Question

I have such code to read a text file using BufferedReader:

BufferedReader reader=null;
    try {
        reader = new BufferedReader(new FileReader("file1.txt"));

        while (reader.ready()) {
            final String line = reader.readLine();
            System.out.println("<"+line+">");
        } catch (..)
    {
        ...
    }

It works correctly but Findbugs reports a warning:

NP_DEREFERENCE_OF_READLINE_VALUE : The
result of invoking readLine() is
dereferenced without checking to see
if the result is null. If there are no
more lines of text to read, readLine()
will return null and dereferencing
that will generate a null pointer
exception.

When I change FileReader to StringReader, i.e.

BufferedReader reader=null;
    try {
        reader = new BufferedReader(new StringReader("ABCD"));

        while (reader.ready()) {
            final String line = reader.readLine();
            System.out.println("<"+line+">");
        } catch (..)
    {
        ...
    }

the readLine method returns null while the ready method always returns true – indeed this is an infinite loop.

This seems that the readLine may return null even if ready returns true. But why does the behavior differ for different Readers?

UPDATE:

I do know the normal way to read a text file (just like Peter and Ali illustrated). but I read that piece of code from my colleague and realized that I don’t know the ready method. Then I read the JavaDoc, but don’t understand block. Then I did a test and posted this question. So, the better way to put this question might be:

When will the input be blocking? How to use the ready method (or why not to use it)? Why do those 2 Readers (FileReader and StringReader) behave differently with regards to the ready method?

Answer 1

The ready method tells us if the Stream is ready to be read.

Imagine your stream is reading data from a network socket. In this case, the stream may not have ended, because the socket has not been closed, yet it may not be ready for the next chunk of data, because the other end of the socket has not pushed any more data.

In the above scenario, we cannot read any more data until the remote end pushes it, so we have to wait for the data to become available, or for the socket to be closed. The ready() method tells us when the data is available.