Introduction to ClassesObjects

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Introduction to Classes/Objects

• CIS 120

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Object Oriented Programming (OOP)

• Combines the power of procedural programming with
  Objects
• More Flexibility
• Modularity
• Clarity
• Reusability
• Through the use of Abstraction
• Encapsulation
• Inheritance
• Polymorphism

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Objects

• Focus is on the Object
• Unit of Programming is the Class
• A class is NOT an Object
• It is used to create them
• A Blueprint to tell the Java Virtual Machine how
  to create an object of that type
• Abstract
• Objects of the class are Instantiated (created)
• Concrete, One specific instance of a class

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Classes

• Programmers create their own reference types
  Classes
• Each Class Contains
• Attributes / Variables Things the object knows
• Represents the state of the object
• Each object will have their own unique values for
  the attrbutes
• Methods Things the object can do
• Operations on the data
• Invoking a method is asking an object of the
  class to perform the task

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Encapsulation

• All of the attributes and methods that are needed
  by that class are included as part of the class
  definition
• Details of how the class performs its tasks are
  kept hidden from the user of the methods
• Client can use the method without knowing how it
  works
• Programmer can change how the method works
  without affecting anyone using the method (as
  long as the method header doesnt change)

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Inheritance

• Creating classes from other pre-existing classes
• Every class in Java inherits from another class
  (the Object class)
• Child objects gain the existing attributes and
  behaviors of the Parent class
• Can Modify them
• Can Add to them

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Class Definitions

• Variables
• Class Variables
• Instance Variables
• Constructors
• Methods
• Class Methods
• Instance Methods

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Variables

• Class Variables
• Defined with keyword static
• Variables where the entire class has one copy of
  this variable
• Defined by the class one copy of these for the
  entire class, not one per object of the class
• Example a counter (more on this later)

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Variables

• Instance Variables
• Variables / Attributes that all objects of the
  class have their own copy of
• Each instance / object of the class will have
  unique data values for the Instance Variables

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Constuctors

• Special Methods used to create an object of the
  class
• Use Keyword new to call the constructor
• Constructors have the same name as the class
  (even start with a capital letter)
• Constructors have NO Return Type not even VOID
• Default Constructor no formal parameters
• Sets default values for variables
• Overloaded Constructor different parameter lists

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Methods

• Class Methods
• Defined with keyword static
• Methods defined by the class that are available
  to be called but are not performed by any one
  specific object of the class
• Performs some type of computation or useful task
• Not performed by a specific object

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Methods

• Instance Methods
• Methods performed by a specific object of the
  class
• Object can be thought of as an actor performing
  the method

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Creating a Class

• Class Header
• What class headers have we created?
• What has it contained?
• Access Control Keyword
• Public, Private, Protected, ltNonegt
• Keyword class
• Name of the class (capitalized)
• Class definition contained in

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Create a Circle Class

• public class Circle
• // all parts of class definition go here
• // end class circle

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Variables

• What data does a circle contain?
• Radius
• Should Radius be an Instance Variable or Class
  Variable

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Create the Radius Variable

• Created the same as any other variable
• Data type name
• Can include Access Control Keywords to limit
  access (Public, Private, Protected, ltNonegt)
• // instance variables
• public double radius
• // end class

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Constructor

• Default Constructor
• You should always have a default constructor
• No Formal Parameters
• Public Circle ( )
• radius 0.0
• // end default constructor

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Method Overloading

• Additional Constructor(s) ? Different Formal
  Parameter Lists
• For Circle, used when there is a Radius
• Public Circle (double newRadius)
• radius newRadius
• // end double constructor

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Methods

• Instance Methods
• getArea ? an object of class Circle should be
  able to determine its radius
• Instance Method needs a specific instance of a
  circle, with a set radius, to call the method
• public double getArea ()
• return radius radius Math.PI
• // end getArea method

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Creating an Object of the Class

• Not done within the Class Definition
• Created within a separate class
• Tester Class
• Application Class
• Within other / 2nd class
• Invoke a constructor using the new keyword
• objectReference new ClassName (actual
  parameters)

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Object Reference

• Reference to the instance of the object in memory
• Like a variable name only for an instance of an
  object
• Object Reference is created on the data stack
• Points to location in Heap memory where object is
  actually created
• Create like declaring a variable
• must at some point call constructor
• Circle myCircle
• myCircle new Circle ()
• Circle myCircle new Circle () // same as 2
  lines above

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Accessing Data and Methods

• After an object has been created
• Data and methods can be accessed using the .
  (dot) operator
• ObjectReference.dataField
• myCircle.radius
• ObjectReference.method(arguments)
• myCircle.getArea()

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Static Variables / Methods

• Static Variables
• All instances of the class share the same data
• Data values are stored in a common memory
  location
• All objects of the class are affected if one
  object changes the value of a static variable
• Example count
• Keep track of how many circle objects have been
  created

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Static Method

• Performed by the class not one instance/object
  of the class
• Example getNumberOfCircles
• Not specific to any one instance
• Call with ClassName.methodName

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Variables vs. Reference Objects

• Variables represent a memory location on data
  stack with a value
• Tell compiler what type of data the variable can
  hold
• Variable of a Reference Type (object reference)
  holds an address to a memory location on the heap
  to where the contents of the object are held

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Variable vs. Reference Object

• Copying a Variable
• Primitive type assignment i j
• Before After
• i i
• j j

1
2
2
2
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Variable vs. Reference Object

• Copying a Reference to an Object
• Object Type Assignment c1 c2
• Before

X
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Access Control Keywords

• Visibility Modifiers
• Control Access to Variables, Methods Classes
• Public makes classes, methods and variables
  accessible from any class, sub-class, package
• Protected makes classes, methods and variables
  accessible from the same class, sub-classes, and
  package (but not other classes)
• ltNonegt makes methods and variables accessible
  only from the same class and package (not
  sub-class or other classes)
• Private makes methods and variables accessible
  only from within its own class (not sub-classes,
  package, or other classes)

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Why use Access Control Keywords?

• In class Circle
• Radius and numberOfCircles are public
• Can be modified directly by ANY user
• This is a BAD idea
• Data may be tampered with
• User might set numberOfCirlces to any number
• Makes the class more difficult to maintain and
  vulnerable to bugs
• Changes to radius would then require a change to
  the class and a change to all other classes that
  use radius

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Private

• Prevent direct modifications of Class Variables
• Make them private
• Data Field Encapsulation
• Make the user call a method to access the data in
  the variable
• Private Variables cannot be directly accessed
  outside of the class that defines the private
  field
• Use Accessor and Mutator methods for private data
  fields

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Accessor Methods

• A get method
• Used to get the value of a private class
  variable
• public returnType getVariableName()
• return variableName
• // end get method

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Mutator Methods

• A set method
• Used to set a new value/update for a private
  class variable
• Public void setVariableName (dataType
  valueVariable)
• variableName valueVariable
• // end set method

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Circle Code of Accessor/Mutator
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Change Testing Class
Method Call
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Passing Objects to Methods

• Objects can be passed to methods as parameters
• Passing an object is actually passing the
  reference to the object
• The memory location of the object on the heap
• A new method object is created, however, it the
  reference is to the same memory location as the
  original object
• 2 References to the Same Memory Location
• Changes made to the object in the method WILL
  affect the object in the calling method

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Scope of Variables

• Local Variables Declared and Used inside a
  method
• Same variable names can be reused in different
  methods (not a good idea)
• Instance and Class Variables
• Scope is the entire class regardless of where
  declared
• Can be declared in any order in the class
• A class variable can be declared only once
• If a class variable has the same name as a local
  variable (really not a good idea) the local
  variable takes precedence

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this keyword

• this.variableName
• this serves as a proxy for the object that
  called the method
• Stands for Refers to the calling object
• It is like a blank waiting to be filled in by the
  object that invokes the method
• Often implicit
• Often omitted

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.toString Method

• Inherited from Object Class
• All classes are child classes from Object
• Intended to output object as a string
• Actually outputs Object Reference
• Hexadecimal Memory Address
• Must be overloaded for each class to allow output
  of class objects

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Circle.toString method

• Instance Method in Circle Class

• Call in Test Class