Reading input from the console input

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Reading input from the console input
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Java's console input

• The console is the terminal window that is
  running the Java program
• I.e., that's the terminal window where you type
  in the command java ProgramName

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Java's console input

• When a Java program starts running, the Java
  runtime system will initialize many variables in
  support for the running program.
• One of these variables is the Java system
  variable
• which represents the console input
• The variable System.in is included in every Java
  program (you don't need to define it).

System.in
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Java's console input

• A Java program can obtains inputs from the
  console through the keyboard
• In other words

• The Java system variable System.in represents
  the keyboard

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A note on the notation "System.in"

• At this moment in the course, we want to learn
  how to read input from the keyboard
• All you need to know is
• It is too early in the course to explain the
  notation System.in
• We will explain this after we have covered
  classes

• The variable named System.in represents the
  keyboard  

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Java's Scanner library functions

• Fact
• The details of what the computer must do to read
  in a number will be discussed in CS255
• The Java programming language provides a
  collection of methods stored in the Scanner class
  that perform read operations      
• (Remember that a class is a container for
  methods)

• There is a lot of work that the computer must do
  to read in a floating point number

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Java's Scanner library functions (cont.)

• Webpage of the Java documentation on Scanner
  class http//download.oracle.com/javase/6/docs/ap
  i/java/util/Scanner.html

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Java's Scanner library functions (cont.)

• We will now learn how to use the methods in the
  Scanner class to read in floating point numbers

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Importing the Scanner class definition

• Recall the Rule of usage of methods in the Java
  library (See http//mathcs.emory.edu/cheung/Cou
  rses/170/Syllabus/04/java-lib.html)

• If a Java program wants to use a method in the
  Java library, the Java program must first import
  the containing class
• All classes in the java.lang package have
  already been imported into a Java program
• (You can use methods in these classes without the
  import clause)

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Importing the Scanner class definition (cont.)

• We can use the following import clause to import
  the Scanner class

import java.util.Scanner
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Preparation before we can read input from the
keyboard

• Before a Java program can read input from the
  keyboard, the program must " construct a Scanner
  object
• It is too early to explain what this means... I
  will only tell you
• how to do it

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Preparation before we can read input from the
keyboard (cont.)

• A Scanner object is constructed using the
  following statement
• The name varName is an identifier
• Example constructing a Scanner object named in

Scanner varName new Scanner(System.in)
Scanner in new Scanner(System.in)
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Reading in a floating point number from the
keyboard

• After having constructed the Scanner object named
  in, you can use the following expression to read
  a floating point number from the keyboard
• You must save (store) the number read in by
  "in.nextDouble()" in a variable with an
  assignment statement

in.nextDouble()
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Reading in a floating point number from the
keyboard (cont.)

• What happens inside the computer

• Just like Math.sqrt(..), the method call
  in.nextDouble() will invoke (run) a method in
  Java's library.
• The task performed by in.nextDouble() is to read
  a floating point number from the keyboard

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Reading in a floating point number from the
keyboard (cont.)

• If you type in "3.5" on the keyboard at the time
  that in.nextDouble() is running, then the call
  will return the value 3.5
• The return value will replace the method call
• The input value 3.5 is then stored in the
  variable a !!!

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Summary steps to read in a floating point number

• This figure summarizes the programming steps to
  read in a floating point number

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Example reading input for the a,b,c-formula

• Programming Example ABC formula

import java.util.Scanner // Import Scanner class
(contains methods // for reading keyboard
input)   public class Abc2 public static
void main(String args) double a, b, c, x1,
x2 // Define 5 variable   Scanner in new
Scanner(System.in) // Construct a Scanner object
  a in.nextDouble() // Read in next number
and store in a b in.nextDouble() // Read in
next number and store in b c in.nextDouble()
// Read in next number and store in c
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Reading in a floating point number from the
keyboard (cont.)
x1 ( -b - Math.sqrt( bb - 4ac ) ) / (2a)
x2 ( -b Math.sqrt( bb - 4ac ) ) / (2a)
  System.out.print("a ") System.out.println(a
) System.out.print("b ") System.out.println(
b) System.out.print("c ") System.out.println
(c) System.out.print("x1 ")
System.out.println(x1) System.out.print("x2
") System.out.println(x2)
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Reading in a floating point number from the
keyboard (cont.)

• Example Program (Demo above code)    
• Prog file http//mathcs.emory.edu/cheung/Courses
  /170/Syllabus/04/Progs/Abc2.java 
• How to run the program

• Right click on link and save in a scratch
  directory
• To compile   javac Abc2.java
• To run          java Abc2

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Good programming practice Prompting user for
input

• The previous program works, but requires the
  users to know exactly what to do
• In other words

• An unaware user may not know that he/she needs
  to enter some input before the program can
  perform its task.

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Good programming practice Prompting user for
input (cont.)

• Good programming courtesy

• When the program needs the user to enter input
  from the keyboard, it must print out a (short)
  prompt message

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Good programming practice Prompting user for
input (cont.)

• Example

import java.util.Scanner // Import Scanner
class (contains methods // for reading
keyboard input) public class Abc2
public static void main(String args)
double a, b, c, x1, x2 // Define 5
variable Scanner in new
Scanner(System.in) // Construct a Scanner
object
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Good programming practice Prompting user for
input (cont.)
System.out.print("Enter a ") //
Prompt message a in.nextDouble() // Read
in next number and store in a System.out.print("E
nter b ") b in.nextDouble() // Read in
next number and store in b System.out.print("Ente
r c ") c in.nextDouble() // Read in
next number and store in c x1 ( -b -
Math.sqrt( bb - 4ac ) ) / (2a) x2 ( -b
Math.sqrt( bb - 4ac ) ) / (2a)
System.out.print("a ") System.out.println(a
) System.out.print("b ")
System.out.println(b) System.out.print("c
") System.out.println(c)
System.out.print("x1 ") System.out.println(
x1) System.out.print("x2 ")
System.out.println(x2)
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Reading other types of input from the keyboard

• The procedure to read other types of inputs from
  the keyboard is similar to the one above

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Reading other types of input from the keyboard
(cont.)

• The only different is that we need to use a
  different method in the Scanner class that read
  the correct type of data.

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Reading other types of input from the keyboard
(cont.)

• Reading an integer number from the keyboard use
  nextInt()

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Reading other types of input from the keyboard
(cont.)

• Note you also need to use an int typed variable
  to store an integer value !!!