Learning Java Part 2

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Learning Java Part 2

• Dorian Miller

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• Hooray it is
• Much needed for drought in NC
• Raining

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Announcements

• Posted on BlackBoard
• Solution to midterm and programs
• Slides on scores/grades
• Java/ Eclipse
• Complete installation by Fridays recitation
• Assistance office hours, email, ITS help
• Program 4 Online store
• JavaScript
• Due Thu Nov 1, 1159pm EST

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Big picture

• Learning a new programming language, Java
• Questions to ask
• Purpose of language
• Variables
• Input/output
• Control structures
• Functions

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Program 4 Online storefront(JavaScript)

• Ecommerce
• Electronic business on the web
• Part of new economy
• Online stores
• Amazon
• Ebay
• New egg
• Brick-and-motor stores online
• Barnes and Nobles

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Online store receipt

• Receipt of items
• Customer selects shopping items
• Click item buttons
• Customer checks out, gets total
• Click total button
• Reset shopping list
• Click clear button

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Program 4 tester

• Previous version
• Text must match exactly
• This version
• 3 lines in green must match. Totals must be to 2
  decimal places
• Total number of lines must match, in example 8
  lines
• Lines in blue will not match, everyone will have
  unique items

• -----------
• Subtotal 92.76
• Tax 6.49
• Total 99.25
• item 4 50.00
• item 3 31.22
• item 2 10.55
• item 1 0.99

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Design requirements

• Design the web page interface
• Store name and description
• List items and price
• Buttons of shopping items must use the same
  function
• Must declare constant arrays (you pick names)
• var PRICE 0.99, 10.55, 31.22, 50.00
• var NAMES "Item 1", "Item 2", "Item 3", "Item
  4"

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Text Area HTML form field

• 2 HTML tag
• Multiple lines of text
• Initial text goes between tags
• Rows and cols sets size of text area

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Text area text

• Text in text area is a string
• Text in receipt is on multiple lines
• Subtotal 92.76
• Tax 6.49
• Total 99.25
• Use special character to create new line
• \n (slash n)
• Example of string
• myString Subtotal 92.76\nTax 6.49\nTotal
  99.25\n
• or
• myString Subtotal 92.76 \n
• myString myString Tax 6.49\n Total
  99.25\n

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Escape characters

• Special characters called escape characters
• \ (slash) indicates start of escape char
• Examples
• \n new line
• \t tab, a larger space
• \\ the \ (slash) character
• \ the quote character

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Input and Output

• Graphical User Interface (GUI)
• Modern interface
• Consists of text fields, buttons, slides, etc
• Input
• Enter text
• Click and drag with mouse
• Output
• Graphical display changes
• Dialog messages

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Text based interface

• Text based interface
• Interface before GUI
• Monitor and keyboard referred to as console
• Example Computer text, User text
• Please enter your login
• Comp110 Console
• Please enter your password
• Enter student name
• Jane Doe

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Why learn text based interface

• Java
• Text based is simplest input/output
• Java capable of GUI
• Experience how computer manages input
• Low level understanding of keyboard
• GUI is based on low level input
• Geeky knowledge to share at cocktail party

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Output

• Output
• Display text on monitor
• Java text output command
• System.out.println(String to display")

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Keyboard Input

• User enters input
• Text
• Other characters (enter key)
• User input
• I typed each character (enter)
• In the last two lines
• Keystrokes stored in buffer
• Array of characters
• I typed each character \n In the last two lines

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Keyboard input in Java program

• Scanner object
• Reads data from the keyboard
• Convention object name starts with capital
  letter. Distinguish from variables and constants
• Step 1 Learn about Scanner documentation
• Search web Java Scanner
• Get description
• Get data and functions

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Step 2 Declare Scanner variable

• Declare variable of type Scanner
• Named console, refers to monitor and keyword
• Syntax
• static Scanner console new Scanner(System.in)
• Scanner is type of object.
• new keyword specify creating new instance of
  object
• System.in specifies input comes from the keyboard
• static keyword explained later

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Scanner variable

• Console
• In this case, should be a global variable
• One object used throughout program to get input
  from keyboard
• public class InputExample
• static Scanner console new Scanner(System.in)
• public static void main(String args)
• // start program

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Step 3 Import Java objects

• Import specifies objects required for program
• Java has 3300 objects
• Loading all would require unnecessary memory
• Import listed at top of program
• Import objects not part of Java lang (System,
  String)
• Package Collection of objects (java.util)
• Package name indicates object grouping
• Example Scanner in util pakage of Java package
• import java.util.Scanner // also could be import
  java.util.
• public class InputExample
• static Scanner console new Scanner(System.in)
  ...

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Step 4 Use the Scanner object

• Read input from the keyboard
• Scanner commands (some)
• console.nextInt() return integer
• console.nextDouble() return double
• console.next() return string
• Notes
• Read input up to the next white-space
• Assigned variable must be same type
• public static void main(String args) // Start
  program
• int myInt
• System.out.println("Enter an integer")
• myInt console.nextInt() // read input from
  keyboard
• System.out.println(Answer myInt)

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Control structures

• Control structures
• If-else statement
• Switch statement
• For loop
• While loop
• We are in luck
• Almost the same as JavaScript!

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Boolean logic, same as JavaScript

• Boolean values
• true, false (keywords, lower case)
• Operators
• and
• or
• ! equal/ not equal
• greater/less than
• Boolean variable
• boolean valid false

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If-else statement

• Same syntax as JavaScript
• if (boolean condition) else
• Use of to group statements
• Same indentation
• Applies to primitive data types
• int, double, boolean, char
• boolean valid false
• if (valid true)
• System.out.println("Yes")
• else
• System.out.println(No")

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Java specific

• Comparing objects
• Possible in JavaScript
• var s1 one string
• var s2 same as other
• If (s1 s2)
• In Java, comparing objects (like a string) is
  different

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Java specific

• In Java, comparing objects (like a string) is
  different
• String s1 one string
• String s2 other string
• Incorrect in Java
• If (s1 s2)
• Compare objects, Use equals function
• true if strings match char for char
• if (s1.equals(s2) true)

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Compare objects

• Use comparison function
• String s1 one string
• String s2 other string
• if (s1.compareTo(s2) 0)
• s1.compareTo(s2) 0 s1 and s2 are equal
• s1.compareTo(s2)
• s1.compareTo(s2) 0 s2 before s1
• See Java String documentation, search web

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For loop

• Same syntax as JavaScript
• int i
• System.out.println(Start)
• for (i1 i
• System.out.println("i " i)
• System.out.println(Blast off)

Output Start i 1 i 2 i 3 i 4 i 5 i 6 i 7 i 8 i
9 i 10 Blast off
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Shortcut For loop

• for loop shortcut (in Java)
• Declare counter variable in for loop
• Scope of counter only valid in for loop
• Easier to manage temporary variable
• // before variable i unknown, out of scope
• for (int i1 i
• // variable i valid in scope of loop
• System.out.println("i " i)
• // after variable i unknown, out of scope

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While loop

• Same syntax as JavaScript
• int j 10
• while (j 0)
• System.out.println("j " j)
• j j-2

Output j 10 j 8 j 6 j 4 j 2 j 0
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Exercise

• Task
• Ask the user to input 5 integers.
• Sum the numbers and display the sum
• Tips
• Use a for loop to count the integers
• Use the Scanner object to get user input
• Use System.out.println to display messages

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Solution

• import java.util.Scanner
• public class InputExample
• static Scanner console new Scanner(System.in)
• public static void main(String args)
• int sum 0 // Start of program
• int x
• for (int i1 i
• System.out.println("enter number " i)
• x console.nextInt()
• sum sum x
• System.out.println("Sum is " sum)

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Using Eclipse

• Run program
• Compile errors
• Mistakes while writing program
• Runtime errors
• Problems while program executes
• Java Files

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Run program

• Run for first time
• Select the class in Package Explorer
• Right click, select Run
• Run again
• Click the run button (arrow, green curcle)

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Compile errors

• All compile errors must be resolved before
  running program
• Errors listed next to X icon (missing semicolon)
• List of errors See Problems tab

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Compile errors

• Common errors
• Missing semicolon
• Variable not declared or misspelled
• Incorrect type assigned
• Compiler errors can be helpful
• Makes sure program is complete before running
• Compiler messages can be confusing

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Runtime errors

• Runtime errors
• Problems while program executes
• Causes program to crash (stop)
• Examples
• Divide by zero, invalid computation
• Scanner.nextInt() crashes when keyboard input is
  not an integer (float or string)

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Runtime error

• Scanner.nextInt() crashes when keyboard input is
  not an integer (float or string)
• Indicates
• line of code where error occurred (line 31)
• Type of error, cryptic but useful
• enter number 1
• 2
• enter number 2
• string
• Exception in thread "main" java.util.InputMismatch
  Exception
• at java.util.Scanner.throwFor(Unknown Source)
• at java.util.Scanner.next(Unknown Source)
• at java.util.Scanner.nextInt(Unknown Source)
• at java.util.Scanner.nextInt(Unknown Source)
• at InputExample.main(InputExample.java31)

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Java Files

• Where is my program
• Java files saved in workspace\project folder
• Need Java files to submit program
• Need Java files to share program with others

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Java Files

• Workspace\project folder
• Right click on project in Package Explorer
• Select Properties
• Copy the location (highlight text and copy)
• Open the folder

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Project properties

• Workspace\Project folder. Example
• C\....\workspace\ProjectName

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Java files

• Overall Project
• C\....\workspace\ProjectName
• Class name ExampleInput
• Java souce code files (text you type)
• C\....\workspace\ProjectName\src
• ExampleInput.java
• Java byte-code, the file Java uses to run the
  program (bin stands for binary)
• C\....\workspace\ProjectName\bin
• ExampleInput.class

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Fridays recitation

• Practice Java
• Input, ouput
• Loops
• Using Eclipse
• Using Java
• Submitting Java program