Making BufferedReader Iterable

I was reading recently about a issues around reading an entire file into a string. I pretty much never do that, since I like my code to be robust even in the face of very large files. But it did get me thinking about reading in lines of a file one at a time. I do have reason to do that on occasion.

Originally, I might have done it something like this:

File file = ...
BufferedReader br = new BufferedReader(new FileReader(file));
String line = br.readLine();
while ( line != null ) {
    // Do something with line
    line = br.readLine();
}

That's not bad, but I've been working with Ruby lately and really like the terse yet easy-to-read nature of the code. We can improve the above code with a for loop:

File file = ...
BufferedReader br = new BufferedReader(new FileReader(file));
for (String line = br.readLine(); line != null; line = br.readLine()) {
    // Do something with line
}

Better, but still not great. Now, what if the BufferedReader were Iterable? It isn't, but we can wrap an Iterable around it. First, let's look at usage:

File file = ...
BufferedReaderIterable bri = new BufferedReaderIterable(new BufferedReader(new FileReader(file)));
for (String read : bri) {
    // Do something with the line
}

Very nice. So, all we need is to implement BufferedReaderIterable. Happily, it's quite simple:

public class BufferedReaderIterable implements Iterable<String> {

    private Iterator<String> i;

    public BufferedReaderIterable( BufferedReader br ) {
        i = new BufferedReaderIterator( br );
    }
    public Iterator iterator() {
        return i;
    }

    private class BufferedReaderIterator implements Iterator<String> {
        private BufferedReader br;
        private java.lang.String line;

        public BufferedReaderIterator( BufferedReader aBR ) {
            (br = aBR).getClass();
            advance();
        }

        public boolean hasNext() {
            return line != null;
        }

        public String next() {
            String retval = line;
            advance();
            return retval;
        }

        public void remove() {
            throw new UnsupportedOperationException("Remove not supported on BufferedReader iteration.");
        }

        private void advance() {
            try {
                line = br.readLine();
            }
            catch (IOException e) { /* TODO */}
        }
    }
}

It would be nice if we could make the construction cleaner. What if BufferedReaderIterable had a constructor that took a File?

public class BufferedReaderIterable implements Iterable<String> {

    private BufferedReader mine;
    private Iterator<String> i;

    public BufferedReaderIterable( BufferedReader br ) {
        i = new BufferedReaderIterator( br );
    }

    public BufferedReaderIterable( File f ) throws FileNotFoundException {
        mine = new BufferedReader( new FileReader( f ) );
        i = new BufferedReaderIterator( mine );
    }
    public Iterator<String> iterator() {
        return i;
    }

    private class BufferedReaderIterator implements Iterator<String> {
        private BufferedReader br;
        private String line;

        public BufferedReaderIterator( BufferedReader aBR ) {
            (br = aBR).getClass();
            advance();
        }

        public boolean hasNext() {
            return line != null;
        }

        public String next() {
            String retval = line;
            advance();
            return retval;
        }

        public void remove() {
            throw new UnsupportedOperationException("Remove not supported on BufferedReader iteration.");
        }

        private void advance() {
            try {
                line = br.readLine();
            }
            catch (IOException e) { /* TODO */}
            if ( line == null && mine != null ) {
                try {
                    mine.close();
                }
                catch (IOException e) { /* Ignore - probably should log an error */ }
                mine = null;
            }
        }
    }
}

Now, usage is even cleaner:

File file = ...
BufferedReaderIterable bri = new BufferedReaderIterable(file);
for (String read : bri) {
    // Do something with the line
}

[Editorial Note: Updated to put the template angle brackets back in after Eric Burke noticed Blogger was eating them]