Java 语言程序设计

Java 语言程序设计

华中师范大学计算机科学系

王明安

2

华中师范大学计科系

第五讲面向对象编程（ OOP ） OOP 概念举例

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OOP 概念 OOP(Objective-Oriented Program) 结构化程序设计

Algorithms + Data Structures = Programs OOP

Objective(Data Structures)-Oriented 程序由对象组成，算法封装在对象里对象就是数据结构每个对象有自己的任务和方法算法通过对象自己的方法和调用别的对象的方法实

现

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OOP 概念思考对象的时候，需要采用形象思维，

而不是程序化的思维。根据问题来描述问题，而不是根据方案。很难很好地设计出对象，只有数量相当

少的“专家”能设计出最好的对象，然后让其他人享用。

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OOP 概念 “ 纯粹”的面向对象程序设计方法

所有东西都是对象。程序是一大堆对象的组合；每个对象都有自己的存储空间，可容纳其他

对象。每个对象都有一种类型。同一类所有对象都能接收相同的消息。

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OOP 概念抽象和建模

抽象的目的通过抽象实现简化通过抽象实现概括

抽象的方法分级分类进行抽象

用抽象的结果来建立模型

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OOP 概念问题

无数的建模方法没有正确的模型，只有较好的不正确的模型充分反映用户需求的模型

用户工程师

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Java How to Program, fourth Edition

Object-oriented programming (OOP) Encapsulates （封装） data (attributes) and functions

(behavior) into packages called classes Data and functions closely related

Information hiding Implementation details are hidden within the classes th

emselves Unit of Java programming: the class

A class is like a blueprint – reusable Objects are instantiated ( 例示， created) from the class For example, a house is an instance of a “blueprint cla

ss” C programmers concentrate on functions

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华中师范大学计科系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 （继承） data and methods from ba

se 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

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举例class 课堂

{class 教室 ; class 教师 ;class 学生 [];class 科目 ;String 上课时间 ;boolean 课堂状态 ;

……}

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举例class 教室

{class 房屋 ;class 讲台 ;class 黑板 ;class 桌椅 ;

int 桌椅数 ; String 编号；}

class 多媒体教室 extends 教室{

class 投影机 ;

class 屏幕 ;class 计算机 ;

class 扩音设备 ;}

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举例class 教师 extends

人{String 系别 ;String 职称 ;……}

class 学生 extends 人{ class 成绩 ; String 系别 ; String 学号 ; ……}

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举例class 课堂

{

课堂 (String r_num, String t_name, String obj_name, String c_time)

{ 多媒体教室 room = new 多媒体教室 ; 教师 teach = new 教师 ;学生 [] student;科目 object = new 科目 ;

room.set 编号 (r_num); teach.set 名字 (t_name);

student = new 学生 [65]; object.set 名称 (obj_name);

上课时间 = c_time;}

……}

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举例main()

{课堂 J_class = new 课堂 (“740

2”, “ 王明安” , “JAVA”, “401”);

……}

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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

3 import java.text.DecimalFormat; // used for number formatting

4

5 // This class maintains the time in 24-hour format

6 public class Time1 extends Object {

7 private int hour; // 0 - 23

8 private int minute; // 0 - 59

9 private int second; // 0 - 59

10

11 // 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 }

18

19 // 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 }

27

28 // Convert to String in universal-time format

29 public String toUniversalString()

30 {

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1.4 "00" signifies two digits to be printed

2. Override toString

31 DecimalFormat twoDigits = new DecimalFormat( "00" );

32

33 return twoDigits.format( hour ) + ":" +

34 twoDigits.format( minute ) + ":" +

35 twoDigits.format( second );

36 }

37

38 // Convert to String in standard-time format

39 public String toString()

40 {


42

43 return ( ( hour == 12 || hour == 0 ) ? 12 : hour % 12 ) +

44 ":" + twoDigits.format( minute ) +

45 ":" + twoDigits.format( second ) +

46 ( hour < 12 ? " AM" : " PM" );

47 }

48 }

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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

51 import javax.swing.JOptionPane;

52

53 public class TimeTest {

54 public static void main( String args[] )

55 {

56 Time1 t = new Time1(); // calls Time1 constructor

57 String output;

58

59 output = "The initial universal time is: " +

60 t.toUniversalString() +60 t.toUniversalString() +

61 "\nThe initial standard time is: " +

62 t.toString() +

63 "\nImplicit toString() call: " + t;

64

65 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();

70

71 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

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Program Output

76 JOptionPane.showMessageDialog( null, output,

77 "Testing Class Time1",

78 JOptionPane.INFORMATION_MESSAGE );

79

80 System.exit( 0 );

81 }

82 }

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1. package statement

1.1 import

1.2 Constructor

1.3 Method definitions

1 // Fig. 26.3: Time2.java

2 // Time2 class definition

3 package com.deitel.chtp3.ch26; // place Time2 in a package

4 import java.text.DecimalFormat; // used for number formatting

5

6 // This class maintains the time in 24-hour format

7 public class Time2 extends Object {

8 private int hour; // 0 - 23

9 private int minute; // 0 - 59

10 private int second; // 0 - 59

11

12 // 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 ); }

16

17 // 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 }

26

27 // set the hour

28 public void setHour( int h )

29 { hour = ( ( h >= 0 && h < 24 ) ? h : 0 ); }

30

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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 ); }

34

35 // set the second

36 public void setSecond( int s )

37 { second = ( ( s >= 0 && s < 60 ) ? s : 0 ); }

38

39 // Get Methods

40 // get the hour

41 public int getHour() { return hour; }

42

43 // get the minute

44 public int getMinute() { return minute; }

45

46 // get the second

47 public int getSecond() { return second; }

48

49 // Convert to String in universal-time format

50 public String toUniversalString()

51 {


53

54 return twoDigits.format( getHour() ) + ":" +

55 twoDigits.format( getMinute() ) + ":" +

56 twoDigits.format( getSecond() );

57 }

58

59 // Convert to String in standard-time format

60 public String toString()

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1.3 Method

definitions

61 {


63

64 return ( ( getHour() == 12 || getHour() == 0 ) ?

65 12 : getHour() % 12 ) + ":" +

66 twoDigits.format( getMinute() ) + ":" +

67 twoDigits.format( getSecond() ) +

68 ( getHour() < 12 ? " AM" : " PM" );

69 }

70 }

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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

73 import java.awt.*;

74 import java.awt.event.*;

75 import javax.swing.*;

76 import com.deitel.chtp3.ch26.Time2;

77

78 public 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;

85

86 public void init()

87 {

88 t = new Time2();

89

90 Container c = getContentPane();

9192 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 );9899 minuteLabel = new JLabel( "Set minute" );100 minuteField = new JTextField( 10 );

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2. Define actionPerformed

2.1 Notice e.getSource

101 minuteField.addActionListener( this );

102 c.add( minuteLabel );

103 c.add( minuteField );

104

105 secondLabel = new JLabel( "Set Second" );

106 secondField = new JTextField( 10 );

107 secondField.addActionListener( this );

108 c.add( secondLabel );

109 c.add( secondField );

110

111 display = new JTextField( 30 );

112 display.setEditable( false );

113 c.add( display );

114

115 tickButton = new JButton( "Add 1 to Second" );

116 tickButton.addActionListener( this );

117 c.add( tickButton );

118

119 updateDisplay();

120 }

121

122 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 }

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2.2 Method

definitions

131 else if ( e.getSource() == minuteField ) {

132 t.setMinute(


134 minuteField.setText( "" );

135 }

136 else if ( e.getSource() == secondField ) {

137 t.setSecond(


139 secondField.setText( "" );

140 }

141

142 updateDisplay();

143 }

144

145 public void updateDisplay()

146 {

147 display.setText( "Hour: " + t.getHour() +

148 "; Minute: " + t.getMinute() +

149 "; Second: " + t.getSecond() );

150 showStatus( "Standard time is: " + t.toString() +

151 "; Universal time is: " + t.toUniversalString() );

152 }

153

154 public void tick()

155 {

156 t.setSecond( ( t.getSecond() + 1 ) % 60 );

157

158 if ( t.getSecond() == 0 ) {

159 t.setMinute( ( t.getMinute() + 1 ) % 60 );

160

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2.2 Method

definitions

161 if ( t.getMinute() == 0 )

162 t.setHour( ( t.getHour() + 1 ) % 24 );

163 }

164 }

165}

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Program Output

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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

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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.


5 import java.text.DecimalFormat;

6

7 public class ThisTest {


9 {

10 SimpleTime t = new SimpleTime( 12, 30, 19 );

11

12 JOptionPane.showMessageDialog( null, t.buildString(),

13 "Demonstrating the \"this\" Reference",


15


17 }

18 }

19

20 class SimpleTime {

21 private int hour, minute, second;

22

23 public SimpleTime( int hour, int minute, int second )

24 {

25 this.hour = hour;

26 this.minute = minute;

27 this.second = second;

28 }

29

30 public String buildString()

29

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 }3738 public String toString()39 {40 DecimalFormat twoDigits = new DecimalFormat( "00" );41 42 return twoDigits.format( this.hour ) + ":" +43 twoDigits.format( this.minute ) + ":" +44 twoDigits.format( this.second );45 }46 }

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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++

31


Finalizers (II) 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

32


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

33


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

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1. Class definition

1.1 static variable

1.2 Constructor

1.3 Method finalize

1.4 Get methods

1 // Fig. 26.5: Employee.java2 // Declaration of the Employee class.3 public class Employee extends Object {4 private String firstName;5 private String lastName;6 private static int count; // # of objects in memory78 public Employee( String fName, String lName )9 {10 firstName = fName;11 lastName = lName;1213 ++count; // increment static count of employees14 System.out.println( "Employee object constructor: " +15 firstName + " " + lastName );16 }1718 protected void finalize()19 {20 --count; // decrement static count of employees21 System.out.println( "Employee object finalizer: " +22 firstName + " " + lastName +23 "; count = " + count );24 }2526 public String getFirstName() { return firstName; }2728 public String getLastName() { return lastName; }2930 public static int getCount() { return count; } 31 }

35

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.


36

37 public class EmployeeTest {


39 {

40 String output;

41

42 output = "Employees before instantiation: " +

43 Employee.getCount();

44

45 Employee e1 = new Employee( "Susan", "Baker" );

46 Employee e2 = new Employee( "Bob", "Jones" );

47

48 output += "\n\nEmployees after instantiation: " +

49 "\nvia e1.getCount(): " + e1.getCount() +

50 "\nvia e2.getCount(): " + e2.getCount() +

51 "\nvia Employee.getCount(): " +


53

54 output += "\n\nEmployee 1: " + e1.getFirstName() +

55 " " + e1.getLastName() +

56 "\nEmployee 2: " + e2.getFirstName() +

57 " " + e2.getLastName();

58

59 // mark objects referred to by e1 and e2

60 // for garbage collection

36

2. System.gc

Program Output

61 e1 = null;

62 e2 = null;

63

64 System.gc(); // suggest that garbage collector be called

65

66 output += "\n\nEmployees after System.gc(): " +


68

69 JOptionPane.showMessageDialog( null, output,

70 "Static Members and Garbage Collection",



73 }

74 }

37


课后练习利用整型数组建立堆栈类和队列类

堆栈先进后出属性：堆栈大小，栈顶位置，容器（整型数组）方法：压栈，出栈，是否空，是否满

队列先进先出属性：队列大小，当前队长，容器（整型数组）方法：出队列，入队列，是否空，是否满

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Java 语言程序设计