1 / 25

Object-Oriented Programming

Explore the advanced topics of object-oriented programming, including dynamic inheritance, contract programming, and multiple dispatch. Learn about encapsulation, inheritance, and subtype polymorphism. Understand the role of classes and their various purposes in modern programming languages.

christopherstanley
Download Presentation

Object-Oriented Programming

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Object-Oriented Programming “The Rest of the Story”, CS 4450 – Chapter 16

  2. Topics • What is OOP? • Objects Without Classes • “Dynamic inheritance” in Javascript, Python • Contract Programming • Multiple Dispatch

  3. What is Object Oriented Programming? • Encapsulation • Inheritance • Subtype Polymorphism • or “equivalent”

  4. Encapsulation • Separating interface from implementation • Different mechanisms • Classes & Interfaces • Nested functions and closures

  5. The Role of Classes • Most OO languages have some kind of class construct • Classes serve a variety of purposes, depending on the language: • Group fields and methods together • Are instantiable: the running program can create as many objects of a class as it needs • Serve as the unit of inheritance: derived class inherits from base class or classes Modern Programming Languages

  6. Classes • More purposes: • Serve as a type: objects (or references to them) can have a class or superclass name as their static type • House static fields and methods: one per class, not one per instance • Serve as a labeled namespace (scope); control the visibility of contents outside the class definition Modern Programming Languages

  7. Without Classes • Imagine an OO language with no classes • With classes, you create objects by instantiating a class • Without classes, you could create an object from scratch by listing all its methods and fields on the spot • Or, you could clone an existing prototype object and then modify parts of it Modern Programming Languages

  8. With classes: instantiation x = new Stack(); x = { private Node top = null; public boolean hasMore() { return (top!=null); } public String remove() { Node n = top; top = n.getLink(); return n.getData(); } …} y = x.clone();y.top = null; Without classes: raw object creation Without classes: prototype cloning Modern Programming Languages

  9. Prototypes • A prototype is an object that is copied to make similar objects • When making copies, a program can modify the values of fields, and can add or remove fields and methods (“dynamic OOP”) • Prototype-based languages (Self, Javascript to some degree) use this concept instead of classes Modern Programming Languages

  10. Without Classes • Instantiation is only one use of classes • Other things prototype-based languages must do without: • Classes as types: most prototype-based languages are dynamically typed • Inheritance: “static inheritance” requires classes • Prototype languages use “dynamic inheritance” • aka “delegation” Modern Programming Languages

  11. Inheritance • Inheritance is a way to defer attribute or method selection to another type • a “superclass” • This is static inheritance • since relationships among types are defined in source code • There is another way…

  12. Delegation“Runtime Inheritance” • Object (not type)-based inheritance • Instead of a superclass, an object has a delegate • aka “prototype” (just another object) • Delegates are set at runtime • Any unresolved name lookup defers to the delegate object • and so on up the delegate chain… • A similar effect to subtype polymorphism, but without types! • See prototype.ds (Javascript), prototype.py

  13. About Dynamic Typing • An object may or may not be able to respond to a particular request (“message”) — no compile-time check • Total freedom: program can try using any method for any object • Polymorphism isa “given” in dynamically-typed languages • Polymorphism is only a conscious concern in statically-typed languages • “Polymorphism is a way gaining some of the freedom of dynamic type checking without giving up the benefits of static checking.” (p. 128) Modern Programming Languages

  14. Contract ProgrammingA Perspective for Polymorphic Interfaces • Methods are contracts with the user • Users must meet pre-conditions of a method • What the method expects from the client • Index in a certain range, for example • Method guarantees certain post-conditions • but only if the pre-conditions were met CS 3370 - Defensive Programming

  15. Parties in ContractsClients and Suppliers • Clientsmust satisfy pre-conditions • They may benefit from post-conditions • But don’t have to • Suppliersmust satisfy post-conditions • They may assume the pre-conditions • But don’t have to • This affects inheritance… CS 3370 - Defensive Programming

  16. Liskov Substitutability Principle • All derived objects must be able to substitute for a base class object • In contexts where public methods of the base class are used • Derived classes must not change the “rules of the game” CS 3370 - Defensive Programming

  17. Contracts and Inheritance • Contracts are set by the base class • Derived classes must obey the base contract • Otherwise substitutability is compromised • Clients program to the contract • By using and understanding the base class interface and its conditions • And by using base/interface pointers • and letting polymorphism do its work invisibly • Example: contravar.cpp, contravar.d CS 3370 - Defensive Programming

  18. Contracts and InheritanceSummary • “The problem for instances of B is how to be perfectly substitutable for instances of A. The only way to guarantee type safety and substitutability is to be equally or more liberal than A on inputs, and to be equally or more strict than A on outputs.”– Wikipedia • “Require no more; Promise no less” CS 3370 - Defensive Programming

  19. Dynamic DispatchThe Mechanics of Subtype Polymorphism • In Java/C++/C#, the static type of a reference may be a superclass or interface of the actual runtime class • At runtime, the language system must find the right method for the actual class • the dynamic type • That’s dynamic dispatch: the hidden, implicit branch-on-class to implement method calls • Optional in C++ (must use virtual keyword) Modern Programming Languages

  20. Multiple Dispatch • All of the examples we’ve seen of subtype polymorphism (or delegation) key off a single type (or object) to find the right method • p.f(x,y) considers only the dynamic type of p • x and y are not considered at runtime • Some languages (CLOS, Groovy, Perl 6) use all types involved to resolve the method lookup • “runtime overloading” • No one class/object “owns” the method

  21. Double Dispatch Example Definitions for f(): What is the “most derived” function for the calls: c.f(x), c.f(w)?

  22. An Ordering Perspective • List parameter combinations in “odometer order” (most general to most specific): • A,V * • A,W • A,X * • B,V • B,W • B,X * • C,V * • C,W • C,X • Reverse, keeping only existing methods: • C,V • B,X • A,X • A,V • To dispatch, test parameters in the order above for is-a, using RTTI • See doubledisp.cpp

  23. Switching the Parameter Order • V A * • V B • V C * • W A • W B • W C • X A * • X B * • X C • X B * • X A * • V C * • V A * • See doubledisp-B.cpp

  24. Multiple Dispatch • Any number of hierarchies/parameters may be used • Again, applicable methods are considered in “most derived” order • Supported natively by CLOS • Common Lisp Object System • “Generic Methods”

  25. Multiple Dispatch Example • See multimeth.lsp, multimeth.cpp, multimeth.d

More Related