Class and Object

 

   ~ Object - represents either objects in the real world like automobiles, houses, and employee records or abstractions like colors, shapes, and words.

   ~ Objects have state, behaviour and identity.

      State : The condition of an object at any moment, affecting how

                  it can behave

      Behaviour : Define by a set of methods, invoking a method on an

                  object means that asking the object to perform a task.

  Identity : Each object is unique.
~ Objects are categorized into classes.

~ Each individual object is an instance of a class.

~Class - the definition of a kind of object. It is like a plan or a blueprint for constructing specific objects.

A Class as a blueprint

 Objects that are instantiations of the class Automobile.

~ Class is a caterogy of objects that share the same :

Attribute

   + A named property of a class that describes a range of values

      that instances of the attribute might hold.

   + Attribute are the way classes encapsulate data.

   + Example :

          Class : Employee

          Attributes : employee_name, employee_id,

                             position, department etc...

Operation

   + A behavior of an object

   + Implemented in classes are methods

   + Methods are the identified and invoked by their signature,

      including name, parameters, and return type

   + Example :

           Class : Employee

           Attributes : employee_name, employee_id,

                              position, department, salary etc...

           Operations : setEmployeeDetails

                               calculateSalary

                               printEmployeeDetails

~ Programmers often use a simpler graphical notation to summarize some of the main properties of a class. This notation is called a UML class diagram or simply a class diagram.

*UML - Universal Modeling Lamguage

A Class Outline as a UML Class Diagram

~ Declaration of object & assigning of an object reference to the variable.

            ClassName objectRefVar = new ClassName ( ) ;   

         Example :   Vehicle car = new Vehicle ( ) ;  

~After an object is created, its data can be accessed and its methods invoked using the dot operator (.). 

  + objectRefVar . dataField - References a data field in the object

  + objectRefVar . method (arguments) - Invokes a method on the object

     Example : + Car . colour - References the colour in Vehicle

                        class

                     + Car . printColour ( ) ;

                     - Invokes the printColour() method on Car .

                         Methods are invoked as operations on objects.