Chapter 3 Introduction to Classes and Objects

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Chapter 3Introduction to Classes and Objects

• Definitions
• Examples

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Definitions

• Object instance of a class
• Objects have attributes and behaviors
• Attributes data
• Behaviors methods or operations

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Definitions

• Encapsulation combining attributes and methods
  into one group (object)
• Information hiding
• Implementation details are hidden within the
  objects themselves
• Abstraction another term for information hiding

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Definition

• Default package set of classes that are
  compiled in the same directory on disk.
• We say that all such classes are considered to be
  in the same package
• Classes in the same package are implicitly
  imported into the source code files of other
  classes in the same package.
• Import is not required when one class in a
  package uses another class in the same package.

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Definitions

• Java classes contain
• Methods implement operations
• Fields implement attributes
• Accessor methods
• Methods that allow access to internal data fields
• Example
• getInterestRate()
• //method name generally starts with get
• //highly recommended that it starts with get

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Definitions

• Mutator methods methods that change values in
  data fields
• Example
• setInterestRate (double x)
• //method name generally starts with set
• //highly recommended that mutator methods start
• //with set

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Variables

• Local variables
• Variables declared in the body of a particular
  method
• Fields
• Variables declared inside a class declaration but
  outside the bodies of the classs method
  declarations
• Also known as instance variables (if not static)

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Fields instance variables

• Can be declared as public, private, protected,
  default
• Generally should be declared as private to
  promote data hiding
• Methods generally declared as public

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Example class (Circle.java)

• public class Circle
• private double radius 1.0 //field
• double findArea()
• return radius radius 3.13159

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Declaring a reference to an object and creating
an object

• Declaration Classname objectReference
• Circle myCircle //myCircle is a reference to a
  Circle //object
• Create a Circle object
• myCircle new Circle()
• Declare and create
• Circle myCircle new Circle()

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Circle Class

• CircleClass.java
• CircleProgram.java

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Constructors

• Method with the same name as the class
• Allows construction of objects with different
  initial data values initializes object
• Do not have a return type
• Invoked using the new operator
• Circle (double r) Circle ()
• radius r radius 1

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Constructors

• Default constructor
• Constructor with no parameters
• If your class has no constructors, Java will
  automatically create a default constructor
• If your class has a constructor, Java will not
  create a default constructor
• You should include default constructors in your
  class.

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Instance Variables

• Normally should be initialized in constructors
• Alternative Initialize when declarations are
  made
• If instance variables are initialized in the
  declaration and you have constructors --- Be
  sure to define a default constructor even if the
  body of the default constructor is empty.
• The following
• AClass anObject new AClass()
• is illegal if no default constructor

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AClass anObject new AClass()

• public class AClass
• private int value 1
• public AClass (int x)
• value x
• public void setValue(int x)
• value x
• public int getValue()
• return value

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Static Methods methods that do not require a
calling object

• Uses the keyword static in method heading
• You cannot do anything inside a static method
  that refers to a calling object
• You cannot access instance variables from inside
  a static method
• You cannot call a non-static method inside a
  static method )unless you first create a new
  object and use it as the calling object)
• You can invoke a static method with an object
• Confusing and is not normally done.

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Method Syntax

• access specifier
• qualifier
• return type
• method name
• (argument list)

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Access Specifiers protecting class variables
and methods
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Static Variables

• Static variables belong to the class as a whole
  and not just to one object
• private static int turn
• Used to communicate between objects
• One object can change the static variable and
  another object can read that change
• Should normally be declared as private (unless
  declared as constants)

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Final Instance Variables

• Keyword final is used to specify that a variable
  is not modifiable (it is a constant)
• private final int INCREMENT 5
• Use all caps when denoting constants

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Method arguments

• All arguments are pass by value methods always
  receive a copy of the argument
• If argument is an object reference (different
  story)

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Class methods static methods

• Example
• Integer.parseInt(s)
• main (String args)
• An instance of the class is not necessary to
  access the static method
• Static methods can only call static methods and
  can only access static variables
• Method is usable by any code that has access to
  the method regardless of whether any objects of
  the class have been created.