支持面向对象的编程

1 .Chapter 12 Support for Object-Oriented Programming

2 .Copyright © 2015 Pearson. All rights reserved. 1- 2 Chapter 12 Topics Introduction Object-Oriented Programming Design Issues for Object-Oriented Languages Support for Object-Oriented Programming in Smalltalk Support for Object-Oriented Programming in C++ Support for Object-Oriented Programming in Objective-C Support for Object-Oriented Programming in Java Support for Object-Oriented Programming in C# Support for Object-Oriented Programming in Ruby Implementation of Object-Oriented Constructs Reflection

3 .Copyright © 2015 Pearson. All rights reserved. 1- 3 Introduction Many object-oriented programming (OOP) languages Some support procedural and data-oriented programming (e.g., C++) Some support functional program (e.g., CLOS) Newer languages do not support other paradigms but use their imperative structures (e.g., Java and C#) Some are pure OOP language (e.g., Smalltalk & Ruby) Some functional languages support OOP, but they are not discussed in this chapter

4 .Copyright © 2015 Pearson. All rights reserved. 1- 4 Object-Oriented Programming Three major language features: Abstract data types (Chapter 11) Inheritance Inheritance is the central theme in OOP and languages that support it Polymorphism

5 .Copyright © 2015 Pearson. All rights reserved. 1- 5 Inheritance Productivity increases can come from reuse ADTs are difficult to reuse—always need changes All ADTs are independent and at the same level Inheritance allows new classes defined in terms of existing ones, i.e., by allowing them to inherit common parts Inheritance addresses both of the above concerns--reuse ADTs after minor changes and define classes in a hierarchy

6 .Copyright © 2015 Pearson. All rights reserved. 1- 6 Object-Oriented Concepts ADTs are usually called classes Class instances are called objects A class that inherits is a derived class or a subclass The class from which another class inherits is a parent class or superclass Subprograms that define operations on objects are called methods

7 .Copyright © 2015 Pearson. All rights reserved. 1- 7 Object-Oriented Concepts (continued) Calls to methods are called messages The entire collection of methods of an object is called its message protocol or message interface Messages have two parts--a method name and the destination object In the simplest case, a class inherits all of the entities of its parent

8 .Copyright © 2015 Pearson. All rights reserved. 1- 8 Object-Oriented Concepts (continued) Inheritance can be complicated by access controls to encapsulated entities A class can hide entities from its subclasses A class can hide entities from its clients A class can also hide entities for its clients while allowing its subclasses to see them Besides inheriting methods as is, a class can modify an inherited method The new one overrides the inherited one The method in the parent is overriden

9 .Object-Oriented Concepts (continued) Three ways a class can differ from its parent: 1. The subclass can add variables and/or methods to those inherited from the parent 2. The subclass can modify the behavior of one or more of its inherited methods. 3. The parent class can define some of its variables or methods to have private access, which means they will not be visible in the subclass Copyright © 2015 Pearson. All rights reserved. 1- 9

10 .Copyright © 2015 Pearson. All rights reserved. 1- 10 Object-Oriented Concepts (continued) There are two kinds of variables in a class: Class variables - one/class Instance variables - one/object There are two kinds of methods in a class: Class methods – accept messages to the class Instance methods – accept messages to objects Single vs. Multiple Inheritance One disadvantage of inheritance for reuse: Creates interdependencies among classes that complicate maintenance

11 .Copyright © 2015 Pearson. All rights reserved. 1- 11 Dynamic Binding A polymorphic variable can be defined in a class that is able to reference (or point to) objects of the class and objects of any of its descendants When a class hierarchy includes classes that override methods and such methods are called through a polymorphic variable, the binding to the correct method will be dynamic Allows software systems to be more easily extended during both development and maintenance

12 .Copyright © 2015 Pearson. All rights reserved. 1- 12 Dynamic Binding Concepts An abstract method is one that does not include a definition (it only defines a protocol) An abstract class is one that includes at least one virtual method An abstract class cannot be instantiated

13 .Copyright © 2015 Pearson. All rights reserved. 1- 13 Design Issues for OOP Languages The Exclusivity of Objects Are Subclasses Subtypes? Single and Multiple Inheritance Object Allocation and Deallocation Dynamic and Static Binding Nested Classes Initialization of Objects

14 .Copyright © 2015 Pearson. All rights reserved. 1- 14 The Exclusivity of Objects Everything is an object Advantage - elegance and purity Disadvantage - slow operations on simple objects Add objects to a complete typing system Advantage - fast operations on simple objects Disadvantage - results in a confusing type system (two kinds of entities) Include an imperative-style typing system for primitives but make everything else objects Advantage - fast operations on simple objects and a relatively small typing system Disadvantage - still some confusion because of the two type systems

15 .Copyright © 2015 Pearson. All rights reserved. 1- 15 Are Subclasses Subtypes? Does an “is-a” relationship hold between a parent class object and an object of the subclass? If a derived class is-a parent class, then objects of the derived class must behave the same as the parent class object A derived class is a subtype if it has an is-a relationship with its parent class Subclass can only add variables and methods and override inherited methods in “compatible” ways Subclasses inherit implementation; subtypes inherit interface and behavior

16 .Copyright © 2015 Pearson. All rights reserved. 1- 16 Single and Multiple Inheritance Multiple inheritance allows a new class to inherit from two or more classes Disadvantages of multiple inheritance: Language and implementation complexity (in part due to name collisions) Potential inefficiency - dynamic binding costs more with multiple inheritance (but not much) Advantage: Sometimes it is quite convenient and valuable

17 .Copyright © 2015 Pearson. All rights reserved. 1- 17 Allocation and DeAllocation of Objects From where are objects allocated? If they behave line the ADTs, they can be allocated from anywhere Allocated from the run-time stack Explicitly create on the heap (via new ) If they are all heap-dynamic, references can be uniform thru a pointer or reference variable Simplifies assignment - dereferencing can be implicit If objects are stack dynamic, there is a problem with regard to subtypes – object slicing Is deallocation explicit or implicit?

18 .Copyright © 2015 Pearson. All rights reserved. 1- 18 Dynamic and Static Binding Should all binding of messages to methods be dynamic? If none are, you lose the advantages of dynamic binding If all are, it is inefficient Maybe the design should allow the user to specify

19 .Copyright © 2015 Pearson. All rights reserved. 1- 19 Nested Classes If a new class is needed by only one class, there is no reason to define so it can be seen by other classes Can the new class be nested inside the class that uses it? In some cases, the new class is nested inside a subprogram rather than directly in another class Other issues: Which facilities of the nesting class should be visible to the nested class and vice versa

20 .Initialization of Objects Are objects initialized to values when they are created? Implicit or explicit initialization How are parent class members initialized when a subclass object is created? Copyright © 2015 Pearson. All rights reserved. 1- 20

21 .Copyright © 2015 Pearson. All rights reserved. 1- 21 Support for OOP in Smalltalk Smalltalk is a pure OOP language Everything is an object All objects have local memory All computation is through objects sending messages to objects None of the appearances of imperative languages All objected are allocated from the heap All deallocation is implicit Smalltalk classes cannot be nested in other classes

22 .Support for OOP in Smalltalk (continued) Inheritance A Smalltalk subclass inherits all of the instance variables, instance methods, and class methods of its superclass All subclasses are subtypes (nothing can be hidden) All inheritance is implementation inheritance No multiple inheritance Copyright © 2015 Pearson. All rights reserved. 1- 22

23 .Copyright © 2015 Pearson. All rights reserved. 1- 23 Support for OOP in Smalltalk (continued) Dynamic Binding All binding of messages to methods is dynamic The process is to search the object to which the message is sent for the method; if not found, search the superclass, etc. up to the system class which has no superclass The only type checking in Smalltalk is dynamic and the only type error occurs when a message is sent to an object that has no matching method

24 .Copyright © 2015 Pearson. All rights reserved. 1- 24 Support for OOP in Smalltalk (continued) Evaluation of Smalltalk The syntax of the language is simple and regular Good example of power provided by a small language Slow compared with conventional compiled imperative languages Dynamic binding allows type errors to go undetected until run time Introduced the graphical user interface Greatest impact: advancement of OOP

25 .Introduction to Smalltalk Expressions Four kinds 1. literals (numbers, strings, and keywords) 2. variable names (all variables are references) 3. message expressions 4. block expressions

26 .Message Expressions Two parts: the receiver object and the message itself The message part specifies the method and possibly some parameters Replies to messages are objects Multiple messages to the same object can be strung together, separated by semicolons

27 .Messages Messages can be of three forms Unary (no parameters) e.g., myAngle sin (sends a message to the sin method of the myAngle object) Binary (one parameter, an object) e.g., 12 + 17 (sends the message “+17” to the object 12; the object parameter is “17” and the method is “+”) Keyword (use keywords to organize the parameters) e.g., myArray at: 1 put: 5 (sends the objects “1” and “5” to the at:put: method of the object myArray)

28 .Methods General form: message_pattern [| temps |] statements a message pattern is like the formal parameters of a subprogram for unary messages, it is just the name for others, it lists keywords and formal names temps are just names -- Smalltalk is typeless!

29 .Assignments Simplest form: name1 ¬ name2 It is simply a pointer assignment RHS can be a message expression e.g., index ¬ index + 1