Outline for today 12199

1
Outline for today (1/21/99)

• Recap and todays buzzwords
• Storage bins in programs - variables
• video Guy Steele on Extending the Java
  programming langauge
• Data types
• Input and output
• Simple arithmetic and operators
• Syntax summary

2
Recap
3
Todays buzzwords

• Variables
• storage bins in programs - can usually only
  hold one thing
• Memory location/address
• a number referring to the place in memory in
  which a variable is stored
• Data types
• A variable needs to have a type, like a number, a
  character
• Arithmetic Operators
• A symbol representing arithmetic operations on
  values (e.g., - / )
• Operator precedence
• In an arithmetic expression, a rule that governs
  which operators will be applied first.
• Assignment operator
• An operator that takes one value and copies it
  into another

4
Variables

• Storage areas for keeping values for later use
• Each variable is unique, and must have a unique
  name (no two variables can have the same name for
  now)
• Each variable (for now) can only have one value
  in it
• You can change the value in a variable at will
• (thats why it is a variable - it varies.)

5
Variable names

• Must start with a alphabet
• Can have any number of characters and numbers and
  the underscore _
• Spaces and punctuations are not permitted
• Which of the following are valid names?
• abcd Hello_world
• 1234 hello,world
• 3abCd5 Hello world

6
Naming tips

• Never use single-character names like x, y, i
  although they are common in arithmetic
• Use descriptive names like count, index, price,
  amount
• If necessary use multi-word names but ensure the
  words can be read easily, e.g.,
• priceOfBook, amount_of_loan

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Lets watch a video!

• Guy Steele, Sun Microsystems
• One of the people in Sun as part of the
• Java Development Team
• Extending the Java Programming Language

8
Data types

• Every variable must have a unique type
• the range of values of a variable depends on its
  type
• Variables retain the same type during the life of
  the program.
• Type names are reserved words in Java and can not
  be used for anything else

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Basic Data Types in Java

• Number types - can have numeric values
• int integer values e.g., 10 35598
• long, short, byte (different sizes of integers)
• float values with decimal points e.g., 45.768
  (about 7 digits)
• double decimal values upto about 13 decimal
  places
• Character types - any ASCII character
• char character data e.g., 'a' 'b' 'c' 'd'
• Logical types - only logical values (true, false)
• boolean

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Giving types to variables

• Each variable must be declared.
• Variable declaration specifies the type and name
  of a variable. e.g.
• float price
• int numberOfItems
• boolean paid
• You can declare multiple variables in one
  statement
• double loanAmount, interestPaid

11
Getting values in variables

• Initialization (assign value when declaring)
• int numberOfItems 25
• boolean finished false
• Assignment (assign value later)
• float amountOfLoan
• amountOfLoan 35000.00
• The is called the assignment operator.

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More on Assignment Operator

• The assignment operator calculates the right
  side, and puts the result in the variable in the
  left side
• Examples
• totalScore lab1Score lab2Score
• averagePoints totalPoints / numberOfGames
• total total itemPrice / How? /
• Added slide on conversions

13
Implicit and Explicit Conversions

• Sometimes data is converted implicitly (you may
  not realize that there is a type conversion)
• double totalAmount 100
• Often data needs to be converted explicitly (you
  say how you want data to be converted)
• int totalAmount (int)someDoubleVariable

14
Input and Output

• Getting input from the user is usually hard in
  Java, because you need to consider many possible
  problems beforehand
• In this class we use a library instead (the ccj
  library from the book)
• To use this library, you put the following line
  at the top of your code
• import ccj.

15
Producing output

• We already know how to output simple text
• System.out.println("Hello, A201 folks!\n")
• To output values of variables, you can use
• System.out.println(averageScore)
• To mix and match
• System.out.println("The class average is "
  averageScore)

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Getting input

• With the ccj library, its easy to get input.
• To read an integer, you use the method
  Console.in.readInt(). e.g.
• System.out.println(Enter total number of
  students )
• int noStudents Console.in.readInt()
• Similar for reading doubles.

17
Arithment Operations in Java

• Addition (), Subtraction (-), Multiplication
  (), Division (/)
• Individual operations can be blocked using
  parentheses
• Order of evaluation is left to right with the
  following precedence
• ( ) have the highest precedence, innermost first
• and / have the same precedence
• and - have the same precedence lower than and
  /

18
Try some!

• What do the following statements do?
• int result 7 - 3 5
• int result (7- 3) 5
• int result 5 (5 - (5 5))
• int result (5 5) - 5 5
• int result 10 / 4
• float result 10 / 4
• float result 10.0 / 4

19
Syntax summary

• Variable names
• start with a letter, only letters and numbers and
  _
• Types of variables
• int, short, long, byte, float, double, char,
  boolean
• Variable declaration and initialization
• lttypegt ltnamegt ltvaluegt
• You have to declare a variable before using it.
• Importing a library
• import ccj.