What is the buffer size in BufferedReader?

What is the sense of buffer size in the constructor?

BufferedReader(Reader in, int size)

As i have written the program:

import java.io.*;
class bufferedReaderEx{
    public static void main(String args[]){
        InputStreamReader isr = null;
        BufferedReader br = null;
            try{
                isr = new InputStreamReader(System.in);
//              System.out.println("Write data: ");
//              int i = isr.read();
//              System.out.println("Data read is: " + i);
                //Thus the InputStreamReader is useful for reading the character from the stream
                System.out.println("Enter the data to be read by the bufferedReader: ");
                //here isr is containing the lnefeed already so this is needed to be flushed.
                br = new BufferedReader(isr, 2);
                String str = br.readLine();
                System.out.println("The data is : :" +  str);
            }catch(IOException e){
                System.out.println("Can't read: " + e.getMessage());
            }
    }
}

Output:

Enter the data to be read by the bufferedReader: Hello world and hello world again
The data is: Hello world and hello world again

Then what does the buffer size means as i intended that it would be reading only two characters. but it was not that.

BufferedReader buffers the input, just as the name says. This means that it reads from the input source into a buffer before passing it onto you. The buffer size here refers to the number of bytes it buffers.

Reading input from most sources is very slow. A buffer of just 2 bytes is going to hurt performance, as your program is very likely going to be waiting on input most of the time. With a buffer size of 2, a read of 100 bytes will result in reading 2 bytes from the in-memory buffer (very fast), filling the buffer (very slow), reading 2 bytes from the buffer (very fast), filling the buffer (very slow), etc - overall very slow. With a buffer size of 100, a read of 100 bytes will result in reading 100 bytes from the in-memory buffer (very fast) - overall very fast. This is assuming the buffer is contains the 100 bytes when reading though, which in a case like yours is a reasonable assumption to make.

Unless you know what you're doing, you should use the default buffer size which is quite large. One reason for a smaller buffer is when you are running on a limited-memory device, as the buffer consumes memory.