I Am Not Getting the Result I Expect Using readLine() in Java

Question

I am using the code snippet below, however it's not working quite as I understand it should.

public static void main(String[] args) {
 BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

 String line;
 try {
  line = br.readLine();
  while(line != null) {
   System.out.println(line);
   line = br.readLine();
  }
 } catch (IOException e) {
  e.printStackTrace();
 }
}

From reading the Javadoc about readLine() it says:

Reads a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

Throws: IOException - If an I/O error occurs

From my understanding of this, readLine should return null the first time no input is entered other than a line termination, like '\r'. However, this code just ends up looping infinitely. After debugging, I have found that instead of null being returned when just a termination character is entered, it actually returns an empty string (""). This doesn't make sense to me. What am I not understanding correctly?

Answer 1

No input is not the same as the end of the stream. You can usually simulate the end of the stream in a console by pressing Ctrl+D (AFAIK some systems use Ctrl+Z instead). But I guess this is not what you want so better test for empty strings additionally to null strings.

Answer 2

I don't think you understood it correctly. When they were talking about a (null) being returned it was in reference to when reading a document. Where the reader would eventually run out of lines and a (null) indicating the end of file (EOF).

However since you are using the command line there is no end of file, because it can keep going on and on. Maybe I can help you code a better solution if I know what you are reading from the command line? Is this program meant to take user input or scrape a console?

Answer 3

From my understanding of this, readLine should return null the first time no input is entered other than a line termination, like '\r'.

That is not correct. readLine will return null if the end of the stream is reached. That is, for example, if you are reading a file, and the file ends, or if you're reading from a socket and the socket closses.

But if you're simply reading the console input, hitting the return key on your keyboard does not constitute an end of stream. It's simply a character that is returned (\n or \r\n depending on your OS).

So, if you want to break on both the empty string and the end of line, you should do:

while (line != null && !line.equals(""))

Also, your current program should work as expected if you pipe some file directly into it, like so:

java -cp . Echo < test.txt

Answer 4

There's a nice apache commons lang library which has a good api for common :) actions. You could use statically import StringUtils and use its method isNotEmpty(String ) to get:

while(isNotEmpty(line)) {
    System.out.println(line);
    line = br.readLine();
}

It might be useful someday:) There are also other useful classes in this lib.