What is the use of System.in.read()?

Two and a half years late is better than never, right?

int System.in.read() reads the next byte of data from the input stream. But I am sure you already knew that, because it is trivial to look up. So, what you are probably asking is:

  • Why is it declared to return an int when the documentation says that it reads a byte?

  • and why does it appear to return garbage? (I type '9', but it returns 57.)

It returns an int because besides all the possible values of a byte, it also needs to be able to return an extra value to indicate end-of-stream. So, it has to return a type which can express more values than a byte can.

Note: They could have made it a short, but they opted for int instead, possibly as a tip of the hat of historical significance to C, whose getc() function also returns an int, but more importantly because short is a bit cumbersome to work with, (the language offers no means of specifying a short literal, so you have to specify an int literal and cast it to short,) plus on certain architectures int has better performance than short.

It appears to return garbage because when you view a character as an integer, what you are looking at is the ASCII(*) value of that character. So, a '9' appears as a 57. But if you cast it to a character, you get '9', so all is well.

Think of it this way: if you typed the character '9' it is nonsensical to expect System.in.read() to return the number 9, because then what number would you expect it to return if you had typed an 'a'? Obviously, characters must be mapped to numbers. ASCII(*) is a system of mapping characters to numbers. And in this system, character '9' maps to number 57, not number 9.

(*) Not necessarily ASCII; it may be some other encoding, like UTF-16; but in the vast majority of encodings, and certainly in all popular encodings, the first 127 values are the same as ASCII. And this includes all english alphanumeric characters and popular symbols.


System is a final class in java.lang package

code sample from the source code of api

public final class System {

   /**
     * The "standard" input stream. This stream is already
     * open and ready to supply input data. Typically this stream
     * corresponds to keyboard input or another input source specified by
     * the host environment or user.
     */
    public final static InputStream in = nullInputStream();

}

read() is an abstract method of abstract class InputStream

 /**
     * Reads the next byte of data from the input stream. The value byte is
     * returned as an <code>int</code> in the range <code>0</code> to
     * <code>255</code>. If no byte is available because the end of the stream
     * has been reached, the value <code>-1</code> is returned. This method
     * blocks until input data is available, the end of the stream is detected,
     * or an exception is thrown.
     *
     * <p> A subclass must provide an implementation of this method.
     *
     * @return     the next byte of data, or <code>-1</code> if the end of the
     *             stream is reached.
     * @exception  IOException  if an I/O error occurs.
     */
    public abstract int read() throws IOException;

In short from the api:

Reads some number of bytes from the input stream and stores them into the buffer array b. The number of bytes actually read is returned as an integer. This method blocks until input data is available, end of file is detected, or an exception is thrown.

from InputStream.html#read()


May be this example will help you.

import java.io.IOException;

public class MainClass {

    public static void main(String[] args) {
        int inChar;
        System.out.println("Enter a Character:");
        try {
            inChar = System.in.read();
            System.out.print("You entered ");
            System.out.println(inChar);
        }
        catch (IOException e){
            System.out.println("Error reading from user");
        }
    }
}


import java.io.IOException;

class ExamTest{

    public static void main(String args[]) throws IOException{
        int sn=System.in.read();
        System.out.println(sn);
    }
}

If you want get char input you have to cast like this: char sn=(char) System.in.read()

The value byte is returned as int in the range 0 to 255. However, unlike in other languages’ methods, System.in.read() reads only a byte at a time.

Tags:

Java

Recent Posts