java - What is the buffer size in BufferedReader?

Question

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.

Answer 1

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.