Lecture 8

Fall 2018

Narges Norouzi

Recap

• Strings
  • Creation
  • Methods
• Characters
  • Methods
  • ASCII Encoding

Object Oriented Programming

Introduction to OOP

• Objectives:

  • Know the difference between functional programming and OOP

  • Know basic terminology in OOP

  • OOP Design

Structured vs. oo programming

​Structured Programming:

structured programming

• Using function

  • Function & program is divided into modules

  • Every module has its own data & functions which can be called by other modules.

Object-Oriented approach

• OO approach goes one step further

• Lets the programmer represent problem space elements

• The elements in the problem space and their representations in the solution space are referred to as "objects"

OO Programming

• Objects have both data and methods

• Objects send and receive messages to invoke actions

• Objects of the same class have the same data elements and methods

• Key idea in object-oriented programming:

  • The real world can be accurately described as a collection of objects that interact.

Object Oriented Programming

Oebjects

• An object has:

  • state - descriptive characteristics

  • behaviors - what it can do (or what can be done with it)

• Bank Account object:

  • state: account number and current balance

  • behaviors: ability to make deposits and withdrawals

• Note that the behavior of an object might change its state

Classes

• An object is created using a class

• A class is the blueprint of an object

• The class uses methods to define the behaviors of the object

• A class represents a concept in problem domain

• Multiple objects can be created from the same class

• The class that contains the main method of a Java program represents the entire program

example: a rabbit

• You could (in a game, for example) create an object representing a rabbit

• It has data:

  • How hungry it is

  • How frightened it is

  • Where it is

• And methods:

  • eat, hide, run, dig

what's a class made of?

• Members of a class:

  • Attributes or Fields (instance variables, data)

    • For each instance of the class (object), values of attributes can vary, hence they are instance variables

  • Methods (instance methods)

• ​Person class

  • Attributes: name, address, phone number

  • Methods: change address, change phone number

• Alice object

  • Name is Alice, address is ...

Example of a class

class as an object factory

• A class is like a factory that creates objects of that class

• We ask a class to create an object by using the keyword:

  • new

• We can also ask the class to initialize the object
  • and pass data to help initialize it

basic terminology

• Object
  • Usually a person, place or thing (a noun)
• Method
  • An action performed by/on an object (a verb)
• Attribute
  • Description of objects in a class
• Class
  • A category of similar objects
  • Does not hold any values of the object’s attributes

5 minutes break

LINK

Clock class abstraction 

• A Clock represents a 12-hour clock

• Its internal state includes: hour, minute, second

• Its interface allows telling the clock that a second has elapsed and querying the clock for the time:

Structure of clock class

Constructors

• Objects must be initialized before use.

  • We must specify what is the initial state of the object before we can use it.

• Space for holding the object state data is only allocated when the object is constructed.
• We specify the way an object is initialized using a constructor, which is a special method invoked every time we create a new object.
• The name of the constructor is always identical to the class name

clock constructor

"new" operator (1)

• "new" creates a new object from

  specified type:

  • ​new String();

  • new Book();

"new" operator (2)

• Returns the reference to the created object

  • String s = new String();

  • Dog d = new Dog();

  • Rectangle rectangle = new Rectangle();​

• ​Primitive types are not referenced:
  • new int();

object references

• When you declare an object, you declare its reference:
  • String s;
  • Book b;
• Exception:  
  • Primitive types
• Primitive types are not actually objects
  • They cannot have references

create objects

• "String str;" will not create an object
• It just creates a reference
• You cannot use “str” variable, “str” is null
  • null value in java
• You should connect references to real objects
• How to create objects?
  • new
  • Or static value in case of String: String str = “Hello”;

Time for another Demo?