Scanner class for input

1
Scanner class for input

• Instantiate a new scanner objectScanner in new
  Scanner(System.in)
• Getting input using scanner
• int i scanner.nextInt()
• double d scanner.nextDouble()
• String s scanner.nextLine()
• Differences from IO.read methods
• There is no popup window
• You just enter by typing at the bottom
• You need to prompt the user what to enter

2
Command line Input

• Using command line from Jgrasp
• Click on build, then on run arguments
• Enter the arguments that you want separated by
  spaces
• Run the program and the program will use those
  arguments
• Using the real command line
• In Windows, click on start, then on run, then
  type cmd and click OK. You will be able to type
  Windows commands
• Type java ltname of your programgt ltarguments
  separated with spacesgt
• Your program will run and use those command line
  arguments

3
Command Line Example

• In Jgrasp, click on build, and then on run
  arguments, and enter 1 2 3 4 5 into the text box
  at the top
• Enter the following program
• public class CommandLine
• public static void main(String args)
  int sum 0 for (int i0
  iltargs.length i) sum
  Integer.parseInt(argsi)
  System.out.println("Sum " sum)
• Compile and run, the output should be sum 15

Note parseInt is a static method that converts a
string to an integer
4
Multiple Classes
So far our applications had only a single class

• Some applications can have hundreds of classes
  that work together to solve a problem
• Each class is entered as a separate .java source
  file.
• Compiling the main program will also compile all
  the classes it needs.
• Design Challenge How do we define an appropriate
  number of classes to create a structure for an
  application. This is a skill you learn in other
  Computer Science classes
• Example Creating a phone book (Following slides)
• Class for a person
• Class for a list of persons
• Main class

5
The Person class

• public class Person
• private String name, number // Instance
  Variables
• public Person(String name, String number) //
  Constructor
• this.name name
• this.number number
• public String toString() // Return nicely
  formatted string
• String spaces
  
• String s (name spaces).substring(0,spaces.le
  ngth())
• String t (number spaces).substring(0,spaces.
  length())
• return s t
• // End of toString method
• // End of Person class

Note We dont want outside class methods
accessing the instance variables, hence
private Note The toString() method is handy as
you will see Note The strings s and t mean that
all Person outputs will be formatted the same way
6
Class for list of Persons

• public class PersonList
• private Person persons
• private int howMany
• public PersonList(int size) personsnew
  Personsize howMany0
• public boolean addPerson(String person, String
  number)
• if (howMany persons.length) return false
• else personshowMany new Person(person,
  number)
• return true
• public String toString()
• String str Phone Book Listing\n
• for (int i0 ilthowMany i) str
  personsi .toString() \n
• return str

7
Main class
Input a group of persons, and then print out the
list

• public class Main
• private static PersonList personList
• public static void main(String args)
• String phone, name
• Scanner in new Scanner(System.in)
• personList new PersonList(10)
• do
• System.out.print(Enter name )
• name in.nextString()
• System.out.print(Enter number )
• phone in.nextString()
• while (personList.addPerson(name, phone))
• System.out.println(personList)

8
Final Thoughts

• Using multiple classes makes the application more
  general
• The Person class can be used by other programs
• The details of the Person class is internal.
  Users only have to know how to call its methods
• We did this from day one without knowing
  itExamples System.out.println(), Math.round(),
  and IO.readString()
• The private modifier is important
• Normally make instance variables visible only
  where they are declared
• This makes for easier modifications to an
  application
• It enables the principle of limiting scope
• The purpose of a constructor is to initialize an
  object
• General Rule Use the static modifier only when
  necessary