Chapter 2: Java OO II. Yang Wang wyang AT njnet.edu.cn

Transcription

1 Chapter 2: Java OO II Yang Wang wyang AT njnet.edu.cn

2 Outline Abstraction Abstract Class Interface Inheritance Polymorphism

3 Abstraction

4 Abstraction What is Abstraction? An abstraction is a general idea rather than one relating to a particular object, person, or situation. - From Collins The Significance of Abstraction Model Implementation Language Tools for Abstraction in Java Abstract Class Interface

5 Basic Abstraction Class <=> Object superclass vs. subclass extends create a big tree but still model is bundled with full implementation

6 Abstraction Abstract Class Abstract Class:Java Class providing part of implementations Including abstract methods Cannot be used to create objects Must be inherited and implemented

7 Abstraction Abstract Class An Abstract Class for Benchmark

8 Abstraction Abstract Class A Completed, Specified Class for Benchmark

9 Abstract Class Rules Non-Abstract class Can t have Abstract methods Abstract class Can t be initialized with New A class must be Abstract when it has Abstract methods Non-Abstract class subclass can be Abstract Abstract class constructor must be protected

10 Example java.lang.numer bytevalue() shortvalue() intvalue() longvalue() floatvalue() DoubleValue Byte Integer Long Float BigInteger 10

11 Quiz Number numberref = new Integer(0); Double doubleref = (Double) numberref;

12 Abstraction Interface The language basis of Java is Class But the basis of OOD is Type Class Type + Implementation Java prompts Interface-based OOD Designer only cares of design, or saying, interface;

13 Abstraction Interface Most Interfaces do: Distinguish one type with other types Showing that one type can have the ability to do sth: Cloneable:an object of this type can be cloned Comparable: objects of this type can be compared Runnable: an object of this type can be run in a thread Serializable: an object of this type can be serialized

14 Abstraction Interface Defining an Interface Constant Method

15 Abstraction Interface

16 Abstraction Interface Fields in Interface public,static,final Must be initialized

17 Abstraction Interface Implementing an Interface A Class can implement multiple interfaces Multiple Implementation All methods in an interface should be implemented Multiple Implementation!= Multiple Inheritance

18 Abstraction Interface Think Does M-Implementation bring the same problem as M-Inheritance?

19 Abstraction Interface Using Interface to declare the type of objects

20 Abstraction Interface Interface-based Programming Agile: First, lets care about the design, the type, the interface, and ignore the implementation, the class. Reliable: assure the correctness based on the use of types

21 Abstraction Interface Marker Interface Nothing defined in an interface It is just a marker Such as Cloneable

22 Abstraction Interface Abstract Class is a Strong is-a relationship, ( 苹果是 一种 水果 ) Interface is a Weak is-a relationship( 苹果是 一类可以吃的东 西 )

23 Abstraction Interface Abstract Class name is a Noun Interface name is a Noun or a Adj.

24 Abstraction Interface Abstract Class name is a Noun Interface name is a Noun or a Adj.

25 Abstraction Interface Abstract Class is used for common feature in sub classes Interface is used for common feature in classes

26 Abstraction Interface Fields in Abstract Class have no restrictions. Fields in Interface must be public static final.

27 Abstraction Interface Methods in Abstract Class have no restrictions. Methods can be in Interface must be public abstract

28 Abstraction Interface Constructors in Abstract Class must be protected. Constructors can not be in Interface

29 Abstraction Interface Multiple inheritance is allowed for Interface, not for abstract class. Abstract class provides part of implementation, while interface has no implementation.

30 Abstraction Interface Combining Abstract Class and Interface

31 Inheritance

32 Inheritance Class The Significance of Inheritance Code reuse Enhancement of scalability Types of Inheritance Inheritance of class Enhancement of maintainability Inheritance of interface

33 Inheritance Class Example - House

34 Inheritance Class Example - GeorgianHouse

35 Inheritance Class

36 Inheritance Class Constructions in Inheritance Constructors in subclasses should invoke constructors in superclasses explicitly for initialization. If step 1 is not satisfied, the default constructor in superclass will be invoked. If no default constructor is defined in superclass, step 1 must be satisfied. The invocation of constructors in superclass should be placed foremost.

37 Inheritance Class Thinking in Java A robust programming language should have a sound initialization process. Each field in a class should be initialized.

38 Inheritance Class Overriding( 覆盖 ) Overriding NOT Overloading( 重载 )// what is overloading Overriding means a method in subclass will replace a method with same signature in superclass //what is signature Overriding let the subclass perform differently with superclass

39 Inheritance Class Example

40 Inheritance Class Limits of Overriding The access rights should be enlarged or unchanged, not be reduced. // why? The return type should be reduced or unchanged, not be enlarged. // why? If return type in superclass is Class A, the return type in sublcass should be A or subclass of A If return type in superclass is a primary type, it should be unchanged in subclass.

41 Inheritance Class Thinking in Java Overriding MUST retain compatibility (not breaking the behavior specification in superclass) Think House spark_house = new GeorgianHouse(); // Right? GeorgianHouse spark_house = new House(); //Right?

42 Inheritance Class Examples

43 Inheritance Class Hiding

44 Inheritance Class A: G,orgianHo:s, Ho:s, Th,,;t,nd,d class: G,origanHo:s, Th,,;t,nd,d class: G,origanHo:s, B: G,orgianHo:s, G,orgianHo:s, Th,,;t,nd,d class: G,origanHo:s, Th,,;t,nd,d class: G,origanHo:s, C: G,orgianHo:s, Ho:s, Th,,;t,nd,d class: G,origanHo:s, Th, s:p,r class: Ho:s, D: G,orgianHo:s, Ho:s, Th,,;t,nd,d class: G,origanHo:s, Th,,;t,nd,d class: Ho:s, -or Fi,ld, look at th, d,claration typ,; -or M,thod, look at th, r:n tim, typ,;

45 Inheritance Class Conversion of Types Objective:convert an object of one class to another For example: a Parrot to a Bird, ghouse to House Classification of Conversion Upcasting 上溯造型 Downcasting 下溯造型

46 Inheritance Class Upcasting Upcasting is safe and unrestricted Downcasting : reverse of Upcasting, may be unsafe may cause casting exceptions Use instanceof to check the safety

47 Inheritance Class Keyword: super

48 Inheritance Class Keyword: final final before a class means it is not inheritable final before a method means it is not overridable

49 Inheritance Class Class Inheritance:How and when? Is-a (Inheritance) Has-a(Composition) How to design an extensible class

50 Inheritance Class Multiple Inheritance for Interfaces Inheritance of Interfaces also has: Hiding of fields Override of Methods

51 Polymorphism

52 Polymorphism Definition of Polymorphism Greek, means Multiple Forms Refers to the existence of different methods with same names Intuition: in OOD, same objects have the same behavior (method name), but have different way of behaving and lead to different result(return type) Question: What will the exact behaving result be for a certain object? Significance: Improve the flexibility and versatility in OOP Types of Polymorphismversatility Static polymorphism - polymorphism in compile-time Dynamic polymorphism - polymorphism in run-time

53 Polymorphism Static Polymorphism Polymorphism which can be determined in compilation Overloading

54 Polymorphism Dynamic Polymorphism Behavior and result be can only determined in run-time Dynamic Binding The binding of method invocation and method body in run-time Classification of Dynamic Polymorphism Inheritance Polymorphism Interface Polymorphism

55 Polymorphism

56 Polymorphism Inheritance Polymorphism

57 Polymorphism Interface Polymorphism

58 Review of OO

59 Answer:val 是 non-static variable, 需要通过实例例化后的对 象访问 package cn.edu.seu.cose.java.wyang; public class Alpha { int val; package cn.edu.seu.cose.java.wyang; public class Beta { public static void main(string args[]){ Alpha.val = 1;

60 Answer: a 没有实例例化 (new),1.2 变成整形需要强制转换 package cn.edu.seu.cose.java.wyang; public class Alpha { int val; package cn.edu.seu.cose.java.wyang; public class Beta { public static void main(string args[]){ Alpha a; a.val = 1.2;

61 Answer:Alpha 缺少缺省构造函数 package cn.edu.seu.cose.java.wyang; public class Alpha { int val; public Alpha (int i){ val = i; package cn.edu.seu.cose.java.wyang; public class Beta { public static void main(string args[]){ Alpha a = new Alpha(); a.val = 1;

62 Answer:val = 0, 初始化块缺少 { package cn.edu.seu.cose.java.wyang; public class Alpha { int val; val = 0; public Alpha (){ val = 1; package cn.edu.seu.cose.java.wyang; public class Beta { public static void main(string args[]){ Alpha a = new Alpha(); a.val = 1;

63 Answer:val 是 default 权限, 不不能跨 package package cn.edu.seu.cose.java.wyang; public class Alpha { int val; public Alpha (){ val = 1; package cn.edu.seu.cose.java.xzhang; public class Beta { public void ChangeVal(Alpha a){ a.val = 1;

64 nswer:val 是 protected 权限, 不不能被跨 package 的 非 子类访问 package cn.edu.seu.cose.java.wyang; public class Alpha { protected int val; public Alpha (){ val = 1; package cn.edu.seu.cose.java.xzhang; public class Beta { public void ChangeVal(Alpha a){ a.val = 1;

65 Answer:protected 权限, 跨 package 只有 子类能访问, 所以 首先需要 extends, 同时访问对象的 protected field 或 method 时, 必须该对象引 用的声明类型和当前类相同, 否则 无法访问 val 成员 这 里里还有 一个隐含的错误, 需要从另 一个 package 中 import alpha package cn.edu.seu.cose.java.wyang; public class Alpha { protected int val; public Alpha (){ val = 1; package cn.edu.seu.cose.java.xzhang; public class Beta extends Alpha{ public void ChangeVal(Alpha a){ a.val = 1;

66 Answer:private 值其他类不不可 见 package cn.edu.seu.cose.java.wyang; public class Alpha { private int val; public Alpha (){ val = 1; package cn.edu.seu.cose.java.wyang; public class Beta extends Alpha{ public void ChangeVal(){ val = 2;

67 Answer:b,d 不不是合法的重载,a,c 是合法的重载, 这 里里要 注意 Java 是强类型和重载的要求 a. public class Beta { void changeval(int a,int b){ void changeval(short a, int b){ c. public class Beta { void changeval(int a){ int changeval(int a, String b){ b. public class Beta { void changeval(int a){ int changeval(int b){ d. public class Beta { void changeval(int a, String b){ private void changeval(int a, String b){

68 Answer:a 正确 overriding 的问题, 返回值需要兼容 package cn.edu.seu.cose.wyang; public class Alpha { int changeval(int a){ a. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { short changeval(int a){ b. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { void changeval(int a){ c. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { long changeval(int a){

69 Answer:b,c 正确,overriding 的问题权限兼容性 package cn.edu.seu.cose.wyang; public class Alpha { int changeval(int a){ a. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { private int changeval(int a){ b. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { public int changeval(int a){ c. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { protected int changeval(int a)

70 Answer:a,c 正确, 如果签名不不 一致, 就不不是 overriding, 而是 overloading,a 是 overloading,c 是 overriding package cn.edu.seu.cose.wyang; public class Alpha { int changeval(int a){ a. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { void changeval(int a, int b){ b. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { void changeval(int a){ c. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { short changeval(int a){

71 Answer:overriding 里里 unvisible 的问题, 不不 一定只有 private 权限是 unvisible, 只要是权限不不可访问就可以, 这 里里由于 default 权限在跨 package 时不不可 见, 所以三个答案都正确,b,c 里里的 Beta 无法看到 Alpha 里里的 changeval 函数, 所以不不存在冲突, 注意这 里里也不不是 overloading package cn.edu.seu.cose.wyang; public class Alpha { int changeval(int a){ a. package cn.edu.seu.cose.wyang; public class Beta extends Alpha { short changeval(int a){ b. package cn.edu.seu.cose.xzhang; public class Beta extends Alpha { void changeval(int a){ c. package cn.edu.seu.cose.xzhang public class Beta extends Alpha { long changeval(int a){

72 Answer: 由于 display 方法是 default, 跨 package 不不可 见后, 固然两个函数变成相互独 立的了了, 但是声明类型的 方法如果不不可 见, 那么也就不不可调 用 package cn.edu.seu.cose.wyang; public class Alpha { String name = Alpha ; void display(){ System.out.println(name); package cn.edu.seu.cose.xzhang; public class Beta extends Alpha{ String name = Beta ; void display(){ System.out.println(name); public static void main(string args[]){ Alpha a = new Beta() a.display()

73 Answer:hiding,Field 是 hiding, 看声明类型决定访问谁 package cn.edu.seu.cose.wyang; public class Alpha { String name = Alpha ; void display(){ System.out.println(name); public class Beta { String name = Beta ; void display(){ System.out.println(name); public class Test { public static void main(string arg Beta b = new Beta(); Alpha a = b; System.out.println(a.name); Sytsm.out.println(b.name); a.display(); b.display(); a. Alpha Alpha Beta Beta b. Beta Beta Beta Beta c. Alpha Beta Beta Beta d. Alpha Beta Alpha Beta

74 .Answer: 抽象类不不可实例例化 package cn.edu.seu.cose.wyang; public class Alpha { int val = 0 void changeval(){ val = 1; abstract void getval(); public static void main(string args[]){ Alpha a = new Alpha(); a.changeval();

75 Answer: 类不不允许多重继承, package cn.edu.seu.cose.wyang; public class Alpha { public class Beta { public class Gamma extends Alpha, Beta {

76 Answer: 接 口不不允许实现, 需要由实现接 口的类来完成实现 package cn.edu.seu.cose.wyang; interface intalpha { void display(){ System.out.println( intalpha ); public class Alpha implements intalpha{ public static void main(string args[]){ intalpha a = new Alpha(); display();

77 Answer: 接 口 method 的缺省权限为 public, 类中 method 缺省权限为 default, 所以类中实现接 口的 方法需要声明 public, 否则在访问权限上不不兼容 package cn.edu.seu.cose.wyang; interface intalpha { void display(){ public class Alpha implement intalpha{ void display(){ System.out.println( Alpha ); public static void main(string args[]){ intalpha a = new Alpha(); display();

78 Answer: 接 口多重继承问题, 有歧义时, 必须明确从哪 一个接 口来访问 variable, 不不能通过类来访问有歧义的 variable, 对于 method, 取公共 子集的返回类型, 如果 无法取得, 则失败 package cn.edu.seu.cose.wyang; interface intalpha { String val= Alpha ; interface intbeta { int val=1; public class Alpha implement intalpha, intbeta{ public void display(){ System.out.println(Alpha.val); a. Alpha b. Beta c. Null d. 以上都不不是 public static void main(string args[]){ intalpha a = new Alpha(); a.display();