OOP vs. Languages: Exploring Classes, Inheritance, Encapsulation, Polymorphism, Slides of Advanced Computer Programming

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Object Orientation

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 1

Definitions

 Give definitions for the following:

– Object-oriented language

– Object-oriented programming

 Then again, why bother?

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 2

Outline

 16.2 Object-oriented programming

– OO in ML

– Non-OO in Java

 16.3 Object-oriented language features

– Classes

– Prototypes

– Inheritance

– Encapsulation

– Polymorphism

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 4

Chapter Sixteen Modern Programming Languages, 2nd ed. 5

public class Node { private String data; private Node link; public Node(String theData, Node theLink) { data = theData; link = theLink; } public String getData() { return data; } public Node getLink() { return link; } }

A previous Java example: a node

used to build a stack of strings

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Chapter Sixteen Modern Programming Languages, 2nd ed. 7

datatype message = GetData | GetLink;

datatype response = Data of string | Object of message -> response;

fun node data link GetData = Data data | node data link GetLink = Object link;

Same OO idea in ML.

We have a type for messages and a type for responses.

To construct a node we call node, passing the first two

parameters.

Result is a function of type message->response.

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Node Examples

 Objects responding to messages

 null has to be something of the object type

(message->response); we could use

Chapter Sixteen Modern Programming Languages, 2nd ed. 8

• val n1 = node "Hello" null; val n1 = fn : message -> response
• val n2 = node "world" n1; val n2 = fn : message -> response
• n1 GetData; val it = Data "Hello" : response
• n2 GetData; val it = Data "world" : response

fun null _ = Data "null";

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Chapter Sixteen Modern Programming Languages, 2nd ed. 10

datatype message = IsNull | Add of string | HasMore | Remove | GetData | GetLink;

datatype response = Pred of bool | Data of string | Removed of (message -> response) * string | Object of message -> response;

fun root _ = Pred false;

Expanded vocabulary of

messages and responses,

for both node and

stack

Root class handles all

messages by returning

Pred false

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Chapter Sixteen Modern Programming Languages, 2nd ed. 11

fun null IsNull = Pred true | null message = root message;

fun node data link GetData = Data data | node data link GetLink = Object link | node _ _ message = root message;

fun stack top HasMore = let val Pred(p) = top IsNull in Pred(not p) end | stack top (Add data) = Object(stack (node data top)) | stack top Remove = let val Object(next) = top GetLink val Data(data) = top GetData in Removed(stack next, data) end | stack _ message = root message;

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Inheritance, Sort Of

 Here is a peekableStack like the one in

Java from Chapter Fifteen:

 This style is rather like a Smalltalk system

– Message passing

– Messages not statically typed

– Unhandled messages passed back to superclass

Chapter Sixteen Modern Programming Languages, 2nd ed. 13

fun peekableStack top Peek = top GetData | peekableStack top message = stack top message;

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Thoughts

 Obviously, not a good way to use ML

– Messages and responses not properly typed

– No compile-time checking of whether a given

object can handle a given message

 (Objective CAML is a dialect that integrates

OO features into ML)

 The point is: it’s possible

 OO programming is not the same as

programming in an OO language

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 14

Java

 Java is better than ML at supporting an

object-oriented style of programming

 But using Java is no guarantee of object-

orientation

– Can use static methods

– Can put all code in one big class

– Can use classes as records—public fields and

no methods, like C structures

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 16

Classes Used As Records

Chapter Sixteen Modern Programming Languages, 2nd ed. 17

public class Node { public String data; // Each node has a String... public Node link; // ...and a link to the next Node }

public class Stack{ public Node top; // The top node in the stack }

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Polymorphism

 In Chapter Fifteen: Worklist interface

implemented by Stack, Queue, etc.

 There is a common trick to support this kind

of thing in non-OO solutions

 Each record starts with an element of an

enumeration, identifying what kind of

Worklist it is…

Chapter Sixteen Modern Programming Languages, 2nd ed. (^) Docsity.com 19

A Non-OO Worklist

Chapter Sixteen Modern Programming Languages, 2nd ed. 20

public class Worklist { public static final int STACK = 0; public static final int QUEUE = 1; public static final int PRIORITYQUEUE = 2; public int type; // one of the above Worklist types public Node front; // front Node in the list public Node rear; // unused when type==STACK public int length; // unused when type==STACK }

The type field says what kind of Worklist it is.

Meanings of other fields depend on type.

Methods that manipulate Worklist records must branch on

type…

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