Objects

1
Objects

• Robert Stumpf, Professor
• Computer Information Systems
• California State Polytechnic University Pomona

2
What are Objects

• Basically they are a collection of data values
  handled as a unit
• The values are usually of the form of int,
  double, and other data types
• The values may also be references to other
  objects such as Strings

3
What are Objects

• A collection of objects are called a class
• What makes objects special is that behaviors can
  be associated with them
• These behaviors are implemented as Java methods
• All objects in a given class have the same methods

4
What are Objects

• What is good about objects is that they model
  real objects
• For example an object might be a guest (role), an
  account (abstract thing), a vehicle (real thing),
  a purchase (transaction), or a mouse click (event)

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What are Objects

• Note that real objects and model objects both
  have attributes and behavior
• A model object is an object modeled in Java
• The attributes are the data values or references
  to other objects
• The behaviors are the methods

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What are Objects

• The fact that an object contains behavior and
  data values is called encapsulation
• To invoke the behavior, a message is sent to the
  object reference
• The message must have the same spelling as the
  method name

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Summary of What Objects Are

• Objects encapsulate data values and behavior
• They model real objects
• They receive messages to invoke their behaviors

8
The Purpose of a Class

• A class in Java is a mechanism used to define an
  object
• Sometimes they are called templates for objects
• They act as a blue print of an object
• When an object is instantiated (created) the
  template is used

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The Purpose of a Class

• Thus in Java the word class is a reserved word
• In addition there is a visibility modifier for
  each class and members (data and methods) of the
  class
• For now one chooses private and public

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The Purpose of a Class

• A visibility modifier tells the system how to
  protect the members from the outside world (other
  objects)
• It is recommended to protect the data members,
  thus use private visibility
• The class and the methods are usually set to
  public visibility which allows them to be used by
  other objects

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The Purpose of a Class

• There is one additional feature of classes that
  is often very confusing
• It is permissible to assign data and behavior to
  the class instead of the object
• The reserved word static is used to describe
  these class variables and methods
• The word static means it is the same for all
  objects

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The Purpose of a Class

• Examples of static data and methods
• public static double final Pay_Rate 8.75
• public static void main(String args)
• Note that static members are usually public in
  that they are often used by other objects

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The Purpose of a Class

• Examples of using static data and methods
• double root Math.sqrt(x)
• double pay MyClass.Pay_Rate hours
• Note that the variables and messages are
  associated with the class name

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The Purpose of a Class

• There are some examples which are very confusing
• For example
• System.exit(0)
• The System class is defined in the java.lang
  package and is always available

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The Purpose of a Class

• There are some examples which are very confusing
• For example
• System.out.println(Hello)
• The System class has an instance variable called
  out that is instantiated for you
• Thus println is really an instance method

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Summary of what Classes are

• Recall a class is a group of objects classified
  together
• A class definition is used by the Java system as
  a template to create objects
• A class may also have data and methods and they
  are called static data and static methods

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Summary

• Objects are an important part of the Java
  language
• They allow us to model real world objects
• This results in a very meaningful way to program
  in that allows us to create a system to reflect
  the structure of the real world

18
Thank You

• Java Objects Model Real World Objects
• Any questions should be directed to Professor
  Robert Stumpf
• Email rvstumpf_at_csupomona.edu
• Web Sitehttp//www.csupomona.edu/rvstumpf