Declarations, Variables, and Java Arithmatic

1
Declarations, Variables, and Java Arithmatic

• Homework Solutions
• 1) Object-Oriented Programming 7) 21
• 2) C 8) 63
• 3) i) the programmer 9) CPU
• ii) the compiler 10) System.out.println(Hell
  o World!)
• iii) the JVM (Java Virtual Machine) 11)
  Classes
• 4) 8 12) Java will ignore them they are
• 5) 11002 comments for the human reader.
• 6) FE
• Concepts from last nights reading
• Whitespace code layout
• Classes
• Strings in quotes
• System.out.println() does some calculations
• Compiler errors vs. logic errors

2
Rules for Variable Names

• Compiler Rules
• only letters, numbers, and underscores.
• first character may not be a number
• cannot use Java keywords
• Accepted conventions
• first character is lowercase char. (Because
  class names are capitalized.)
• Use camel-case (accountBalanceBeforeInterest)
• Good practice
• self-descriptive name
• not too long

3
Bad Variable Names

• Illegal variable names
• myMoms
• larry_at_yahoo.com
• 3brothers
• boolean
• Frowned upon names
• AdamsAccount
• ssgheuf
• x (depends on context)
• thisIsMyVariableThereAreManyLikeItButThisOneIsMine

4
Good Variable Names

• acctBalance
• xPosition
• yVel
• temp
• row
• col
• balBefInterest

5
Declaring variables

• All variables have a type
• All types have default values
• Variables are containers
• Declarations
• int myNum //0
• double myDecimal //0.0
• boolean finished //false
• char letter //ignore
• MyCoolObject obj1 //null
• Assignments
• myNum -5
• myDecimal 5.449
• finished true
• letter j
• obj1 obj2

6
Initialize the variable!

• Usually you declare and initialize together
• int myNum 89
• double myDecimal -78.22
• boolean finished false
• char letter h
• MyCoolObject obj1 obj2

7
Variable Math

• Result type depends on component types.
• Java will preserve precision.
• 3 4.0 //will be a double
• 4.0 3 //will be a double
• 3 2 //will be an int
• 2.0 4.0 //will be a double

8
Casting -- Changing Variable Types

• int a 8
• double b 2.0
• double x a b
• int y (int)(ab)
• int z a (int)b

9
Java Operators

• Mathematical Operators
• -
• Assignment operators
• - /
• Unary operators
• --

10
Example Problem

• What is the output of the following code?
• int x 4
• double y 5.3
• x y
• y
• x-y
• System.out.println(x)
• OUTPUT 2

11
Example Problem

• What is the output of the following code?
• int x 2
• double y 3.1
• x y
• x - y
• x y
• x / y
• x (int)y
• System.out.println(x)
• OUTPUT 0