Methods, classes, and Objects

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Methods, classes, and Objects

• Dr. Jim Burns

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Question

• Which of the following access modifiers is the
  default modifier?
• public
• private
• protected
• package

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Question 2

• Which of the following access modifiers is needed
  to enable a class to be used without being
  instantiated?
• Public, private, static, void, protected

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Question 3

• Methods are invoked using parameters
• System.out.printlin(With raise, salary is
  newamount)
• Method headers have arguments
• Println(string args)
• Do the parameters and arguments occupy the same
  locations in memory?

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Question 4

• In the statement
• System.out.printlin(With raise, salary is
  newamount)
• Println() is the method name?
• What is System?
• What is out?

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Using Methods, classes, and Objects

• Methods are similar to procedures, functions, or
  subroutines
• Statements within a method execute only when the
  method is called
• To execute a method, you call it from another
  method
• The calling method makes a method call
• Execution starts with the main method

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Simple methods.

• Dont require any data items (arguments or
  parameters), nor do they return any data items
  back
• You can create any method once and use it many
  times in different contexts

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Example

• Public class First
• Public static void main(String args)
• System.out.println(First Java application)

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Method Declaration

• Is the first line or header of a method and
  contains
• Optional access modifiers
• The return type for the method
• The method name
• An opening parenthesis
• An optional list of method arguments separated by
  commas
• A closing parenthesis

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Access Modifiers

• public accessible anywhere
• private accessible only within the class in
  which it is defined
• protected accessible only to derived
  (inherited) classes only
• package is the default
• static does not require instantiation before it
  can be used and remains in place after use,
  without being destroyed

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Implementation hiding

• The encapsulation of method details within a
  class
• You dont know the algorithm being used inside
  the method
• prinAndInter intrRate prin prin
• prinAndIntr (1intrRate) prin

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Implementation hiding

• We use televisions, telephones, computers,
  automobiles without understanding much about
  their internal mechanisms

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Method declaration

• modifiers name(arg_type arg1, arg_type arg2,
  arg_type arg3)
• HOWEVER,
• modifiers name(arg_type arg1, arg2, arg 3)
• IS INCORRECT!!
• Each arg must be preceded by its own data type

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Argument types

• Can be any of the primitive types
• Can be any of the predefined class types
• Assume a class customer exists
• Public void approveCredit(customer oneCustomer)

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Consider the following

• Public static void predictRaise(double
  moneyAmount)
• Double newAmount
• newAmount moneyAmount 1.1
• System.out.println(With raise, salary is
  newAmount)

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Now consider this

• Public static void predictRaise(double
  moneyAmount)
• Double newAmount
• newAmount moneyAmount 1.1
• System.out.println(With raise, salary is
  newAmount)
• moneyAmount 10000

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Would this change ..

• The value of salary in
• predictRaise(salary)
• ???

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No

• Because Java supports passing of values rather
  than passing addresses, so
• The values of the parameters in the calling
  statement are copied into the arguments of the
  header.
• The parameters and their associated arguments
  occupy different locations in memory
• C supports both pass-by-address and
  pass-by-value

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Further, at the end of

• the usage of predictRaise(salary) the storage
  used by the instantiation of the method is
  released back, unless the method is declared
  static, so the assignment of 10000 to moneyAmount
  is lost

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• public class DemoRaise
• public static void main(String args)
• double mySalary 200.00
• double moneyAmount 800.00
• System.out.println("Demonstrating some
  raises")
• predictRaise(400.00)
• predictRaise(mySalary)
• predictRaise(moneyAmount)
• public static void predictRaise(double
  moneyAmount)
• double newAmount
• newAmount moneyAmount 1.10
• System.out.println("With raise, salary
  is " newAmount)

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Creating Methods that Require Multiple Arguments

• Public static void predictRaiseUsingRate(double
  money, double rate)
• double newAmount
• newAmount money (1 rate)
• System.out.println(With raise, new salary is
  newAmount)

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In the above

• The order in which the paramters are passed must
  be consistent with the order in which the
  arguments are declared in the header
• money
• rate

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Creating Methods that Return Values

• Public static double predictRaise(double
  moneyAmount)
• double newAmount
• newAmount moneyAmount 1.1
• Return newAmount
• Whats different???

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This method would be invoked using

• Double mySalary
• Double myNewSalary
• mySalary 1000
• myNewSalary predictRaise(mySalary)

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Could also do something like

• System.out.println(New salary is
  calculateRaise(mySalary))
• OR
• SpendingMoney calculateRaise(mySalary)
  expenses

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Learning about Class Concepts

• When thinking in objects, everything is an
  object, and every object is a member of a class
• Your desk is a member of the class that includes
  all desks and your car is a member of the class
  that includes all cars
• These are called is-a-relationshipsrelationships
  in which the object is a member of the class

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An object

• Is an instantiation of a class (one tangible
  example of a class)
• A Buick, a Toyota Camry, and a Volkswagon Jetta
  are all instantiations of the class automobile

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Creating a Class

• Begins with a header declaration
• Continues with the body
• Data fields (instance variables)
• Methods
• Fields are named variables that you declare
  within a class, but outside of any method
• Data fields are also called instance variables

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Class header declaration (one or more)

• An optional access modifier
• The keyword class
• Any legal identifier you choose for the name of
  your class
• Public class employee

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Fields (also called data fields)

• Are instance variables
• Are accessible to all methods within the class
• Are usually declared private
• If private, they can only be accessed (read or
  changed) by methods within the class
• This is called information hiding

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Information Hiding

• Class fields are declared private
• Class methods are declared public
• The only way another class can access a classs
  fields is by use of one of its public methods

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Creating Instance Methods in a Class

• Public class employee
• private int empNum
• public int getEmpNum()
• return empNum
• This is an instance method that assumes empNum
  has been defined

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Instance Methods

• Unlike class methods, instance methods do not
  employ the static modifier.
• Static is used for class-wide methods, but not
  for methods that belong to objects
• When you are creating a main method within a
  class, many of the methods invoked by the main
  will be declared static so they can be invoked
  without calling them from within the main

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More on Instance Methods

• However, if you are creating a class from which
  objects will be instantiated, most methods will
  probably be non-static because you will associate
  the methods with individual objects

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Declaring Objects and Using their Methods

• Declaring a class does not create any objects
• A class is just an abstract description of an
  object
• Suppose a class Employee has been defined
• Employee someEmployee
• Creates an instance of Employee

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Notice in the above

• That class names will begin with a cap
• Variables and instances of classes (objects) do
  not
• Just as a convention

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Declarations

• int someValue
• Complier sets aside space for someValue (4 bytes)
  at compile time
• Employee someEmployee
• Again, compiler is notified that you will use the
  identifier someEmployee as an instance of the
  class Employee

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Instantiations can be done as follows.

• Employee someEmployee
• someEmployee new Employee()
• Here you informed the compiler that the
  identifier someEmployee will be an instance of
  Employee and second, you told the compiler to set
  aside enough space for the object (instance)
  someEmployee at run time

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This is dynamic storage allocation

• The actual object someEmployee does not exist
  until the new operator is encountered
• You can use
• Employee someEmployee new Employee()
• Here you are declaring and establishing code to
  create a new instance of the class Employee at
  the same time

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Organizing Classes

• Public class employee
• private int empNum
• private String empLastName
• private String empFirstName
• private double empSalary
• // Methods will go next

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The following is also equally acceptable

• Public class employee
• private int empNum
• private String empLastName,
  empFirstName
• private double empSalary
• // Methods will go next

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Order..

• You can place the data and methods in any order
  or mix them up

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Class Employee

• public void setEmpLastName(String name)
• empLastName name
• public String getEmpFirstName()
• return empFirstName
• public void setEmpFirstName(String name)
• empFirstName name
• public double getEmpSalary()
• return empSalary
• public void setEmpSalary(double sal)
• empSalary sal

• public class Employee
• private int empNum
• private String empLastName
• private String empFirstName
• private double empSalary
• public int getEmpNum()
• return empNum
• public void setEmpNum(int emp)
• empNum emp
• public String getEmpLastName()
• return empLastName

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An introduction to using Constructors

• Employee chauffeur new employee()
• Here you are calling a method named Employee()
  that is provided by default by the Java compiler
• It is a constructor that is provided by the
  compiler

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Employee chauffeur new employee()Can also be
written

• Employee chauffeur
• Chauffer new employee()
• The first statement declares an object named
  chauffeur
• The second statement instantiates the object
  named chauffeur
• Memory is allocated when the second statement is
  encountered

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Default constructors

• Are provided by the compiler when the user does
  not provide one
• They require no arguments

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When using the default constructor, it will
specify the following values for data fields

• Numeric fields are set to 0
• Character fields are set to Unicode \u0000
• Boolean fields are set to false
• Fields that are nonprimitive objects themselves
  are set to null (or empty)

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If you dont like these default values, or if you
want to perform additional tasks when you create
an instance of a class,

• Then you must write your own constructor
• Any constructor you write must have the same name
  as the class it constructs and constructor
  methods cannot have a return type

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• public class ComputeCommission
• public static void main(String args)
• char vType 'S'
• int value 23000
• double commRate 0.08
• computeCommission(value, commRate,
  vType)
• computeCommission(40000, 0.10, 'L')
• public static void computeCommission(int
  value,
• double rate, char vehicle)
• double commission
• commission value rate
• System.out.println("\nThe " vehicle
• " type vehicle is worth " value)
• System.out.println("With " (rate
  100)
• " commission rate, the commission
  is "