Introduction to Console Input

1
Introduction to Console Input

• Primitive Type Wrapper Classes
• Converting Strings to Numbers
• System.in Stream
• Wrapping System.in in a Buffered Reader Object
• Reading Strings
• Numeric Input

2
Primitive Type Wrapper Classes

• Java uses primitive types, such as int and char,
  for performance reasons.
• However, there are times when a programmer needs
  to create an object representation
• for one of these primitive types.
• Java provides a Wrapper class for each primitive
  type. All these classes are in
• the java.lang package

• Wrapper classes are used to provide constants
  and general methods for the primitive data
• types.

3
String to Number Conversion

• Each of the Wrapper classes Double, Float,
  Integer, and Long has a method to convert the
• string representation of a number of the
  corresponding primitive type into its numeric
  format

• Examples
• int numStudents Integer.parseInt(500)
• String inputLine 3.5
• double studentGPA Double.parseDouble(inputLine)
  

4
System.in stream

• In Java I/O is handled by streams.
• An input stream is an object that takes data from
  an input source and delivers that data to
• a program.
• An output stream is an object that takes data
  from a program and delivers it to an
• output destination. e.g., System.out
  that corresponds to the monitor
• In Java, console input is usually accomplished by
  reading from the input stream System.in
• of the class java.lang.System
• System.in represents the standard input stream
  (i.e., it corresponds to the keyboard).
• System.in has no methods for reading characters,
  strings, or numbers. It has a read method
• to read a single byte at a time. Java
  uses Unicode in which each character is two
  bytes.

5
Wrapping System.in in a BufferedReader Object

• To be able to read characters, strings, or
  numbers, System.in must be wrapped in other
  objects.
• To turn System.in into a Reader object (i.e., an
  object that is capable of reading one character
  at
• a time), wrap it in an
  InputstreamReader object
•  
• InputStreamReader reader new
  InputStreamReader(System.in)
•  
• To turn the object referenced by reader into an
  object with the ability to read entire lines at a
  time,
• wrap the object in a Buffered Reader
  object
•  
• BufferedReader stdin new
  BufferedReader(reader)
•  
• The steps of turning System.in into a Buffered
  Reader object can be combined into a single
• statement
•  
• BufferedReader stdin
• new BufferedReader(new
  InputStreamReader(System.in))
•  
• Note Both the Buffered Reader class and the
  InputstreamReader class are defined in the
• java.io package.

6
Reading Strings

• A Buffered Reader object contains a readLine
  method that reads one input line at a time, and
• returns that line as a string.
• The reading of input may cause an error. Input
  errors are called lOExceptions.
• A programmer may write code to handle
  lOExceptions or he must declare that lOExceptions
  are
• not handled by a method by using the clause
  throws lOException in the method header.
• Example
•  
• import java.io.
• public class ReadString
• public static void main(String args)throws
  IOException
• BufferedReader stdin new
• BufferedReader(new
  InputStreamReader(System.in))
• System.out.println(Enter a line of text)
• String message stdin.readLine()
• System.out.println(You entered
  message)

7
Numeric Input

• The Java library contains no classes to read
  numbers directly.
• One way of processing numeric input is to read it
  as a string using the readLine method.
• The string is then converted to its corresponding
  numeric value using the parse method of an
  appropriate
• Wrapper class.
• Examples
• String inputLine stdin.readLine()
• int numStudents Integer.parseInt(inputLine)
• double speed Double.parseDouble(stdin.readLine
  ())
• floatHeight Float.parseFloat(stdin.readLine().
  trim())
•  
• import java.io.
• public class ReadIntegers
• public static void main(String args)throws
  IOException
• BufferedReader stdin new
• BufferedReader(new
  InputStreamReader(System.in))
• System.out.println(Enter two integers
  on separate lines)
• int num1 Integer.parseInt(stdin.readLi
  ne())
• int num2 Integer.parseInt(stdin.readLi
  ne())
• System.out.println(Sum (num1
  num2))