Chapter 2: Java Fundamentals contd

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Chapter 2 Java Fundamentals contd
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Outline

• 2.1 The Parts of a Java Program
• 2.2 The print and println Methods, and the Java
  Standard Class Library
• 2.3 Variables and Literals
• 2.4 Primitive Data Types
• 2.5 Arithmetic Operators
• 2.6 Combined Assignment Operators
• 2.7 Conversion Between Primitive Types
• 2.8 Creating Named Constants with final
• 2.9 The String Class
• 2.10 Scope
• 2.11 Comments
• 2.12 Programming Style
• 2.13 Reading Keyboard Input
• 2.14 Dialog Boxes
• 2.15 Common Errors to Avoid

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The operator

• Returns the remainder of the division
• Examples
• 45 is 4
• 306 is 0
• 227 is 1
• 3205100 is 5
• 320510 is 5

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Exercise

• Write the following in a Java file
• double amount 137/5
• System.out.println(Amount is amount )
• amount 137.0/5
• System.out.println(Amount is amount )

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Integer Division

• Dividing an integer by an integer gives an
  integer ? the remainder is ignored
• Examples
• 5/4 is 1
• 17/3 is 5

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Operator Precedence

• What is the result of
• Polynomial 123 6/2 -2
• Is it ?
• (12)3 6/(2-2)
• 1(23) (6/2)-2
• (12)3 (6/2)-2

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Precedence Rules

• Always evaluate , / and before and
• Always negate before any calculations
• , / and have same precedence
• and have same precedence
• If equal precedence then evaluate from left to
  right except for negations where we evaluate from
  right to left

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Precedence examples

• Polynomial 123 6/2 2
• Polynomial has the value of 163-28
• Polynomial 1 5 2 // 2
• Polynomial (3) (5) //8

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Grouping with parentheses

• You can use parentheses to force the evaluation
  of a formula
• Examples
• x ( y zz ) instead of xy zz
• x ( y ( z 165 ) 85 ) 65
• Average (a b c ) /3

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The Math class

• value Math.pow( x,y) // now value holds x to
  the power of y
• value Math.sqrt( x) //now value holds the
  square root of x

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Combined Assignment Operators
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Operator Precedence

• What is the result of
• Polynomial 123 6/2 -2
• Is it ?
• (12)3 6/(2-2)
• 1(23) (6/2)-2
• (12)3 (6/2)-2

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Precedence Rules

• Always evaluate , / and before and
• Always negate before any calculations
• , / and have same precedence
• and have same precedence
• If equal precedence then evaluate from left to
  right except for negations where we evaluate from
  right to left

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Precedence examples

• Polynomial 123 6/2 2
• Polynomial has the value of 163-28
• Polynomial 1 5 2 // 2
• Polynomial (3) (5) //8

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Grouping with parentheses

• You can use parentheses to force the evaluation
  of a formula
• Examples
• x ( y zz ) instead of xy zz
• x ( y ( z 165 ) 85 ) 65
• Average (a b c ) /3

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The Math class

• value Math.pow( x,y) // now value holds x to
  the power of y
• value Math.sqrt( x) //now value holds the
  square root of x

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Combined Assignment Operators
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2.7 Conversion between Primitive Data Types

• Before a value is stored in a variable, Java
  checks the Data Type of the value and the
  variable
• If the data types are compatible then Java
  performs the conversion automatically ? No Error
• If the data types are not compatible then Java
  issues an error.

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2.7 Conversion between Primitive Data Types

• A widening conversion is the conversion of a
  small value to a larger one
• A narrowing conversion is the conversion of a
  large value to a smaller one

• double largest
• float
• long
• int
• short
• byte smallest

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Widening conversion

• Example 1
• double x
• int y 10
• x y
• Example 2
• int x
• short y 2
• x y

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Narrowing Conversion

• We have to perform casting i.e. the name of the
  smaller data type is put in parentheses in front
  of the value
• Example
• int number
• double pi 3.14
• number (int) pi

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Cast operator

• Used to convert from one primitive data type to
  another
• Must be used for narrowing conversions

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Example

• int pies 10, people 4
• double piesPerPerson
• piesPerPerson pies /people
• piesPerPerson (double) pies/people
• piesPerPerson pies/(double) people
• piesPerPerson(double)(pies/people)

(double)(10/4) (double)(2) 2.0 because it is
an integer division
10/4 2 because it is an integer division
10.0/4 2.5 because one of the numbers is a
double
10/4.0 2.5 because people is double
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Mixed Integer Operations

• The result of an arithmetic operation that
  involves only byte, short, or int variables is
  always an int even if both variables are of data
  type short or byte
• Example
• short x 5, y 7
• short z xy // this statement gives an error
• short z (short) ( xy ) //correct

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Mixed Integer Operations

• If one of the operators operands is a double
  then the result of the operation is a double
• If one of the operators operands is a float then
  the result of the operation is a float
• If one of the operators operands is a long then
  the result of the operation is a long

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Creating named constants with final

• A named constant is a variable whose value is
  read-only and cannot be changed
• To create a named constant add the word final to
  declaration
• An initialization value is required when
  declaring a constant
• Example
• final double INTEREST_RATE 0.069

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More about named constants

• When naming a constant, the variable name should
  be written in all uppercase characters.
• Math.PI is a constant that holds the value of pi
  ( i.e. 3.14159 )
• Math.PI is already declared and initialized so it
  ready to use. Example
• double area Math.PI radius radius

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The String class

• A String literal is any text enclosed in
  quotations
• A String is the DataType of a variable that can
  store String literals
• Example of a String variable
• String name CS 0007
• System.out.println( name )

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The String class

• To determine how many letters are stored in a
  String variable (name) use name.length()
• Example
• String mycourse CS 0007
• int number mycourse.length()

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String methods

• charAt(index)
• index is an integer and specifies the character
  position in the String
• This method returns the character at the
  specified position
• Example
• char letter
• String myText This is my Text
• letter myText.charAt(8)

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String methods
myText.length returns 15 because there are 15
characters
myText.charAt(8) returns m because m is the
letter at position 8
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String methods

• toLowerCase()
• This method returns a new String that has all of
  the characters of the original String but in
  lowercase
• Example
• String bigName I am BIG!!
• String smallName
• bigName.toLowerCase()
• // now smallName holds i am big!!

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String methods

• toUpperCase()
• Same as toLowerCase() but it converts all the
  characters to uppercase
• Example
• String smallName I am Big!!
• String bigName
• smallName.toUpperCase()
• // now bigName holds I AM BIG!!

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Example

• String message "Java is Great Fun!"
• String upper message.toUpperCase()
• String lower message.toLowerCase()
• char letter message.charAt(2)
• int stringSize message.length()
• System.out.println(message)
• System.out.println(upper)
• System.out.println(lower)
• System.out.println(letter)
• System.out.println(stringSize)

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Scope

• The variable scope is the part of the program
  that has access to it

public class Scope public static void
main(String args) System.out.println(
value) // ERROR! int value 100
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Scope

• public class Scope
• public static void main(String args)
• int number 100
• System.out.println(number)
• int number 200 //ERROR

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Comments

• Java provides three methods for commenting code.

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Programming Style

• Although Java has a strict syntax, whitespace
  characters are ignored by the compiler.
• The Java whitespace characters are
• space
• tab
• newline
• carriage return
• form feed

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Programming Style

• public class Compact public static void
  main(String args)int shares220 double
  averagePrice14.67 System.out.println("There
  were "shares" shares sold at "averagePrice "
  per share.")

Compiles !!!
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Indentation

• Programs should use proper indentation.
• Each block of code should be indented a few
  spaces from its surrounding block.
• Two to four spaces are sufficient

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Programming Style
/ This example is much more readable than
Compact.java. / public class Readable
public static void main(String args)
int shares 220 double averagePrice
14.67 System.out.println("There were "
shares " shares sold
at " averagePrice "
per share.")
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Dialog Boxes

• A dialog box is a small graphical window that
  displays a message to the user or requests input.
• A variety of dialog boxes can be displayed using
  the JOptionPane class.
• Two of the dialog boxes are
• Message Dialog - a dialog box that displays a
  message.
• Input Dialog - a dialog box that prompts the user
  for input.

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Using the import Statement

• The JOptionPane class is not automatically
  available to your Java programs.
• The following statement must be before the
  programs class header
• import javax.swing.JOptionPane
• This statement tells the compiler where to find
  the JOptionPane class. 

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Dialog Boxes
The JOptionPane class provides static methods to
display each type of dialog box.
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Message Dialogs

• JOptionPane.showMessageDialog method is used to
  display a message dialog.
• JOptionPane.showMessageDialog(null, "Hello
  World")
• The second argument is the message that is to be
  displayed.

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Input Dialogs

• An input dialog is a quick and simple way to ask
  the user to enter data.
• The dialog displays a text field, an Ok button
  and a Cancel button.
• If Ok is pressed, the dialog returns the users
  input.
• If Cancel is pressed, the dialog returns null.

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Input Dialogs

• String name
• name JOptionPane.showInputDialog(
• "Enter your name.")
• The argument passed to the method is the message
  to display.
• If the user clicks on the OK button, name
  references the string entered by the user.
• If the user clicks on the Cancel button, name
  references null.

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NamesDialog.java

• import javax.swing.JOptionPane
• public class NamesDialog
• public static void main(String args)
• String firstName // The user's first name
• String middleName // The user's middle
  name
• String lastName // The user's last name
• // Get the user's first name
• firstName
• JOptionPane.showInputDialog("What is "
  
• "your first name?
  ")

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NamesDialog.java

• // Get the user's middle name.
• middleName
• JOptionPane.showInputDialog(
• "What is " "your middle name? ")
• // Get the user's last name.
• lastName
• JOptionPane.showInputDialog("What is " "your
  last name? ")

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Example

• // Display a greeting
• JOptionPane.showMessageDialog(null, "Hello "
  firstName " " middleName " " lastName)
• System.exit(0)

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The System.exit() Method

• A program that uses JOptionPane does not
  automatically stop executing when the end of the
  main method is reached.
• Java generates a thread, which is a process
  running in the computer, when a JOptionPane is
  created.
• If the System.exit method is not called, this
  thread continues to execute.

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The System.exit() Method

• The System.exit method requires an integer
  argument.
• System.exit(0)
• This argument is an exit code that is passed back
  to the operating system.
• This code is usually ignored, however, it can be
  used outside the program
• to indicate whether the program ended
  successfully or as the result of a failure.
• The value 0 traditionally indicates that the
  program ended successfully.

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Converting a String to a Number

• The JOptionPanes showInputDialog method always
  returns the user's input as a String
• String containing a number, such as 127.89, can
  be converted to a numeric data type.

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The Parse Methods

• Parse methods convert strings to numeric data
  types
• They are
• Byte.parseByte
• Integer.parseInt
• Short.parseShort
• Long.parseLong
• Float.parseFloat
• Double.parseDouble

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The Parse Methods- Examples

• byte bVar Byte.parseByte("1")
• int iVar Integer.parseInt("2599")
• short sVar Short.parseShort("10")
• long lVar Long.parseLong("15908")
• float fVar Float.parseFloat("12.3")
• double dVar Double.parseDouble("7945.6")

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PayrollDialog.java

• import javax.swing.JOptionPane
• public class PayrollDialog
• public static void main(String args)
• String inputString // For reading input
• String name // The user's name
• int hours // The number of hours
  worked
• double payRate // The user's hourly
  pay rate
• double grossPay // The user's gross pay

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PayrollDialog.java

• // Get the user's name.
• name JOptionPane.showInputDialog("What is "
  "your
  name? ")
• // Get the hours worked.
• inputString
• JOptionPane.showInputDialog(
• "How many hours
• did you work this week? ")
• // Convert the input to an int.
• hours Integer.parseInt(inputString)

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PayrollDialog.java

• // Get the hourly pay rate.
• inputString
• JOptionPane.showInputDialog("What is
  " your hourly pay rate? ")
• // Convert the input to a double.
• payRate Double.parseDouble(inputString)
  
• // Calculate the gross pay.
• grossPay hours payRate

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PayrollDialog.java

• // Display the results.
• JOptionPane.showMessageDialog(null, "Hello
  " name ". Your gross pay is " grossPay)
• // End the program.
• System.exit(0)