Object Oriented Programming

Object Oriented

• object incorporate both data structure and behavior
• System functionality is expressed in terms of object services

Object

• behavior + state + identity
  • state
    • state of an object is all the data that it encapsulated
    • objects has a number of attributes
    • some attributes are mutable (address, reference) at the immutable
  • behavior
    • an object can understand certain messages, the messages is normally fixed like the set of attribute it has

• identity -objects are not defined just by the current value of their attributes

  • the state of an object may change but is still the same object logically

• Firstly, don't think water and object will have, instead think what will it do for the system

• objects are potentially reusable components
• An object is the thing you can interact with, i.e. You can send your request and get reaction.
• NB: How object behave may change as the current internal state of the object varies
• pros: no centralized data, only communicate by messaging. Independent encapsulated easy for maintenance

message

• a message includes a selector, i.e. the method that returns the message.
• A message may have arguments

interface

• the public interface of a object defines which messages will be accepted
• an object can also send message to self, this is managed by public or private interface
• public interface: any part of the system can use
• private interface: object itself or other privileged components of the system
• An object can have several interfaces that from different view of point

Class

• each object is an instance of a class
• class defines the message understandable as well as how object will response
• each instance has his own state, but share a same static state of the class
• A class encapsulates data and behavior, hiding the Implementation details

Inheritance

• inheritance is the sharing of attributes and operations in the hierarchy relationship
• subclass(superclass)
• Object class is always coupled to its superclass

Polymorphism

Doctor dc1 = new Surgeon("Mingjie");
Doctor dc2 = new GeneralPracticer("mingjie");

public void callDoctor(Doctor dc) {
  // You can pass dc1 or dc2 as argument in this function
}

Dynamic Binding

The print method is bind dynamically to the subclass in the main method

public class Printer {
  public void print() {
    System.out.println("Printer");
  }
}

public class LaserPrinter extends Printer{
  public void print() {
    System.out.println("LaserPrinter");
  }
}

public static void main(String[] args) {
  Printer p = new LaserPrinter();
  p.print(); // ==> "LaserPrinter"
}

UML

• UML is a language for specifying, visualizing and documenting