Java 程式設計入門. 講師 : 陳昭源 CSIE, NTU September 28, 2005

Transcription

1 Java 程式設計入門 講師 : 陳昭源 CSIE, NTU September 28, 2005

2 Outline Floating Point & Decimal Numbers Inheritance Polymorphism Design Principles Abstract Exercise Interface September 28, 2005 Page 2

3 Floating Point & Decimal Numbers IEEE 754 Floating Point Representation Floating Point (32 bits) s exponent 指數 mantissa 尾數 1 8 bits 23 bits Double Precision Floating Point (64 bits) s exponent 指數 mantissa 尾數 1 11 bits 52 bits 小數部份使用二進位 ( 以 2 為底數 ) 來表示 ; 最高位對應 2-1, 次高位元對應 2-2, 依此類推 許多數字無法使用精確表示 September 28, 2005 Page 3

4 Floating Point & Decimal Numbers BigDecimal 類別 (java.math) constructor BigDecimal(double val) vs. BigDecimal(String val) methods compareto(bigdecimal val) vs. equals(object x) HW05 bonus: 精準度控制 小數部分最多採記到公厘以下兩位, 四捨五入 參考 BigDecimalSample.java September 28, 2005 Page 4

5 Inheritance Inheritance 語法 : class ClassName extends BaseClass Each class in Java inherits from Java standard root class, Object, without any explicitly claim. Each class in Java can inherit from only one (non-interface) base class. Base Class (SuperClass): 基底類別 父類別 Derived Class (Subclass): 衍生類別 子類別 The derived class would inherit all accessible members in the base class, including variables and methods. The new class is like that old class. Note: Constructors can t be inherited! September 28, 2005 Page 5

6 Polymorphism Upcasting The derived class is a type (superset) of the existing (base) class. Objects of the derived class is also of the base class. The base class is always more general than the derived class. Thus, casting from a derived type to a base type is called upcasting, and it is safe. The compiler allow upcasting without any explicit casts or other special notation. Why Upcasting? Polymorphism talk only to the base class (upcasting) instead of many derived classes (overloading) Example: SamplePoly.java September 28, 2005 Page 6

7 Polymprphism How does Java do polymorphism? Late Binding Method-call Binding Connecting a method call to the method body is called binding. Early binding: binding before the program is run. Late binding: binding at run time, based on the object type. All method bindings in Java use late binding unless the method is static or final/private. September 28, 2005 Page 7

8 Design Principles Principle #01 Favor Composition Over Inheritance! One of the clearest ways to determine whether you should use composition or inheritance is to ask whether you will ever need to upcast from your new class to the base class. If you must upcast, then inheritance is necessary; otherwise, you should look closely at whether you need inheritance. September 28, 2005 Page 8

9 Design Principles Advantages/Disadvantages of Composition Advantages Objects are accessed solely through its interface. Black-box reuse, and good encapsulation. Fewer implementation dependencies, and each class is focused on just one task. The composition can be defined dynamically at run-time. Disadvantages Systems tend to have more objects. Interfaces must be carefully defined in order to use many different objects as composition blocks. September 28, 2005 Page 9

10 Design Principles Advantages/Disadvantages of Inheritance Advantages New implementation is easy, and easy to modify/extend the implementation being reused. Disadvantages Breaks encapsulation White-box reuse Subclasses may have to be changed if the implementation of the superclass changes. Implementations inherited from superclasses can not be changed at run-time September 28, 2005 Page 10

11 Design Principles Principle #02 Program to an Interface, not an Implementation! More flexible, but design complexity increases. Principle #03 Software Entities Should be Open for Extension, yet Closed for Modification! Add new code to extend the behavior of the system, but not to modify the existing code. How to do? Polymorphism Inheritance Abstract/Interface September 28, 2005 Page 11

12 Abstract Shape draw() erase() Circle draw() erase() Rectangle draw() erase() Triangle draw() erase() September 28, 2005 Page 12

13 Abstract Abstract Class common interface for all subclasses derived from it Objects of this class have no meaning! Abstract Methods An incomplete method only has the declaration and no method body. A class with one or more abstract methods should be qualified as abstract, but an abstract class may not have any abstract methods. Classes inherit from an abstract class must implement all abstract classes in the base class, or they are also abstract. September 28, 2005 Page 13

14 Abstract Class Declaration AccessLevel abstract/final class ClassName extends SuperClass implements Interface { ClassBody } Method Declaration AccessLevel static abstract/final ReturnType MethodName(ParameterList) { MethodBody } September 28, 2005 Page 14

15 Exercise 參閱 Exercise.java 使用抽象類別 Shapes, 完成 Rect 與 Tri 類別 使用 upcasting 概念, 完成 showdetails method September 28, 2005 Page 15

16 Interface Wind void play() void adjust() Woodwind void play() void adjust() Instrument void play() void adjust() Percussion void play() void adjust() Brass void play() void adjust() Stringed void play() void adjust() September 28, 2005 Page 16

17 Interface Interface a pure abstract class, a protocol between classes to establish the form for a class method names, argument lists, and return types but no method bodies Create an Interface 語法 :interface InterfaceName All methods in an interface are public even if they are not specified explicitly. September 28, 2005 Page 17

18 Interface Interface Use an Interface 語法 :class ClassName implements Iface01, Iface02, One class can implement as many interfaces as you want. Each interface, like the base class, is an independent type you can upcast to. If you want to create an object of a class which implements some interfaces, this class should provide all method definitions. September 28, 2005 Page 18

19 Interface The Core Reason to Use Interfaces to be able to upcast to more than one base type! If you know something is going to be a base class, your first choice should be to make it an interface, and only if you re forced to have method definitions or member variables should you change to an abstract class, or if necessary a concrete class. September 28, 2005 Page 19