Variables

1
Variables Data Types

• MSTU 4031 Programming I
• Summer 2003

2
Variables

• What is a variable?
• Variable declaration
• type variable_name value
• Ex. int counter 0
• Type - identifies what type of information the
  variable will store.
• Data Types
• int, byte, short, long
• float, double
• char, boolean
• String

3
Number Types

• Number variables hold values not objects
• Primitive Types
• int integers, no fractional part. -2 billion
  to 2 billion
• 1, -4, 0
• byte -128 to 127
• short -32,768 to -32,767
• long like an int but capable of representing
  numbers between 9E-18 to 9E18
• 600000000000000L (600 trillion)
• (append the letter L after the number value)

4
Number Types

• Floating-Point Number Types
• double double precision (15 significant digits)
  4.9E-324 to 1.7E308
• 0.5, -3.11111, 4.3E24, 1E-14
• float single precision (7 significant
  digits)1.4E-45 to 3.4E38

5
Other Types

• char Character - holds a single character
• h
• boolean Holds a value of true or false
• true
• false

6
Other Types

• String - a sequence of characters
• Dewey (quotation marks not part of the string)
• 4 (an object of type String)
• String is a class

7
Assignment Operator

• Assignment - Assigns a value to a variable
• Ex. variable value int x int p x
  2 p 10 x x p (2 2 10) Or x
  12 //the value 12 is assigned to the integer
  variable x
• (evaluated right to left)

8
Incrementing/Decrementing Variables

• variable (increment)
• Same as variable variable 1
• variable-- (decrement)
• Same as variable variable - 1

9
Arithmetic Operators

• is the addition operator
• - is the subtraction operator
• is the multiplication operator
• / is the division operator
• If both arguments are integers, the result is an
  integer. The remainder is discarded
• Ex. 7 / 4 1
• 7.0 / 4 1.75
• is the modulus operator (to get the remainder)
• Ex. 7 4 3

10
Order of operations

• ()
• /
• -
• (evaluated left to right)

11
Initialization

• Setting a variable to a well-defined value when
  it is created
• int x 0
• String y

12
Kinds of Variables

• Instance variables - known by all methods of the
  class that the variable is declared
• Automatically initialized
• Accessibility should always be private
• private int total
• Final variables (constants)
• Values that do not change
• Good to use for variables you do not want to
  re-assign
• accessSpecifier static final typeName
  variableName expression
• private static final double NICKEL_VALUE 0.05
• Local variables - known by the method they are
  are declared.
• Must initialize

13
Conversions of Data Types

• Converting a floating-point value to an int
  (conversion loses information).
• Ex. double price 50.95 int dollars
  (int)price
• Converting a String to an int
• Ex. int count Integer.parseInt(input)(using
  the parseInt method of the Integer class)
• Converting a String to a double
• Ex. double price Double.parseDouble(input)

14
Formatting Numbers

• NumberFormat class in java.text package
• To have numbers printed with two digits after the
  decimal point
• Ex. 4.25
• The getCurrencyInstance method generates currency
  value strings with local currency symbol and
  appropriate number of digits after the decimal
  point.
• Ex.
• NumberFormat formatter NumberFormat.getCurrencyIn
  stance()
• System.out.print(formatter.format(amount)) //see
  Apples.java
• Be sure to import java.text.

15
Math functions

• Methods of the java.lang.Math class operate on
  numbers not objects.
• Math.round(4.5) (returns a long integer)
• Math.sqrt(4)
• Math.pow(x,y)
• See Horstmann pg 95 for more functions