, Inc. All rights reserved Implementing a Time Abstract Data Type with a Class (III) import –If no package specified for class, class put in default package Includes compiled classes of current directory –If class in same package as another, import not required –import when classes not of same package Classes simplify programming –Client only concerned with public operations –Client not dependent on implementation details If implementation changes, client unaffected –Software reuse

 2000 Prentice Hall, Inc. All rights reserved Implementing a Time Abstract Data Type with a Class (IV) Method toString –Class Object –Takes no arguments, returns a String –Used as a placeholder, usually overridden Class DecimalFormat ( java.text ) –Create object of class, initialize with format control string DecimalFormat twoDigits = new DecimalFormat( "00" ); – Each 0 is a placeholder for a digit Prints in form 08, 10, –Method format returns String with proper formatting twoDigits.format( myInt );

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. import 1.1 Class definition extends Object 1.2 Constructor (same name as class) 1.3 Method toUniversalString 1// Fig. 26.1: Time1.java 2// Time1 class definition 3import java.text.DecimalFormat; // used for number formatting 4 5// This class maintains the time in 24-hour format 6public class Time1 extends Object { 7 private int hour; // private int minute; // private int second; // // Time1 constructor initializes each instance variable 12 // to zero. Ensures that each Time1 object starts in a 13 // consistent state. 14 public Time1() 15 { 16 setTime( 0, 0, 0 ); 17 } // Set a new time value using universal time. Perform 20 // validity checks on the data. Set invalid values to zero. 21 public void setTime( int h, int m, int s ) 22 { 23 hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); 24 minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); 25 second = ( ( s >= 0 && s < 60 ) ? s : 0 ); 26 } // Convert to String in universal-time format 29 public String toUniversalString() 30 {

 2000 Prentice Hall, Inc. All rights reserved. Outline 1.4 "00" signifies two digits to be printed 2. Override toString 31 DecimalFormat twoDigits = new DecimalFormat( "00" ); return twoDigits.format( hour ) + ":" + 34 twoDigits.format( minute ) + ":" + 35 twoDigits.format( second ); 36 } // Convert to String in standard-time format 39 public String toString() 40 { 41 DecimalFormat twoDigits = new DecimalFormat( "00" ); return ( ( hour == 12 || hour == 0 ) ? 12 : hour % 12 ) + 44 ":" + twoDigits.format( minute ) + 45 ":" + twoDigits.format( second ) + 46 ( hour < 12 ? " AM" : " PM" ); 47 } 48}

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. import JOptionPane (no need to import Time1 ) 2. main 2.1 toString called implicitly to make object of correct type 49// Fig. 26.1: TimeTest.java 50// Class TimeTest to exercise class Time1 51import javax.swing.JOptionPane; 52 53public class TimeTest { 54 public static void main( String args[] ) 55 { 56 Time1 t = new Time1(); // calls Time1 constructor 57 String output; output = "The initial universal time is: " + 60 t.toUniversalString() + 61 "\nThe initial standard time is: " + 62 t.toString() + 63 "\nImplicit toString() call: " + t; t.setTime( 13, 27, 6 ); 66 output += "\n\nUniversal time after setTime is: " + 67 t.toUniversalString() + 68 "\nStandard time after setTime is: " + 69 t.toString(); t.setTime( 99, 99, 99 ); // all invalid values 72 output += "\n\nAfter attempting invalid settings: " + 73 "\nUniversal time: " + t.toUniversalString() + 74 "\nStandard time: " + t.toString(); 75

 2000 Prentice Hall, Inc. All rights reserved. Outline Program Output 76 JOptionPane.showMessageDialog( null, output, 77 "Testing Class Time1", 78 JOptionPane.INFORMATION_MESSAGE ); System.exit( 0 ); 81 } 82}

 2000 Prentice Hall, Inc. All rights reserved Class Scope Class scope –Instance variables and methods –Class members accessible to methods Can be referenced by name –Outside scope, cannot be referenced by name –Visible ( public ) members accessed through a handle objectReferenceName.VariableName Block scope –Variables defined in a method known only to that method –If variable has same name as class variable, class variable hidden –Can be accessed using keyword this (discussed later)

 2000 Prentice Hall, Inc. All rights reserved Creating Packages Packages –Directory structures that organize classes and interfaces –Mechanism for software reuse Creating packages –Create a public class If not public, can only be used by classes in same package –Choose a package name and add a package statement to source code file –Compile class (placed into appropriate directory) –Import into other programs Naming: Internet domain name in reverse order After name reversed, choose your own structure package com.deitel.chtp3.ch26; –See text for detailed instructions

 2000 Prentice Hall, Inc. All rights reserved Initializing Class Objects: Constructors Constructor –Can initialize members of an object –Cannot have return type –Class may have overloaded constructors –Initializers passed as arguments to constructor –Declaration/initialization of new objects takes form: ref = new ClassName( arguments ); Constructor has same name as class –If no constructor defined, compiler makes default constructor Defaults: 0 for primitive numeric types, false for boolean, null for references If constructor defined, no default constructor –Can have constructor with no arguments

 2000 Prentice Hall, Inc. All rights reserved Using Set and Get Methods Set methods –public method that sets private variables –Does not violate notion of private data Change only the variables you want –Called mutator methods (change value) Get methods –public method that displays private variables –Again, does not violate notion of private data Only display information you want to display –Called accessor or query methods

 2000 Prentice Hall, Inc. All rights reserved Using Set and Get Methods (II) Every event has a source –GUI component with which user interacted –ActionEvent parameter can check its source Method getSource public void actionPerformed( ActionEvent e ) if ( e.getSource() == myButton )

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. package statement 1.1 import 1.2 Constructor 1.3 Method definitions 1// Fig. 26.3: Time2.java 2// Time2 class definition 3package com.deitel.chtp3.ch26; // place Time2 in a package 4import java.text.DecimalFormat; // used for number formatting 5 6// This class maintains the time in 24-hour format 7public class Time2 extends Object { 8 private int hour; // private int minute; // private int second; // // Time2 constructor initializes each instance variable 13 // to zero. Ensures that Time object starts in a 14 // consistent state. 15 public Time2() { setTime( 0, 0, 0 ); } // Set Methods 18 // Set a new time value using universal time. Perform 19 // validity checks on the data. Set invalid values to zero. 20 public void setTime( int h, int m, int s ) 21 { 22 setHour( h ); // set the hour 23 setMinute( m ); // set the minute 24 setSecond( s ); // set the second 25 } // set the hour 28 public void setHour( int h ) 29 { hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); } 30

 2000 Prentice Hall, Inc. All rights reserved. Outline 1.3 Method definitions (notice get and set methods) 31 // set the minute 32 public void setMinute( int m ) 33 { minute = ( ( m >= 0 && m < 60 ) ? m : 0 ); } // set the second 36 public void setSecond( int s ) 37 { second = ( ( s >= 0 && s < 60 ) ? s : 0 ); } // Get Methods 40 // get the hour 41 public int getHour() { return hour; } // get the minute 44 public int getMinute() { return minute; } // get the second 47 public int getSecond() { return second; } // Convert to String in universal-time format 50 public String toUniversalString() 51 { 52 DecimalFormat twoDigits = new DecimalFormat( "00" ); return twoDigits.format( getHour() ) + ":" + 55 twoDigits.format( getMinute() ) + ":" + 56 twoDigits.format( getSecond() ); 57 } // Convert to String in standard-time format 60 public String toString()

 2000 Prentice Hall, Inc. All rights reserved. Outline 1.3 Method definitions 61 { 62 DecimalFormat twoDigits = new DecimalFormat( "00" ); return ( ( getHour() == 12 || getHour() == 0 ) ? : getHour() % 12 ) + ":" + 66 twoDigits.format( getMinute() ) + ":" + 67 twoDigits.format( getSecond() ) + 68 ( getHour() < 12 ? " AM" : " PM" ); 69 } 70}

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. import (notice package containing Time2) 1.1 implements ActionListener 71// Fig. 26.3: TimeTest.java 72// Demonstrating the Time2 class set and get methods 73import java.awt.*; 74import java.awt.event.*; 75import javax.swing.*; 76import com.deitel.chtp3.ch26.Time2; 77 78public class TimeTest extends JApplet 79 implements ActionListener { 80 private Time2 t; 81 private JLabel hourLabel, minuteLabel, secondLabel; 82 private JTextField hourField, minuteField, 83 secondField, display; 84 private JButton tickButton; public void init() 87 { 88 t = new Time2(); Container c = getContentPane(); c.setLayout( new FlowLayout() ); 93 hourLabel = new JLabel( "Set Hour" ); 94 hourField = new JTextField( 10 ); 95 hourField.addActionListener( this ); 96 c.add( hourLabel ); 97 c.add( hourField ); minuteLabel = new JLabel( "Set minute" ); 100 minuteField = new JTextField( 10 );

 2000 Prentice Hall, Inc. All rights reserved. Outline 2. Define actionPerformed 2.1 Notice e.getSource 101 minuteField.addActionListener( this ); 102 c.add( minuteLabel ); 103 c.add( minuteField ); secondLabel = new JLabel( "Set Second" ); 106 secondField = new JTextField( 10 ); 107 secondField.addActionListener( this ); 108 c.add( secondLabel ); 109 c.add( secondField ); display = new JTextField( 30 ); 112 display.setEditable( false ); 113 c.add( display ); tickButton = new JButton( "Add 1 to Second" ); 116 tickButton.addActionListener( this ); 117 c.add( tickButton ); updateDisplay(); 120 } public void actionPerformed( ActionEvent e ) 123 { 124 if ( e.getSource() == tickButton ) 125 tick(); 126 else if ( e.getSource() == hourField ) { 127 t.setHour( 128 Integer.parseInt( e.getActionCommand() ) ); 129 hourField.setText( "" ); 130 }

 2000 Prentice Hall, Inc. All rights reserved. Outline 2.2 Method definitions 131 else if ( e.getSource() == minuteField ) { 132 t.setMinute( 133 Integer.parseInt( e.getActionCommand() ) ); 134 minuteField.setText( "" ); 135 } 136 else if ( e.getSource() == secondField ) { 137 t.setSecond( 138 Integer.parseInt( e.getActionCommand() ) ); 139 secondField.setText( "" ); 140 } updateDisplay(); 143 } public void updateDisplay() 146 { 147 display.setText( "Hour: " + t.getHour() "; Minute: " + t.getMinute() "; Second: " + t.getSecond() ); 150 showStatus( "Standard time is: " + t.toString() "; Universal time is: " + t.toUniversalString() ); 152 } public void tick() 155 { 156 t.setSecond( ( t.getSecond() + 1 ) % 60 ); if ( t.getSecond() == 0 ) { 159 t.setMinute( ( t.getMinute() + 1 ) % 60 ); 160

 2000 Prentice Hall, Inc. All rights reserved. Outline 2.2 Method definitions 161 if ( t.getMinute() == 0 ) 162 t.setHour( ( t.getHour() + 1 ) % 24 ); 163 } 164 } 165}

 2000 Prentice Hall, Inc. All rights reserved. Outline Program Output

 2000 Prentice Hall, Inc. All rights reserved Using the this Reference Each object has a reference to itself –The this reference Implicitly used to refer to instance variables and methods Inside methods –If parameter has same name as instance variable Instance variable hidden –Use this.variableName to explicitly refer to the instance variable –Use variableName to refer to the parameter

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. import 2. main 2.1 Initialize object 2.2 Define class 2.3 Constructor (note use of this ) 1// Fig. 26.4: ThisTest.java 2// Using the this reference to refer to 3// instance variables and methods. 4import javax.swing.*; 5import java.text.DecimalFormat; 6 7public class ThisTest { 8 public static void main( String args[] ) 9 { 10 SimpleTime t = new SimpleTime( 12, 30, 19 ); JOptionPane.showMessageDialog( null, t.buildString(), 13 "Demonstrating the \"this\" Reference", 14 JOptionPane.INFORMATION_MESSAGE ); System.exit( 0 ); 17 } 18} 19 20class SimpleTime { 21 private int hour, minute, second; public SimpleTime( int hour, int minute, int second ) 24 { 25 this.hour = hour; 26 this.minute = minute; 27 this.second = second; 28 } public String buildString()

 2000 Prentice Hall, Inc. All rights reserved. Outline 2.4 Method definitions (note use of this ) Program Output 31 { 32 return "this.toString(): " + this.toString() + 33 "\ntoString(): " + toString() + 34 "\nthis (with implicit toString() call): " + 35 this; 36 } public String toString() 39 { 40 DecimalFormat twoDigits = new DecimalFormat( "00" ); return twoDigits.format( this.hour ) + ":" + 43 twoDigits.format( this.minute ) + ":" + 44 twoDigits.format( this.second ); 45 } 46}

 2000 Prentice Hall, Inc. All rights reserved Finalizers Memory –Constructors use memory when creating new objects –Automatic garbage collection When object no longer used, object marked for garbage collection Garbage collector executes, memory can be reclaimed Memory leaks less common in Java than in C and C++ finalizer method –In every class, returns resources to system Performs termination housekeeping on object –Name always finalize Takes no parameters, returns no value –Defined in class Object as a placeholder Every class gets a finalize method

 2000 Prentice Hall, Inc. All rights reserved Static Class Members Static variables –Usually, each object gets its own copy of each variable –static class variables shared among all objects of the class One copy exists for entire class to use –Keyword static –Only have class scope (not global) –static class variables exist even when no objects do –public static members accessed through references or class name and dot operator –private static members accessed through methods If no objects exist, classname and public static method must be used

 2000 Prentice Hall, Inc. All rights reserved Static Class Members (II) static methods –Can only access class static members –static methods have no this reference static variables are independent of objects Method gc –public static method of class System –Suggests garbage collector execute immediately Can be ignored Garbage collector not guaranteed to collect objects in a specific order

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. Class definition 1.1 static variable 1.2 Constructor 1.3 Method finalize 1.4 Get methods 1// Fig. 26.5: Employee.java 2// Declaration of the Employee class. 3public class Employee extends Object { 4 private String firstName; 5 private String lastName; 6 private static int count; // # of objects in memory 7 8 public Employee( String fName, String lName ) 9 { 10 firstName = fName; 11 lastName = lName; count; // increment static count of employees 14 System.out.println( "Employee object constructor: " + 15 firstName + " " + lastName ); 16 } protected void finalize() 19 { 20 --count; // decrement static count of employees 21 System.out.println( "Employee object finalizer: " + 22 firstName + " " + lastName + 23 "; count = " + count ); 24 } public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public static int getCount() { return count; } 31}

 2000 Prentice Hall, Inc. All rights reserved. Outline 1. Class definition 1.1 Initialize objects 32// Fig. 8.12: EmployeeTest.java 33// Test Employee class with static class variable, 34// static class method, and dynamic memory. 35import javax.swing.*; 36 37public class EmployeeTest { 38 public static void main( String args[] ) 39 { 40 String output; output = "Employees before instantiation: " + 43 Employee.getCount(); Employee e1 = new Employee( "Susan", "Baker" ); 46 Employee e2 = new Employee( "Bob", "Jones" ); output += "\n\nEmployees after instantiation: " + 49 "\nvia e1.getCount(): " + e1.getCount() + 50 "\nvia e2.getCount(): " + e2.getCount() + 51 "\nvia Employee.getCount(): " + 52 Employee.getCount(); output += "\n\nEmployee 1: " + e1.getFirstName() + 55 " " + e1.getLastName() + 56 "\nEmployee 2: " + e2.getFirstName() + 57 " " + e2.getLastName(); // mark objects referred to by e1 and e2 60 // for garbage collection

 2000 Prentice Hall, Inc. All rights reserved. Outline 2. System.gc Program Output 61 e1 = null; 62 e2 = null; System.gc(); // suggest that garbage collector be called output += "\n\nEmployees after System.gc(): " + 67 Employee.getCount(); JOptionPane.showMessageDialog( null, output, 70 "Static Members and Garbage Collection", 71 JOptionPane.INFORMATION_MESSAGE ); 72 System.exit( 0 ); 73 } 74}