COP3252 Advanced Java Programming

1
COP3252Advanced Java Programming

• 15-Jan-08
• Lecture Set 3
• Variables, Math Operators, printf, The Java API,
  Strings, User Input,

2
Variables in Java

• All variables have a name, type, size, and a
  value.
• JAVA is a STRONGLY TYPED language
• You must declare a variables type before it can
  be used.
• JAVA contains 8 primitive variable types
• Primitive predefined by the language and is
  named by a reserved keyword.
• Do not share state (values) with other primitive
  values.

3
Primitive Data Types cont.

• byte 8-bit signed twos compliment integer.
• Min value -128
• Max value 127
• Used when you wish to save memory when using
  small values in large arrays
• short 16-bit signed twos compliment integer.
• Min value -32,768
• Max value 32,767
• Also used to save memory

4
Primitive Data Types cont.

• int 32-bit signed twos compliment integer
• Min value -2,147,483,648
• Max value 2,147,483,647
• Most used variable type. Can handle most
  numerical values in your programs.
• long 64-bit signed twos compliment integer
• Min value -9,223,372,036,854,775,808
• Max value 9,223,372,036,854,775,807
• Used when the integer type is not big enough (low
  level cryptographic functions may use this)

5
Primitive Data Types cont.

• float double-precision 32-bit IEEE 754 floating
  point number (a decimal value)
• Min and Max values beyond the scope of this
  class
• Used to save memory when making large arrays of
  decimal numbers
• NOT SUITABLE FOR PRECISE CALCULATIONS (such as
  currency)
• double double-precision 64-bit IEEE 754
  floating point number (a decimal value)
• Min and Max values beyond the scope of this
  class
• Also should not be used for precise calculations
• Note Currency calculations should be done with
  java.math.BigDecimal (discussed in a later class)

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Primitive Data Types cont.

• boolean One of two possible values true and
  false
• Used for simple flags or tracking conditional
  statements
• Represents one single bit of information (but is
  actually more than one bit in size)
• char 16-bit storage container representing a
  Unicode (computer recognized) character.

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Primitive Data Type Default Values

• When you declare a variable, you dont have to
  assign a value (initialize the variable).
• Example int a double temp
• When a variable is not initialized, the compiler
  will assign it a default value.
• It is bad form to create a variable without
  assigning an initial value.

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Primitive Data Type Default Values cont.
9
Variables Naming Conventions

• Rules and conventions for naming a variable
• Variable names are case sensitive !
• TeSt is not the same as test
• Variable names can have unlimited length
• ..but it is not usually wise to name a variable
  thisIsTheLongestVariableNameICanPossiblyThinkOf
• If it is one word all lowercase
• If it is multiple words capitalize the first
  letter of each subsequent word (
  thisIsAMultiWordVariable)
• Constant values (discussed later) should be in
  ALL CAPS with underscores separating the words
  
• (example final int GRAVITY_VALUE)

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Variables Naming Conventions

• Variable names may only begin with a letter, a
  dollar sign, or an underscore
• but it is considered bad form to start a
  variable name with anything other than a letter
• Variable names CAN NOT start with a number!
• int 123variable is not a valid variable name!

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Variables Naming Conventions

• It must not be one of Javas reserved keywords.
  (See Appendix C of the book for a complete list)
• It must not be a boolean literal true or
  false
• It must not be the reserved word null

12
Variables Example Integers

• One of the first variable types we will use will
  be the primitive integer
• Integers are declared with the keyword int
• Example int aNumber 12
• The example creates an integer called aNumber
  and assigns the value 12 to it.

13
Variables Example Integers

• You can change the value of a variable at any
  time.
• Example
• //Create an integer called number with value 5
• int number 5
• //Change the value of number to 10
• number 10

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Variables Example Integers

• Note After creating the variable, you simple
  refer to it by name (no need to use the type and
  name!)
• Incorrect
• //Create an integer called number with value 5
• int number 5
• //Change the value of number to 10
• int number 10
• Correct
• //Create an integer called number with value 5
• int number 5
• //Change the value of number to 10
• number 10

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Variables Example Integers

• You can create multiple variables of the same
  type on one line.
• Example
• //Create several integers
• //Some will be assigned values, others will not
• int a, b10, c, d, e100

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Variables Example Integers

• You can assign one variable to be the value of
  another variable
• Example
• //Create two integers
• int a 100, b 200
• //Change the value of b to the value of a
• b a
• //The value of b is now 100

17
Printing Variables

• We use the standard System.out statements to
  print the values of our variables to the screen.
• Example
• //Create an int called num and give it the value
  5
• int num 5
• //Print the value of num to the screen
• System.out.println(num)

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Printing Variables

• We can print both text and the value of a
  variable in the same print statement.
• You separate your strings and your variables with
  the plus () sign. This is a shorthand for String
  concatenation.
• You can have any combination of strings and
  variables in a print statement.

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Printing Variables

• //Create an int
• int num 6
• //Print the value with text
• System.out.print(The value of num is num
  .)
• The above code would print the following to the
  screen
• The value of num is 6.

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Math Operators

• The following are a collection of common
  arithmetic operators
• Addition
• Subtraction -
• Multiplication
• Division /
• Remainder (mod division)

21
Math and Variables

• You can perform math on all numerical primitive
  variable types (int, long, double, etc.)
• You can use parenthesis to separate operations
  and determine order of precedence.

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Math Examples

• //Create some ints
• int a 5, b 5
• //create another int that is a b
• int c a b
• //Add 10 more to c
• c c 10
• //The value of c at the end of execution is 20.

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Math Operators - Examples

• See VariableAndMathExamples.java for examples of
  creating and modifying variables, printing
  variables, and arithmetic operations.

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Alternative Print Statements

• As of release 1.5, Java includes the classic
  printf style of statement
• The printf statement is Java is used exactly as
  it is used in the C programming language.
• All examples in class will use print and println
  you will rarely see printf used in this class.
• However, the book tends to make use of it
• If you would like to learn about Javas printf
  statement, see
• Page 45 of the 6th edition book, and chapter 28
• Page 48 of the 7th edition book, and chapter 29

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Formatting with printf(6th chap 28, 7th chap
29)

• Some format specifiers for the printf command
  (all preceded by a sign)
• (others will be discussed on Thursday)

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The JAVA API

• Many things you may want to do in your program
  have already been written for you
• API Application Programming Interface
• Used to describe how computer applications and
  software developers can access sets of
  pre-written functions without requiring access to
  the source code of the functions or the library
  (or without requiring a detailed understanding of
  the functions internal workings).
• Reference http//en.wikipedia.org/wiki/Applicati
  on_programming_interface

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The JAVA API Cont.

• The primary JAVA API is maintained by Sun
  MicroSystems
• There are APIs for all JAVA versions (J2SE, J2EE,
  J2ME)
• Standard edition, enterprise edition, and mobile
  edition
• We are learning the Standard Edition, so you will
  use the following API
• http//java.sun.com/j2se/1.5.0/docs/api/

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The JAVA API Cont.

• The JAVA API will be used extensively in this
  class
• Not everything on the API may make sense at this
  point in the class but you should be able to
  use every available class in the API by the end
  of this course.

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The String Variable

• Used to represent strings of characters
• Strings are objects created from the String class
  (they are not a primitive variable type).
• Since strings are a commonly used variable type,
  JAVA has implemented some ways to make working
  with Strings a little more like working with
  primitives

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Creating a String

• Strings are delimited by a set of double quotes.
• Two ways to create a basic String
• //The primitive way
• String aString This is a string
• //The object or constructor way
• String bString new String(This is a string)

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Creating a String

• The String API http//java.sun.com/j2se/1.5.0/do
  cs/api/java/lang/String.html
• Contains the constructors, methods, etc. for the
  object type String

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Using the String Constructors

• //Create a new string with a NULL value
• String aString new String()
• //Create a new string and give it a value
• String bString new String(A String!)
• //Create a string from the previous string
• String cString new String(bString)

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String Example

• See StringsExample.java

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String Methods (Functions)

• The available String methods can be found in the
  API
• String has built in methods for concatenation,
  case switching, find the length, pulling out a
  specific character, etc
• These will be discussed in a later class

35
User Input

• User input allows for interactive programs
• Most programs we will write will include at least
  some section requiring user input
• Input from a command line is simple in most
  languages
• scanf (C programming)
• cin ltlt (C programming)

36
User Input

• Slightly more difficult in JAVA
• Two classes we can use to get user input from the
  command line
• BufferedReader
• Older method but the one I will use most often
  in demonstrations
• Scanner
• Newer method slightly easier to work with

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BufferedReader

• Link to API http//java.sun.com/j2se/1.5.0/docs/
  api/java/io/BufferedReader.html
• Reads text from a character-input stream (such as
  the command line) and buffers characters as to
  provide for efficient reading.

38
User Input

• Using BufferedReaders
• //Create a BufferedReader named br
• BufferedReader br new BufferedReader(new
  InputStreamReader(System.in))
• //Read in a single line from the user and store
  it
• String input br.readLine()
• //Read in another line from the user and store it
• //Note Dont need to declare the String type
  again
• input br.readLine()

39
Importing Classes

• BufferedReader is part of the java.io. package
  (see API)
• JAVA does not automatically load all class
  libraries on its own
• We have to import this class into our programs.
• Import statements must be the very first line of
  actual code in any program we write (put it
  anywhere else and it will not work)
• You can have comments before the statement ----
  but the import statement must be at the top of
  your .java file before any class or method
  declaration.

40
Importing

• We generally import all classes in a package
• Without the import statement, we would not be
  able to use the classes that we find in the Java
  API.
• To import java.ios classes (which includes
  BufferedReader) we would type
• import java.io.

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Buffered Reader Exceptions

• All objects that deal with input and output have
  the ability to throw an exception ( cause an
  error)
• When dealing with BufferedReader, we have to tell
  JAVA that our main program has the ability to
  throw an exception
• This is done by adding throws Exception to the
  end of the main method declaration
• public static void main (String args) throws
  Exception
• We must do this in order for the code to properly
  compile when using the BufferedReader
• We will talk much much more about exceptions in a
  later class --- for now we just need to know that
  the throws Exception statement must be present
  when working with BufferedReader.

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Example

• Using a BufferedReader for user input
• Importing the java.io class and subclasses
• Exception Throwing (will be discussed fully later
  in the semester)
• See UserInput.java example

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BufferedReader Converting Input

• The BufferedReader only reads in input as a
  string
• Even if the user types 12345 it is still
  stored as a string
• It is often necessary to convert the string to
  other variable types (such as an integer)
• There are functions available in JAVA to convert
  variable types.

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Converting a String to an int

• A common conversion is from String to a primitive
  integer.
• We use the Integer class (object form of the
  primitive int) to convert the string.
• See Integer in API http//java.sun.com/j2se/1.5.
  0/docs/api/java/lang/Integer.html

45
Converting a String to an int

• We use the parseInt function of Integer.
• This function takes in a String and returns an
  int.
• Example
• String numbers 123456
• int a Integer.parseInt(numbers)

46
Scanner

• Alternative to BufferedReader
• Available as of JAVA 1.5 (not in JAVA 1.4.2 or
  earlier)
• API Link http//java.sun.com/j2se/1.5.0/docs/api
  /java/util/Scanner.html

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Scanner

• Scanner is contained in java.util.Scanner
• You will have to import that class if you wish to
  use Scanner.
• import java.util.Scanner

48
Scanner

• You must first create a Scanner to obtain input
  from command window.
• This is done with the following command
• //Create a scanner object
• Scanner input new Scanner(System.in)

49
Scanner

• Next, you must tell the scanner to read in from
  the command line. The following line of code
  tells the scanner that it should read from the
  command line and parse it as an integer.
• int num1 input.nextInt()

50
Scanner Example

• import java.util.Scanner
• //Class and main declarations
• //Create the scanner
• Scanner input new Scanner( System.in )
• //Prompt user for input
• System.out.print(Enter digit )
• //Read input as an integer
• int num1 input.nextInt()

51
Scanner Exceptions

• With Scanner, you do not need to throw any
  Exceptions as you do with Buffered Readers. In
  other words, you do not need the throws
  Exception after declaring main.
• Scanner does still throw exceptions (discussed
  later)

52
Scanner and Strings

• You have already see how to read in a String
  using the BufferedReader
• String input br.readLine()
• The above says to create a String variable called
  input and place the input from the Buffered
  Reader into it.

53
Scanner Strings

• To grab a String using Scanner, just use the
  following
• Scanner input new Scanner(System.in)
• String name input.nextLine()

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Scanner Example

• See ScannerExample.java

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On Thursday

• Shortcut operators
• Precedence
• prefix and postfix
• The Math Class
• Type conversions and casting
• Formatting with printf
• Control Structures
• Introduction to classes and objects