The java.io package contains all the classes required for input and output operations. Java uses the concept of stream to make I/O operation fast.

Stream:  It's called a stream because it's like a stream of water that continues to flow. A stream is a sequence of data. In Java a stream is composed of bytes. In java, 3 streams are created for us automatically.

All these streams are attached with console.

  1) System.out: standard output stream

 2) System.in: standard input stream

 3) System.err: standard error stream   

//code
to print output and error message to the console.
                       1.    System.out.println("simple message");  
                       2.    System.err.println("error message");  
  // code to get input
from console.
                   int i=System.in.read();//returns ASCII code of 1st character  
                   System.out.println
((char) i);//will print the character    

OutputStream:       

An Output Stream to write data to a destination, it may be a file,an array,

peripheral device or socket.

 Classes:

OutputStream class is an abstract class. It is the superclass of all classes representing an output stream of bytes. 

  

 InputStream : 

an input stream to read data from a source, it may be a file,an array,peripheral device or socket.

  InputStream class is an abstract class.It is the superclass of all classes representing an input stream of bytes.

       Classes: