Java Programming, Third Edition

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Java Programming, Third Edition

• Chapter Three
• Using Methods, Classes, and Objects

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Objectives

• Create methods with no arguments, a single
  argument, and multiple arguments
• Create methods that return values
• Learn about class concepts
• Create a class

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Objectives (continued)

• Create instance methods in a class
• Declare objects and use their methods
• Organize classes
• Begin to understand how to use constructors

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Creating Methods with Zero, One, and Multiple
Arguments

• Method series of statements that carry out a
  task
• Classes may contain unlimited number of methods
• Methods must include
• A declaration
• An opening curly brace
• A body
• A closing brace
• Review and revise First class

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Creating Methods with Zero, One, and Multiple
Arguments (continued)

• Method declarations must contain
• Optional access modifiers
• The return type for the method
• The method name
• An opening parenthesis
• An optional list of method arguments
• A closing parenthesis

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Creating Methods with Zero, One, and Multiple
Arguments (continued)

• Access modifiers for a method can be
• public, private, protected, package (default)
• Static storage modifier method independent of
  object
• Implement nameAndAddress ( ) method
• Modification public and static
• Logic calls println ( ) three times, passes
  three strings
• Place definition within class, not within other
  method

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Creating Methods with Zero, One, and Multiple
Arguments (continued)

• Using nameAndAddress ( ) in revised First class
• Write method name into main ( )
• Method called during program execution
• Output four lines of text
• println ( ) called indirectly and directly
• Using full name, First.nameAndAddress ( )
• Necessary when method used outside of First class
• Not needed for use within class (First, in this
  case)

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Creating Methods that Require a Single Argument

• Implementation hiding
• Method interface known to user
• Method body is a black box
• Arguments communications to a method
• Each argument must specify data type, local name
  
• Code arguments into predictRaise ( ) method

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Creating Methods that Require a Single Argument
(continued)

• Method declaration
• public void predictRaise (double moneyAmount)
• Method body
• Declare local variable (double newAmount)
• Local variable has local scope (not known to
  caller)
• Calculate adjusted salary using argument
• Assign result to newAmount
• Output new salary using printlin ( )

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Creating Methods that Require a Single Argument
(continued)

• Call syntax
• Use method name
• Place constant or variable name between ( )
• Variable name in call may differ from header
• Examples
• predictRaise(400.00)
• predictRaise (mySalary)
• predictRaise(moneyAmount)

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Creating Methods that Require Multiple Arguments

• Methods can require more than one argument
• Pass multiple arguments by
• Listing the arguments in the call to the method
• Separating them with commas
• Each argument specified by data type and name
• Example add argument to predictRaise ( )
• Rate was previously hard-coded in definition
• New rate variable (double) added to argument list

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Creating Methods that Require Multiple Arguments
(continued)

• Passed arguments match header in type, number,
  order
• Another example computeCommission ( )
• Declaration
• public static void computeCommission(int value,
• 
  double rate, char vehicle)
• Calls using literals and variable names
• computeCommission(40000, 0.10, L)
• computeCommission(value, commRate, vType)

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Creating Methods that Return Values

• void methods return no values
• Example public static void nameAndAddress( )
• Value-returning type also known as methods type
• Two new design elements
• Include data type in header
• Include return statement in method body (last
  line)
• Java restricts return to one type (primitive or
  class)
• Example predictRaise ( ) redesigned to return
  double

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Learning about Class Concepts

• Basic perspective in object-oriented programming
• Everything is an object
• Every object is a member of a class
• Class-object relationship
• Blueprint to concrete instance (instantiation)
• Template ltobjectgt is a ltclass_typegt
• Example "is a" relationships
• My oak desk (object) is a Desk (class)
• Moby (instance) is a Fish (class)

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Learning about Class Concepts (continued)

• Usefulness of class concept
• Objects inherit attributes from classes
• Objects have predictable attributes as class
  members
• Intuitive objects send (or receive) requests
  (messages)
• Two chief ways to utilize classes
• Run as application
• Instantiate objects from class
• Prewritten classes may be called by class client
  (user)

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Creating a Class

• Overview of class creation
• Assign a name to the class
• Determine data and methods required by class
• Writing a three-part class header
• An optional access modifier (public, final,
  abstract)
• The keyword class
• User-selected legal identifier for class name
• Public access least restrictive and inheritance
  friendly
• Illustration declare new Employee class

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Creating a Class (continued)

• Instance variables (data fields or fields)
  attributes
• Four access levels for member data (and methods)
• private, public, protected, package (default)
• Private access promotes information hiding
• Highest level of security
• Data member cannot be directly accessed
• Public method of same class interface to data
  fields
• Illustration declare variable in Employee class

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Creating Instance Methods in a Class

• Static (class) method called independently of
  object
• Instance methods
• Require an instantiated object
• Used to interface with data members
• Implement two public instance methods in Employee
• getEmpNum ( ) accessor returns current attribute
  value
• setEmpNum ( ) mutator sets attribute with passed
  arg

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Declaring Objects

• To declare an object
• Supply a type and an identifier
• Allocate computer memory for the object
• Use the new operator
• Example using condensed syntax
• Employee someEmployee new Employee( )
• Employee ( ) method is a constructor
• Employee objects used in DeclareTwoEmployees class

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Understanding Data Hiding

• Data hiding
• Key concept in object-oriented programming
• Public method interfaces with private data
• DeclareTwoEmployees indirectly accesses private
  data
• Access violation clerk.empNum 789
• Get and set methods provide access channel to
  data
• Get and set provide framework for maintenance
• May be recoded for efficiency and/or more
  protection

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Organizing Classes

• Conventions regarding instance variables
• Place data fields in logical order at class start
• Use a unique, descriptive identifier for
  attributes
• Example empNum
• Conventions regarding instance methods
• Place definitions after listing of data members
• Use descriptive identifier in method name
• Group by one or more criteria (alphabetical
  order)
• Note method prototypes not required in Java

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Using Constructor Methods

• Constructor methods instantiate new class objects
• Must have same name as class
• May not have a return type, not even void
• May include arguments in interface
• Explicit constructor for Employee set salary to
  300.00
• Call Employee chauffer new Employee( )
• If explicit constructor absent, Java constructs
  default

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Summary

• Methods may have 0 or more arguments
• Methods are modular
• Methods have a header and body
• Java does not require method prototypes
• Method access levels private, public, protected

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Summary (continued)

• Static methods are independent of an object
• Instance methods require an object
• Methods may return primitive or class data types
• Methods sending data need return statement and
  type
• Void methods have no return type

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Summary (continued)

• ltobjectgt is a class_type classes are object
  blueprints
• Public class access permits inheritance
• Encapsulation hides method implementation and
  data
• Instance methods interface with private data
• Explicit (or implicit) constructor establishes
  object