Enhancing%20classes

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Enhancing classes
syllabus

• Visibility modifiers and encapsulation revisited
• Static methods and variables
• Method/constructor overloading
• Nested classes

basic programming concepts
object oriented programming
topics in computer science
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Passing Objects to Methods

• Parameters in a Java method are passed by value
  a copy of the actual parameter (the value passed
  in) is stored into the formal parameter (in the
  method header)
• Passing parameters is essentially an
  assignment
• Both primitive types and object references can be
  passed as parameters
• When an object is passed to a method, the actual
  parameter and the formal parameter become aliases

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Example Arrays as parameters

• static void setArrayEl(int arr, int i, int n)
• arri n
• static void setEl(int el, int newVal)
• el newval
• public static void main(String args)
• int samples 1,2,3,4,5,6
• setArrayEl(samples,1,0)
• setEl(samples1,10)

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Overloading Methods

• Method overloading is the process of using the
  same method name for multiple methods
• The signature of each overloaded method must be
  unique
• The signature includes the number, type, and
  order of the parameters overloaded methods
  differ by the number and/or type of parameters
  they get.
• The compiler must be able to determine which
  version of the method is being invoked by
  analyzing the parameters
• The return type of the method is not part of the
  signature

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Overloading Methods
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Overloaded Methods

• The println method is overloaded
• println (String s)
• println (int i)
• println (double d)
• etc.
• The following lines invoke different versions of
  the println method
• System.out.println ("The total is")
• System.out.println (total)

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Constructor Overloading

• Constructors can be overloaded
• An overloaded constructor provides multiple ways
  to set up a new object
• The overloaded constructors differ by the number
  and type of parameters they get.
• When we construct an object, the compiler decides
  which constructor to invoke according to the type
  of the actual parameters
• A constructor with no parameters is called a
  default constructor

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// -----------------------------------------------
- // Constructs a new clock with the specified
hours, // minutes and seconds read. // If one of
the paramenters is not in the allowed // range,
the time will be reset to 000000. // _at_param
hours the hours to be set (0-23) // _at_param
minutes the minutes to be set (0-59) // _at_param
seconds the seconds to be set (0-59) //
------------------------------------------------ p
ublic Clock(int hours, int minutes, int seconds)
if ((seconds gt 0) (seconds lt 60)
(minutes gt 0) (minutes lt 60)
(hours gt 0) (hours lt 24))
this.hours hours this.minutes
minutes this.seconds seconds
else this.hours 0 this.minutes
0 this.seconds 0
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Overloading Constructors

• Clock c1 new Clock()
• // default constructor
• Clock c2 new Clock(23,12,50)
• // Clock(int, int, int) constructor
• The constructor must always check the parameters
  an object should not be initialized to an
  inconsistent state
• What is an inconsistent state in a clock?

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Visibility Modifiers - Classes

• A class can be defined either with the public
  modifier or without a visibility modifier
• If a class is declared as public it can be used
  by any other class
• If a class is declared without a visibility
  modifier it has a default visibility this draws
  a limit to which other classes can use this class
  
• Classes that define a new type of objects, that
  are supposed to be used anywhere, should be
  declared public

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Any other class can use MyClass
public MyClass //
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The Static Modifier, Container Class

• The static modifier can be applied to variables
  or methods
• It associates a variable or method with the class
  rather than an object
• Methods that are declared as static do not act
  upon any particular object they just encapsulate
  a given task, a given algorithm
• Container class we can write a class that is a
  collection of static methods such a class is not
  meant to define a new type of object, it is just
  used as a library for utilities that are related
  in some way

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Example - a Math Class

• // ---------------------------------
• // A library of mathematical methods
• // ---------------------------------
• public class Math
• // -------------------------------------------
  
• // Computes the trigonometric sine of an
  angle
• // -------------------------------------------
  
• public static double sin(double x)
• // ...
• // ----------------------------------------
• // Computes the logarithm of a given number
• // ----------------------------------------
• public static double log(double x)
• // ...
• // ...

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Use of Static Methods

• When we call a static method we should specify
  the class to which method belongs.
• double x Math.sin(alpha)
• int c Math.max(a,b)
• double y Math.random()
• The main method is static it is invoked by the
  system without creating an object
• If a static method calls another static method of
  the same class we can omit the class-name prefix

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Static Variables

• A variable that is declared static is associated
  with the class itself and not with an instance of
  it
• Static variables are also called class variables
• We use static variables to store information that
  is not associated with a given object, but is
  relevant to the class
• We have already seen such usage - constants of a
  class (final static)

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Static Variables - Example

• public class BankAccount
• private long accountNumber // serial number
• private float balance // balance in dollars
• private static int numberOfAccounts 0
• public BankAccount()
• this.accountNumber numberOfAccounts
• numberOfAccounts
• this.balance 0
• this.owner Smith
• public static int getNumberOfAccounts
• return numberOfAccounts

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Static Methods and Instance Variables

• Static methods
• cannot reference instance variables (i.e.,
  access instance variable when an object doesnt
  exist)
• can reference static variables or local variables
  (within the method)
• this has no meaning inside a static method, thus
  its use inside a static method is not allowed
• Instance methods can access both instance and
  static variables

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Static Variables

• Normally, each object has its own data space
• If a variable is declared as static, only one
  copy of the variable exists
• private static float price
• Memory space for a static variable is created as
  soon as the class in which it is declared is
  loaded
• All objects created from the class share access
  to the static variable
• Changing the value of a static variable in one
  object changes it for all others

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Nested Classes

• In addition to a class containing data and
  methods, it can also contain other classes
• A class declared within another class is called a
  nested class

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Nested Classes

• A nested class has access to the variables and
  methods of the outer class, even if they are
  declared private
• In certain situations this makes the
  implementation of the classes easier because they
  can easily share information
• Furthermore, the nested class can be protected by
  the outer class from external use

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Nested Classes

• A nested class produces a separate bytecode file
• If a nested class called Inside is declared in an
  outer class called Outside, two bytecode files
  will be produced
• Outside.class
• OutsideInside.class
• Nested classes can be declared as static, in
  which case they cannot refer to instance
  variables or methods
• A nonstatic nested class is called an inner class

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Example a nested class
// regular class definition public class Farm
private String country // inner class
definition private class Dog public
void makeSound() if (country.equals("Israe
l")) System.out.println("how-how")
else System.out.println("woof-woof")

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Writing code division into methods

• Complicated tasks or tasks that occur often
  within a class should be wrapped in a method
• This results in a readable and manageable code
• This is very helpful when implementing
  algorithms, or when we need helper methods in
  classes

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Division into Methods - Example

• // Prints all the prime numbers
• public class Primes
• public static void main(String args)
• int number 0
• while (true)
• number number 1
• if (isPrime(number))
• System.out.println(number)

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Division into Methods - Example
// Prints all the prime numbers public class
Primes public static void main(String
args) int number 0 while (true)
number number 1 if
(isPrime(number)) System.out.println(n
umber) // Returns true iff
number is prime public static boolean
isPrime(int number) // determines if
number is prime