Copyright © 1997, 2014
Robert W. Hasker

Object-Oriented Programming

• Ch. 6; esp. 6.1-6.7

Object-oriented view of the world

• Consider values like int, double
  • store numbers, nothing more
• world full of more than numbers: objects
  • Example objects: tree, car, bank account, music player
• Those objects have information, behavior
  • Goal: capture this information, behavior in a way that matches the real world
• Sample objects we've already used: String
  • What is the information that's stored?
  • What are some of the operations?
  • How do we use it?
  • Draw picture

Example: time

• class Time
• data: int hrs, min
• methods: set, advance1Minute
  • Use this to access data
  • Add check w/ error message if bad parameters to set
• display(): print time (no newlines!) in 4:03 format
• Using it:
  • Time x = new Time(); x.set(4, 3); x.display(); x.advance1Minute();

Making time fly

• main: start time at 7:55, advance, display result
  • This sort of code known as a driver
• set up second timer a half hour earlier
• Examine these in the debugger
• reference variable: something that refers to an object
  • Will sometimes not distinguish between references and objects, but it's important to know the difference
• int x; - just gives a value
• Time t = new Time(); - sets up a reference
• Another reference: this
• Driver class: code that runs something else
  • for now: class with static main and nothing else
• Add code to advance one minute
• [What isn't tested?]
• Add code to advance 50 minutes.
• Rewrite using for
  • The code

            for(initialization; test; update) {
               actions
            }
    

    is exactly equivalent to

            initialization;
            while ( test ) {
               actions
               update
            }
    

  • General practice: use for when know at start of loop how many times it will be executed

Issue: keeping data private

• Suppose code just sets time to 14, -3
• How to keep that from happening?
• Introduce private, public into Time
• Show what happens with the code.

Terminology

• Time variable: reference
• Object, instance,
• class = type
• attributes: member data
• behaviors: operations, also known as methods
• encapsulation
• Object-orientation; goals:
  • represent data in a way that matches the real world as much as possible
  • encapsulate data: hide details from outside world
    • control access: only allow access by specified operations
  • result: highly maintainable code

UML

• How to document class?
• Text: works for a single class
• Pictures: handle lots of classes
• Notation we will use: Unified Modeling Language
• Box: class name, attributes, methods
• Attributes for time:
  • hours, minutes : int
• Methods for time:
  • advance, display:void
  • set(hours: int, minutes: int):void
  • getHours:int, getMinutes:int
  • toString:String
  • getMinutesPastMidnight:int

Style

• Note indentation
• Note spacing
• Coding standards
  • usefulness
  • importance to programmer
• Oracle's Coding Standard, but note these date back to 1997 and Oracle no longer maintains these documents.
• Fixing style in programs
• Expect the amount of points associated with style to go up.

Example

• Quadcopter
• Driver:

          static int void main(String[] args)
          {
             Quadcopter spy = new Quadcopter();
             spy.setLocation(10.5, 1.2);
             spy.setDestination(-18.2, -45);
             spy.setSpeed(3.5);
             System.out.println("Travel distance: " + spy.distanceToTravel());
             System.out.println("Travel time at " + spy.getSpeed() +
                                ": " + spy.timeToDestination());
          }
  

  1. Draw UML diagram
  2. Write class
  3. Revise timeToDestination to take power (percentage)
• Discuss
  • Local variables, parameters, class variables
  • Using this
  • return without an argument

Examples

• class Name
  • set name - convert to first and last
  • getFirstName, getLastName
  • lookupName: string (last, first order)
  • contains(target) - boolean operation

Review

• Objects, classes, instance variables, methods
• coding standard