CIS3931 Intro to JAVA

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CIS3931 - Intro to JAVA

• Lecture Notes Set 5
• 26-May-05

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Method Overloading

• Enables methods to have the same name
• They must have different parameter lists
• The parameter list allows the compiler/interpreter
  to distinguish between which to use.

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Method Overloading Example

• int MyMethod(int x)
• int MyMethod(double y)
• int MyMethod(int a, double b)
• Since they all have different parameter lists,
  they can all be used in the same class and an
  error will not be raised.

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Is this correct?

• int MyMethod(int x)
• double MyMethod(int a)
• Are these two methods allowed together?

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Math Class

• Very useful. Provides many methods in a pre-built
  class.
• The following are available in the Math class
• Trigonometry functions
• sin, cos, tan, acos, atan, asin

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Exponent Methods

• exp raise e to a power
• sqrt returns the square root
• pow raise a number to a power
• log natural log of a number

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Rounding in Math Class

• ceil round up to nearest integer
• floor round down to nearest integer

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Helpful Math

• random Returns a random number greater than or
  equal to 0.0 and less than 1.0
• abs return absolute value of a number
• min return minimum of two numbers
• max return max of two numbers

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Advice

• The previous were all important, but are only
  half of what the Math class can actually do. Make
  sure you know how to use the methods I mentioned
  here. How to call them, and what they do. I would
  recommend playing around with the Math class and
  experimenting with at least the ones mentioned.
  View the API for the Math class.

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Exception Handling

• What is an exception?
• An exception is an event that occurs during the
  execution of a program that disrupts the normal
  flow of instructions during the execution of a
  program.
• In normal programming, an exception or error
  would cause a program to crash.

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Exception Handling

• JAVA offers methods to handle exceptions
• Exception objects Created by a method when an
  error occurs. This object contains information
  about the error (type, state of program when
  error occurred )
• Creating an exception object throwing an
  exception

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Exception Handling

• When an exception is thrown, the system trying to
  find a way to handle it.
• The system searches through the ordered list of
  methods that have been called prior to the
  exception (call stack)

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Exception Handling

• Block of code than can handle exception
  exception handler
• An exception handler must be able to handle the
  type of exception thrown
• A chose exception handler catches the exception
• If no handler is found, the program will
  terminate.

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Exceptions

• Some reasons an exception may be thrown
• Unable to open a file
• Array is out of range
• Dividing by zero
• Unable to parse a stream

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Exceptions Throw / Catch

• Similar to the raise/handle model in other
  languages
• Exceptions are caused in one of two ways
• Program does something illegal
• Program execute the throw keyword

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Exceptions Throw / Catch

• The following code will crash a program
• public class DivideByZero
• public static void main(String args)
• int i 1, j 0, k
• //Force an error
• k i / j

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Exceptions Throw / Catch

• Result
• Exception in thread main java.lang.ArithmeticExc
  eption / by zero at DivideByZero.main(DivideByZer
  o.java14)

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Exceptions Throw / Catch

• Error message shows that a ArithmeticException
  was thrown by the program
• Program died because there was no exception
  handler available to catch the exception
• Use a try / catch block to handle the exception

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Exceptions Throw / Catch

• Try / Catch Block example
• try
• //Some block of code that you could
  potentially
• //throw an exception
• catch (Exception e)
• //Block of code to handle the exception

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Exceptions Throw / Catch

• When an exception is thrown in the try block,
  control is immediately passed to the associated
  catch block.
• Try blocks must have at least one catch block
  (sometimes multiple are needed).
• Every catch block must have one try block
  associated with it.

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Throw / Catch Advantages

• Error conditions dealt with only where it makes
  sense to deal with them
• Dont have to deal with error at ever level
  between occurrence and where it is handled
• Code can be written as if everything will work
• Separates error handling from the normal flow of
  control
• Reduced program complexity
• Calling functions do not need to error check
  returned values

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Exception handling

• Example program
• Finally statements
• Users can create user defined exceptions
• will go over this next class.

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Scanner

• Good news, Scanner can now be used on program
  since 1.5 was added. This can be used instead of
  Buffered Readers.

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Scanner

• Scanner is contained in java.util.Scanner
• You will have to import that class if you wish to
  use Scanner.
• import java.util.Scanner

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Scanner

• You must first create Scanner to obtain input
  from command window.
• This is done with the following command
• Scanner input new Scanner(System.in)

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Scanner

• Next, you must parse the input into something
  meaningful, as it is now of type Scanner. The
  example below shows how to parse it into an int
• int num1 input.nextInt()

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Scanner Example

• import java.util.Scanner
• //Class and main declarations
• Scanner input new Scanner( System.in )
• System.out.print(Enter digit )
• int num1 input.nextInt()

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Scanner Exceptions

• With Scanner, you do not need to throw any
  Exceptions as you do with Buffered Readers. In
  other words, you do not need the throws Exception
  after declaring main.

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Strings

• You have already played around with Strings
  somewhat
• String input br.readLine()
• The above says to create a String variable called
  input and place the input from the Buffered
  Reader into it.

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Scanner Strings

• To grab a String using Scanner, just use the
  following
• Scanner input new Scanner(System.in)
• String name input.nextLine()

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Strings overview

• A String is an object in Java, it comes from the
  String class
• The String class is part of java.lang and doesnt
  need to be imported since java.lang is imported
  automatically with every java program.

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String Methods

• There are over 50 different methods which can be
  called when you use the String class. Some of the
  more important ones follow.

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charAt()

• This returns the char at the location you
  specify.
• String greet new String(Hello)
• greet.charAt(1) //returns e
• Remember, you must start counting at 0!

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concat()

• This concatenates one string to another string.
• Example
• String st1 Hello
• String st2 World
• String st3 st1.concat(st2)
• //st3 now equals Hello World

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equals()

• Returns a boolean value true or false
• Check to see if 2 strings are equal
• if( st1.equals(st2) )
• System.out.println(Equal)

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equals()

• The equals() method takes case into
  consideration. To ignore case, yet still check
  for equals you should use
• equalsIgnoreCase()

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length()

• Returns the length of the String
• Example
• String st1 Hello
• int Stlen st1.length()
• //Stlen now equals 5

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trim()

• Removes all leading and trailing whitespace
  characters. This does not remove any whitespace
  characters in the middle, just at either end.

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Conversions

• toLowerCase()
• toUpperCase()
• These convert the String to upper/lower case
  letters and ignore all the nonalphabetic
  characters.

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Main

• Recall the following
• public static void main( String args )
• The String args that is in the parameters is an
  array of Strings. This is so the user can enter
  input to start the program.

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Main

• For instance, if the program was started with
• java Project foo bar
• Then args0 foo args1 bar
• args.length 2

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Objects

• Encapsulation of Data along with functions that
  act upon that data
• An object consists of 3 things
• 1. Name which we give it
• 2. Attributes set of data fields and their
  current values
• 3. Methods set of methods that define the
  behavior of the object

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Class

• Blueprint for objects
• Describes and defines objects of the same type
• Contains data and method declarations
• An object is a single instance of a class
• You can create many objects from the same class
  type

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Creating Objects

• Objects are created from a class using the new
  operator.
• They must be attached to a reference variable.

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Example

• Suppose we have a class we built called Square.
  Inside we have methods for computing area and
  circumference.
• Format
• Square MySquare
• My Square new Square()
• Or
• Square MySquare new Square()

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Using the object

• Square MySquare new Square()
• MySquare.side 10
• The above sets side to 10.

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Constructors

• Special member function of a class.
• Purpose is to initialize the members of an
  object.

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How to spot a constructor?

• Its very simple since it must meet the following
  criteria
• 1. It has the same name as the class
• 2. It has no return type.

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Default Constructor

• A constructor without any parameters is the
  default constructor.
• A constructor can have parameters, but no return
  type.
• A constructor is invoked when an object is
  created with new.
• MySquare new Square() //default
• MySquare new Square(2) //also valid

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Visibility

• We can declare members of a class to be public or
  private.
• Public can be accessed inside or outside of the
  class it is in.
• Private can only be used by the object itself.
• You can hide data this way for security, simpler
  interface, etc