Constants

1
Constants

• public class Car
• //This class produces cars with 18 MPG
• private String color
• private String make
• private String model
• private double gas
• public Car (String col, String mk, String
  mod, double galofgas)
• ..
• public void moveForward(double miles)
• gas gas - miles/18.0
• public void moveBackward(double miles)
• gas gas - miles/18.0

2

• public class Car
• private final double MPG 18.0 //value
  cannot change
• 
  //adds readablility to code
• private String make
• private String model
• private double gas
• public Car (String col, String mk, String
  mod, double galofgas)
• ..
• public void moveForward(double miles)
• gas gas - miles/MPG
• public void moveBackward(double miles)
• gas gas - miles/MPG

3
Static Methods

• If a method is declared as static, it may be
  called without using an object of the class.
• For example we called all Car class methods in
  the form
• CarObject.methodNam
  e
• because none of these
  methods were static
• If a method is declared as static, it may be
  called without an object
• ClassName.methodName
  
• Example
• Math.sqrt(4)
• The sqrt method is a static method defined in the
  Math class.

4
Static Variables

• If an instance variable is declared as static, it
  is shared by ALL OBJECTS of the class.
• (eg. Each object created from that class DOES
  NOT have its own storage location allocated for
  the variable all the objects use the one
  variable location.)
• When do you use a static instance variable??
• when information needs to be shared between
  objects
• and a method calls does not make design
  sense
• class constants should be static. A constant
  memory location cannot be changed, so it makes
  sense to just allocate one

5
Class Constants should be static

•  In a method final typeName variableName
  expression
• In a class accessSpecifier static final
  typeName variableName expression
• Why??
• Because, each object of class does not need
  its own copy of a constant
• (each Car object does not need its own memory
  location to store MPG)

6
Simple example

•  public class Player
• private static int topScore 0
• private int myScore 0
• private static final int winScore 21
• public Player() .
• public void gotPoint ()
• myScore myScore 1
• if (myScore gt topScore)
• topScore myScore
• public static boolean winner( )
• return (topScore gt winScore)

7

•  public class Game
• public static void main (String args)
• Player player1, player2, player3
• player1 new Player ()
• player2 new Player ()
• player2 new Player ()
• while ( Player.winner() false)
• //code to play game

8
Arithmetic Operators

• , - , are used for addition, subtraction and
  
• multiplication
  
• / is the division operator
• If both arguments are integers, the result is an
  integer. The remainder is discarded
• For example
• int val1 7
• double value 7.0
• value value / 4 //assigns 1.75 val1
  val1 / 4 //assignes 1
• value val1 / 4 //assigns 1.0

9
Arithmetic Operators

• Get the remainder with (pronounced "modulo")
• For example
• int val1 7
• val1 val1 / 4 //assigns 1
• val1 val1 4 //assigns 3

10
Mathematical Functions
The Math class is a class which contains static
methods which perform mathmatical functions.
11
Type Conversion

• Java will only allow an assignment if NO VALUE
  will be lost and types are compatible
• double total "a lot" //not compatible
• int ww 5.67 //value would be
  lost
• Use cast to force conversion from one type to
  another (if it is possible)
• int ww (int) 5.67 //discards fractional
  part
• int ww Math.round(ww) //rounds to 6.0, then
• 
  //truncates to 6

12
Reading Input

• The simplest (and prettiest) way to read input
  into a Java program is to use a method provided
  by the JOptionPane class (java.lang package).
  The call
• JOptionPane.showInputDialog(prompt)
• causes a dialog box to be displayed with the
  prompt and a textbox for the user to type his/her
  response.
• Whatever the user types into the box is
  returned to the program (in String form).
• Note showInputDialog is a static method of the
  JOptionPane class

13
An Input Dialog
14

• int val
• String in
• in JOptionPane.showInputDialog(Enter a
  positive number)
• Suppose we want to add the user input to the
  value 45.
• we cannot say
• val in 45
• because in is NOT of a numeric type (such as
  int or double).
• We must convert our string to type int. The
  Integer class provides a static method called
  parseInt which accepts a String, and returns its
  int equivalent.
• int size Integer.parseInt(in)
• val size 45

15

• When using Integer.parseInt (or
  Double.parseDouble)
• Conversion throws an exception if user doesn't
  supply a number
• (ie. Program will crash if user provides
  non numerical input)
• JOptionPane also provides a method for output
• JOptionPane.showMessageDialog(null,output
  string)
• AddSystem.exit(0)to the end of any main method
  of any program that uses JOptionPane method