Java Syntax

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Java Syntax
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Introduction

• As mentioned in the chapter, we need a bit of
  'machinery' to display windows and build a GUI
• So we need to start out by building 'toy'
  programs
• Today we discuss the basics of the Java language
• I'll cover this chapter in the precise order that
  the topics are presented
• The authors have really done a great job here

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Comments

• / and / are comment markers
• Comments do not nest
• Comments may span multiple lines
• // is the single line comment marker
• / This is a comment appearing on
• two lines. /
• // And this is a comment on one line.

4
A Word about Packages

• Classes are organized into packages
• Packages are the same as .NET namespaces
• Use the import statement as in
• import java.lang.String.

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Statement Format

• A semicolon terminates a statement
• A physical line means nothing
• Statements can span multiple lines
• There exists no continuation character as in VB
  .NET.

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Variables (Types)

• Java is strongly typed
• EVERY variable must be declared with a type
• There are 8 primitive data types
• Whole numbers
• byte, short, int, long
• Floating point numbers
• float, double
• Character
• char - Note that this is not a string
• Boolean
• true and false

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Whole Number Types

• byte (1 bytes)
• short (2 bytes)
• int (4 bytes)
• long (8 bytes)
• Value has a suffix of "L" for long
• All of these are signed types Java does not
  support unsigned types

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Floating Point Types

• float (4 bytes)
• 6-7 digits of precision
• Suffix constant values with "F"
• double (8 bytes)
• 15 digits of precision
• Suffix constant values with "D"
• These types are not acceptable for financial
  calculations because of rounding errors
• Use the BigDecimal class instead

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Numeric Overflow

• Division by 0 evaluates to constant value
  Double.POSITIVE_INFINITY
• Square root of a negative number produces NaN
  (Not a Number)
• Use double.isNaN(x) to check

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The Character Type (1)

• The data type is char
• This is not the same as a String String is a
  class
• Surround literal values with a single quote
  character
• Java uses Unicode encoding rather than ASCII
  encoding

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The Character Type (2)

• Use C-like escape sequences for non-printable
  characters
• \b - backspace
• \t - tab
• \n - linefeed
• \r - carriage return
• \" - double quote
• \' - single quote
• \\ - backslash

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The Boolean Type

• Represents the values true and false
• Note that these are reserved words
• Unlike in C, integers are not converted to
  boolean values
• Internally, 0 is false
• All other values are true

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Variable Naming

• Variables must begin with a letter and not
  contain special characters
• Underscore ( _ ) character is legal
• CASE SENSITIVE
• All variables must be declared with an explicit
  type
• Suggest using camel-case for variable names
• First word is all lower case
• Capitalize the first character of subsequent
  words

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Declaring Variables

• Note that variables MUST be initialized before
  they are used
• Variable declarations can appear anywhere
• They need not appear at the beginning of a module
• type varname ,varname
• int counter
• float radius 3.8
• boolean isValid true
• char firstLetter a
• int counter1, counter2

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Constants

• The final keyword denotes a constant
• A value can be assigned to a constant only once
• Constant names typically appear in upper case
• Example
• final double PI 3.14

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Operators (Arithmetic)

• (Addition)
• Subtraction
• (Multiplication)
• / (Division)
• If both operands are integers then integer
  division is performed
• Integer division by 0 throws an exception
• Otherwise floating point division is performed
• (Integer remainder)
• Same as Mod function in VB

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Increment and Decrement Operators

• Increment ()
• x // adds 1 to x
• Decrement (--)
• x-- // subtracts 1 from x
• Prefix vs. postfix
• y 2 x
• x is incremented before the multiplication
• y 2 x
• x is incremented after the multiplication

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Operators ( Relational)

• Relational operators return true or false
• (Equality)
• ! (Inequality)
• gt (Greater than)
• lt (Less than)
• gt (Greater than or equal to)
• lt (Less than or equal to)

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Operators (Logical)

• (And)
• (Or)
• Logical operators support short circuiting
• Evaluates from left to right
• Once a sub-expression is deemed to be False,
  evaluation of subsequent sub-expressions is
  terminated
• See table 3-4 on page 49 for operator precedence
  and operator associatively

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Operators (Bitwise)

• Bitwise operators operate on whole number types
  (We don't do much bit fiddling in business
  applications)
• (And)
• (Or)
• (XOr)
• (Not)
• gtgt (Shift right)
• ltlt (Shift left)

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Mathematical Functions

• Provided by the Math class
• Note that these methods are static methods (more
  about this later)
• Math.sin, Math.cos, Math.tan, etcMath.PI,
  Math.E are constants
• Note that Java has no exponent () operator
• Use the pow method of the Math class
• double y pow(x, a)

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Type Conversion (1)

• When evaluating an expression, Java converts
  operands to a common type before evaluating the
  expression
• Conversion is to the least restrictive type

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Type Conversion (2)

• Implicit type conversion is only performed from a
  more restrictive type to a less restrictive type
• See Table 3-1 on page 49
• Use a cast to perform explicit type
  conversionOut of bounds casts will produce
  incorrect results
• Same as CType function in .NET
• Example to convert a double to an int
• Fractional value is truncated rather than
  roundeddouble x 3.1415int nx (int)x

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Operator Precedence

• Refer to Table 3-4 on page 49
• All operators have an order of precedence
• Note that operators have associativity

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Strings Introduction (1)

• String is not a type in Java Its a built-in
  class
• Strings are immutable
• We will talk about string pools and why immutable
  strings are good later
• Character positions are 0-based
• Java performs automatic garbage collection on
  strings
• This is true of all other class instances

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Strings Introduction (2)

• A string is a sequence of characters
• Unlike C, you need not suffix a string with a
  null byte
• () is the string concatenation operator unlike
  () in VB .NET
• Note that the () operator is overloaded in this
  context.
• When concatenation is performed, non-string
  variables are implicitly converted to strings
• Java (like .NET) can convert any type to a string
• Refer to the list on page 53 for additional
  functions

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String Methods (1)

• See list on page 53
• substring extracts a sub string from a string
• First argument contains starting character
  position (0-based)
• Second argument contains the number of characters
• String myName "Michael Ekedahl"
• String s myName.substring(0, 7)

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String Methods (2)

• equals method compares two strings for value
  equality
• equalsIgnoreCase compares for value equality but
  comparison is case insensitive
• Do not use as it tests only for reference
  equality rather than value equality
• Example
• String s1 "Name"
• String s2 "Name"
• Boolean b
• b s1.equals(s2)

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String Methods (3)

• charAt gets the character as a position
• Returns a Char data type
• Position is 0 based
• length gets the length of a string
• The value is 1-based
• Length returns an int
• compareTo compares two strings for equality or
  inequality

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String Examples

• Declare an empty string
• String s
• Declare an initialized string
• String s Hello
• Concatenate a string
• s s World

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Getting Input

• Writing to standard output is easy. Reading from
  standard input is not
• To get by, we show how to create an input dialog
• Import the javax.swing. package
• Display the input dialog as in
• String input JOptionPane.showInputDialog(
  "Greetings")

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Exiting a Program

• By default, Java does not return an exit code to
  the operating system.
• java.lang.System package contains an exit method
  to return a value from a Java program
• System.Exit(3)

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Output Formatting

• java.text package has three method that return a
  standard formatter
• NumberFormat.getNumberInstance()
• To format numbers
• NumberFormat.getCurrencyInstance()
• To format currency values
• NumberFormat.getPercentInstance()
• To format percentage values
• Call the format method to actually perform the
  formatting

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Output Formatting Example

• Format a number as currency
• String input
• JOptionPane.showInputDialog("Greetings")
• NumberFormat formatter
• NumberFormat.getCurrencyInstance()
• double dblInput Double.parseDouble(input)
• System.out.println(
• formatter.format(dblInput))

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Numeric Precision

• Primitive types don't work well for financial
  calculations because of rounding errors
• The java.math package contains two classes to fix
  the problem
• BigInteger works with Integer values
• BigDecimal works with floating point values
• Cannot use arithmetic operators with these types.
  Use methods such as Add, Subtract, Multiply, and
  Divide

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BigDecimal Example

• BigDecimal bd1 BigDecimal.valueOf(1)
• BigDecimal bd2 BigDecimal.valueOf(2)
• BigDecimal result
• result bd1.divide(bd2,
• BigDecimal.ROUND_HALF_UP)
• System.out.println(result)

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Block Scope

• A block contains one or more statements
  surrounded by braces
• Variables declared in a block have a scope of the
  block containing the declaration
• A variable cannot be redefined in an inner block

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Conditional Statements (1)

• Note that the condition MUST be surrounded by
  parenthesis
• Note the if and else are all lower case
• Unlike VB, marks the block of the if
  statement
• if (condition)
• Statements
• else
• Statements

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Conditional Statements (2)

• The else if version has a similar syntax
• if (condition)
• Statements
• else if (condition )
• Statements
• else
• Statements

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Conditional Statements (Example)

• A simple If statement
• if (value gt Math.PI)
• System.out.println(Greater than or )
• else
• System.out.println(Less than)

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Multiple Selection

• The switch statement is a limited form of the if
  statement
• Syntax is the same as in C
• Similar to a VB Select Case statement
• Condition is tested once for multiple alternatives

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Switch Statement Syntax

• switch (expression)
• case value1
• statements
• break
• case value2
• statements
• break
• default
• statements
• break

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Indeterminate Loops

• Use an indeterminate loop when you dont know in
  advance how many times the loops statements will
  execute
• Java supports two forms
• A while loop executes statements while a
  condition is true
• Statements in the loop body may never execute
• A do loop executes statements first and then
  tests the condition
• Statements in the loop body will execute at least
  once

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Indeterminate Loops (while)

• Condition evaluates to a boolean value
• Statements only execute while the condition is
  true
• mark the block of statements
• while (condition)
• statements

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Indeterminate Loops (do)

• Executes statement body before testing the
  condition
• do
• statements
• while (condition)

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Determinate Loops

• Determinate loops are used when the iteration
  count is known in advance
• Commonly used to iterate through the elements of
  an array
• Use whole numbers as floating point errors will
  cause serious problems
• Any for loop can be written as a while loop but
  not the other way around

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Determinate Loops (for)

• Syntax
• for (statement1 expression1 expression2)
• statements
• Statement1 initializes the counter
• Expression1 supplies the terminal condition
• Expression2 updates the counter
• A semicolon separates each segment

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For Loop example

• Print the counting numbers from 1 to 10
• int i
• for (i 1 i lt 10 i)
• System.out.println(i)

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Arrays

• Arrays contain multiple elements of the same type
• Arrays are 0-based
• Unlike C arrays, you cannot perform pointer
  arithmetic
• Java arrays perform bounds checking
• No need to worry about walking over memory
• Compared to VB, use instead of ( )

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Declaring Arrays

• Declare an array variable (note that the
  following does not initialize the array)
• int Alist

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Array Initializers

• Arrays are initialized with the new keyword
• New as the same meaning as it does in .NET
• Note the following initializes the array with 100
  elements having subscripts from 0 to 99
• int AList new int100

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Array Initializers

• Arrays can be initialized using C-like syntax
• Note that new keyword is omitted using this
  syntax
• int Alist 1, 2, 3, 4, 5
• Or
• Alist new int 1, 2, 3, 4, 5
• The preceding syntax can be used to reinitialize
  an existing array

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Copying Arrays (1)

• Again, this works the same way in .NET
• Assignment of an array does not copy the data
  Rather the two variables point to the same array
  (memory)
• int AList 1 ,2 ,3 ,4 , 5
• int BList AList
• AList and BList both reference the same array
  memory

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Copying Arrays (2)

• Call System.arrayCopy to copy array data thereby
  allocating new memory
• System.arraycopy(from, fromIndex, to toIndex,
  count)

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Arrays and Command Line Parameters

• Main accepts an array of string as an argument
• Note that Java does not store the program name as
  the first argument as we would in C
• Print the first command line argument
• Public static void main(String args)
• System.out.println(args0)

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Sorting Arrays

• Call the Sort method of the Arrays class
• Note Java uses a version of the QuickSort
  algorithm
• int Alist 32, 24, 33, 18, 12
• Arrays.Sort(Alist)

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MultiDimensional Arrays

• Add additional subscripts for each dimension
• Declare 2-dimensional array
• int TableArray
• Declare and initialize the array. Note the use of
  nested
• int TableArray 1,2 , 3, 4

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Static Variables

• static keyword declares a variable that is shared
  across classes
• public static final single PI 3.14

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Block Scope

• Braces form a block
• Variables declared in a block have block scope
• Variables can be declared in nested blocks
• Variables in an outer block cannot be redefined
  in an inner block
• C does allow this but .NET does not
• VB 6 did not have block scope at all

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The Help System

• http//java.sun.com/j2se/1.4.2/docs/api/index.html
  

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For next time

• Read chapter 4