Introduction ObjectOriented Programming Behavioral Abstraction Compilation

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IntroductionObject-Oriented ProgrammingBehaviora
l AbstractionCompilation
Lecture 17
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IntroductionObject-Oriented Programming
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The Journey So Far
Procedural Programming
using Pseudocode
You are here
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The Destination
Procedural Programming
Object Oriented Programming
using Pseudocode
using Java
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Introduction

• We are going to introduce two concepts at the
  same time
• Object-Oriented Programming
• Programming in a real language
• Why?
• Pseudocode is a good preparation for learning any
  language.
• The Object Oriented paradigm is a significant
  factor in modern programming
• We feel that an introductory course should at a
  minimum give students a good taste of real
  programming
• Many students have asked for exposure to a real
  language
• Excellent preparation for CS 1312

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Introduction

• We will not abandon the principles you have
  learned so far
• modularity
• abstraction
• the necessity to visually validate the logic
• Today we will give some basic ideas about
• Object Oriented Programming
• Working with a real language Java

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Object Oriented Programming

• A different programming style
• A different way of thinking
• Origins
• Problems with very large systems becoming
  unmanageable (gt 100,000 lines of code)
• Graphical User Interfaces (e.g. Macintosh,
  Windows) required a different way of programming
  due to complex interactions of windows, mouse
  clicks, etc.
• Xerox PARC Smalltalk
• C as an enhancement to C
• Java as a Web language

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To be more specific...
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The Scenario

• Recall the concept of a Queue
• Defined by a set of behaviors
• Enqueue to the end
• Dequeue from the front
• First in, first out

Items
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Where is the Queue?

• The logical idea of the queue consisted of
• Data stored in some structure (list, array, etc.)
• Modules to act on that data
• But where is the queue?
• Wed like some way of representing sucha
  structure in our programs.

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Why?

• We would like to have reusable drop-in
  programming components we could use to solve
  problems.
• We would like to encapsulate the data and the
  behaviors in order to protect it from misuse.
• We would like clear interfaces between different
  parts of programs
• We would like a programming system that was
  extensible
• We would like a programming language that would
  be excellent for modeling real world objects

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Issues

• As is, there is no way to protect against
  violating the specified behaviors.

Procedure Enqueue
Procedure Dequeue
The Queue Data
Sneak into Middle
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Issues

• If our queue could somehow be packaged we could
  drop in a queue object whenever we needed it.

Queue
Acme Manufacturing
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Issues

• Wed like a way to put a wrapper around our
  structure to protect against intrusion.

Sneak into Middle
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Issues
Contract The party of the first part hereinafter
known as the queue agrees to... The party of the
second part hereinafter known as the application
agrees to...
Clear lines of responsibility
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Issues

• I need a queue that can tell me how many items it
  contains...

Queue
Enqueue
Dequeue
Acme Manufacturing
Magic Process
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Issues
Elevator
Signal Object
Elevator Object
Student Object
Car Object
BankAccount Object
Real World Objects!
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Questions?
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Abstraction in Object-Oriented Programming
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Procedural Abstraction

• Procedural Abstractions organize instructions.

Function Power Give me two numbers (base
exponent) Ill return to you baseexponent ???
Implementation ???
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Data Abstraction

• Data Abstractions organize data.

StudentType
Name (string) GPA (num) Letter Grade
(char) Student Number (num)
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Behavioral Abstraction

• Behavioral Abstractions combine procedural and
  data abstractions.

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The Object-Oriented Paradigm

• Instances of behavioral abstractions are known as
  objects.
• Objects have a clear interface by which they send
  and receive messages (communicate).
• OO is a design and approach issue. Just because
  a language offers object-oriented features
  doesnt mean youre doing OO programming.

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Information Hiding

• Information Hiding means that the user has enough
  information to use the interface, and no more

Example Stick shift We dont care about the
inner workings... We just want to get the car
going!
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Encapsulation

• Encapsulation is the grouping of related things
  together within some structure

Item 1
Item 2
Item3
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Encapsulation via Algorithms

• Algorithms encapsulate modules, data, and
  instructions.

Algorithm
Procedure
Instructions
Function
Data
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Encapsulating Instructions

• Modules encapsulate instructions.

Procedure/Function
Instructions
Instructions
Module call
Instructions
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Encapsulating Data

• Records allow us to do this with data

Record
Field 1
Field 2
Record
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Revisiting the Question Where is the Queue?

• Notice we still have no way of identifying the
  idea were discussing
• The Queue is still in the ether.
• Wed like to encapsulate the data (regardless of
  its actual representation) with the behavior
  (modules).
• Once we do this, weve got a logical entity to
  which we can point and say, there it is!

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Encapsulating Data with Methods

• Abstract data types (ADTs) allow us to
  encapsulate logically related data and modules

Queue
Enqueue
Data
Dequeue
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Abstract Data Types

• An idea, a concept, an abstraction of the big
  picture
• It encapsulates the abstract behaviors and data
  implied by the thing being modeled.

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Achieving Behavioral Abstraction

• Abstract data types (ADTs) are concepts.
• We require some way to implement these common
  abstractions so we can write them once, verify
  that they are correct, and reuse them.
• This would save us from having to re-do work.
  For example, every time we create a queue we did
• List_Node definesa ...q_front isoftype
  ...q_tail isoftype ...
• procedure Enqueue(...)
• procedure Dequeue(...)
• We need an algorithmic construct that will allow
  us to bundle these things together the class.

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OO Summary

• Behavioral abstraction combines data abstraction
  with procedural abstraction.
• The object-oriented paradigm focuses on the
  interaction and manipulation of objects.
• An Abstract Data Type (ADT) allows us to think of
  what were representing as a thing regardless of
  its actual implementation.

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Questions?
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Compiling and the Java Virtual Machine (JVM)
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Working with a real language

• The syntax of Pseudocode is pretty loose
• visual validation encourages a permissive
  approach
• emphasized the importance of structure
• Compilers essentially require perfection.
• The procedure for writing your algorithms wont
  change (much!)
• Write a high level approach focusing on
  algorithm.
• Write the code concentrating on details.
• Check it by hand before compiling.
• Because we're introducing a real language at the
  same time as we're introducing the object
  oriented paradigm we will require you to just
  write some code.
• You may not understand it all immediately
• Eventually we'll explain!

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The First Part

• Initially well use Java to write procedural code
  just to let you get the feel of a real language.
• In fact, well start by programming some of the
  same types of modules that we used in the
  procedural pseudocode earlier in the semester.
• Then, as we introduce the Object Oriented
  paradigm well use Java as it was intended.

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About Java
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Introduction to Java

• What Java is
• A real professional programming language
• (which is good and bad...)
• Portable across any hardware platform that has a
  JVM interpreter (more later)
• An object-oriented language
• What Java is not
• The Ultimate Programming Language
• HTML or another web-content language
• Only useful for web applets
• Just Another Vacuous Acronym

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Real Languages

• Real languages require the programmer to write
  code using a very specific set of rules, keywords
  and syntax.
• Then this code is transformed into actual
  instructions that a specific machine can execute.
  Often a complex process
• A number of strategies have been invented to
  accomplish this process
• Assemblers
• Compilers
• Interpreters
• A traditional problem has been the necessity to
  have different versions of a program for
  different machines.

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A New Idea

• Java was specifically developed to be able to
  write code once and run it anywhere
• How is this magic accomplished?
• Using an intermediate language! Byte code.
• The Byte code is interpreted (executed) using a
  special piece of software (a program) called the
  Java Virtual Machine (JVM)

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Compilation
Source Code .java
Java compiler
Generic Byte Code .class
OS-specificJVM interpreter
OS-specific Object Code
Execute program
Need one of these for every different OS
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Structure of Java Programs

• Initially well write programs that fit in one
  file.
• Create the file with an editor (e.g. emacs)
• Compile with javac producing a .class file
• Execute by running java and sending it the .class
  file
• Our first (tiny) program will be roughly like a
  cross between an algorithm and a procedure.
• Lets take a look...

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Sample Application
We create a file (using an editor) called
HelloWorld.java

• public class HelloWorld
• public static void main(String argv)
• System.out.println(Hello World!)

We compile by typing (at the OS prompt) javac
HelloWorld.java Which produces HelloWorld.class Th
en we execute by typing java HelloWorld
Hello World!
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Demo

• gtjavac HelloWorld.java
• gtjava HelloWorld
• Hello World!
• gt

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Quick Trix

• The name of the file must match the name of the
  class EXACTLY!!!
• File Bubba.java
• Contains
• Everything must be EXACTLY correct!!!

Capitalization counts
class Bubba ...
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Eventually...

• Applications (normal computer programs)
• Each program consists of multiple files.
• Each file contains a class.
• Each class will contain modules which will
  resemble procedures and functions.
• THE MODULES WILL BE CALLED METHODS
• The .class file that you send to java must
  contain a method named main
• It will actually look like this
• public static void main(String argv ) ..
• the JVM will use the file naming convention to
  find the other classes required by this main
  program.

As opposed to Applets
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Some basic syntax issues

• Your TA is smarter than the java compiler
• Lines need to terminate with a
• Easier said than done
• Braces will be used to indicate "blocks" of code
• Which essentially act like a single line

public class HelloWorld public static void
main(String argv) System.out.println(He
llo World!)
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A look ahead...

• Pseudocode
• if some_boolean then
• a lt- a 1
• else
• a lt- a 2
• b lt- 7
• endif

• Java
• if(some_boolean)
• a a 1
• else
• a a 2
• b 7

Note placement
Note Indentation is used in both cases. Means
nothing to compiler. instead of lt- (more
later) Must have parens in Java
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Questions?
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