Accepting Input from
the Keyboard
A stream is required to accept input from the keyboard. A stream
represents flow of data from one
Place to another place. It is like a water-pipe where water flows.
Like a water-pipe carries water from
One place to another, a stream carries data from one place to
another place .A stream can carry
Data . From keyboard to memory or from memory to printer or from
memory to a file. A stream is
Always required if we want to move data from one place to
another.
Basically, there are two types of streams: input streams and output streams.
Input streams are those
streams which receive or read data coming from some other place. Output streams are those Streams which sender write data
to some other place.
All streams are represented by classes in java.io (input and
output) package. This package
Contains a lot of classes, all of which can be classified into
two basic categories :input streams and
Outputs treams.
Keyboard is
represented by a field, called in System class. When we write System.in, we are
Representing a standard input device, i.e. keyboard ,by default.
System class is found in java.lang
(language) package and has three fields' as shown below. All
these fields represent some type of
stream:
Note: that both System.out and System.err can be used
to represent the monitor rand hence any of
These two can be used to send data to the monitor.
What is the
difference between System.out and System.err?
System.out and System.err both represent the monitor by default
and hence can be used to send
Data or results to the monitor .But System.out is used to
display normal messages and results whereas
System.err is used to display error messages.
System.out.println("A normal message");
System.err.println("An error Message");
To accept data from the keyboard ,eg . System.in , we need to
connect it to an input stream as some
. inputstream is needed to read data.
We will discuss the ways to input a String, an integer, and
a real number. System.in is a pre-defined InputStream used
with the console. Unfortunately this class does not have a method for simple
input. But there is a java.io.BufferedReader class that has a method readLine(). A
buffer is a region in memory where input from the terminal is stored until
needed by the program. If you want to perform buffered input on the System.in
stream you would pass the System.in object into the constructor
An InputStreamReader is a
bridge from byte streams to character streams: It reads bytes and decodes them
into characters using a specified
charset. The charset that it uses may be specified by name or
may be given explicitly, or the platform's default charset may be accepted.
Each invocation of one of an InputStreamReader's read()
methods may cause one or more bytes to be read from the underlying byte-input
stream. To enable the efficient conversion of bytes to characters, more bytes
may be read ahead from the underlying stream than are necessary to satisfy the
current read operation.
BufferedReader
input = new
BufferedReader
(new InputStreamReader (System.in));
Once
we have created a BufferedReader we can use its method readLine() to
read one line of characters at a time from the keyboard and store it as a
String
object.
String
inputString = input.readLine();
Once
we have the input data in a String object we can use the various methods
available
to a String object to manipulate the data. Since data can get lost/corrupted
during
an input operation the method readLine() canalert the user by
throwing
an exception. There are several ways that Java allows
a
user to handle exceptions. For the time being we will add the phrase
"throws
IOException"
and let the system to do the error handling.
Reading
an
integer: One way to read an integer
is to read the input as a String and then use the method parseInt() of
the wrapper class Integer to convert the String to an integer.
int
num = Integer.parseInt ( inputString);
a
real number: There is a wrapper class
java.lang.Double that can take
the
input as a String and a method parseDouble() to convert the
String
to
a real number.
double
d = Double.parseDouble (inputString );
An
Example :
import
java.io.*;
public
class CalcArea
{
public static void main ( String args[] )
throws IOException
{
System.out.print ( "Enter the radius:
" );
BufferedReader input = new BufferedReader (
new InputStreamReader ( System.in ) );
String inputString = input.readLine();
double radius = Double.parseDouble (
inputString );
double area = 3.14159 * radius * radius;
System.out.println ( "Area is: "
+ area );
}
}
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