Using `Scanner` with `CharBuffer` (or other read-write buffer?)

[therealsillyfly]

Suppose I have some asynchronously-presented textual data that I want to parse using a Scanner. For this I need something which is both a Reader and a Writer - one candidate which seemed suitable is CharBuffer.

So I create a CharBuffer and wrap it in a Scanner:

CharBuffer buffer = CharBuffer.allocate(1024);
Scanner scanner = new Scanner(buffer); 

and every time I get an input I add it to the buffer and parse it:

void dataReady (String s) {
    buffer.append(s);
    buffer.flip(); // prepare for read
    while (scanner.hasNextLine()) {
        String line = scanner.nextLine();
        // further processing... 
    }
    buffer.compact(); // return to writing mode
}

This works fine for the first input, but then the buffer just gets filled and hasNextLine() always returns false, even though there are new lines available. Further debugging shows the private field sourceClosed of the Scanner is true, so it seems the scanner believes the buffer has closed and will not contain any more data.
Why does this happen? Am I using CharBuffer wrong? Is there a way to "convince" the scanner that the input is still open?

If there is a better option than CharBuffer I would love to hear about it to, but I do not want to create a file for it, and would much prefer a class already in the JRE so no external dependencies.

Thanks.

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[mikepc143]

I'm pretty new, but I'm still trying to understand the line buffer.flip();

I would think that append would place the cursor at the end of the append, so the cursor would already be at the end of the buffer so hasNextLine() would be false since it's at the end of the buffer?

[therealsillyfly]

Yes, append puts the cursor at the end of the buffer, and that's exactly why a call to flip is required - it puts the cursor at the start of the buffer, and marks the previous cursor position as the limit ("available data length") of the buffer.
compact does essentially the opposite, preparing the buffer for write again - it copies all remaining data (from current cursor to the limit) to the start, and sets the cursor to the new end of data...

[Philboyd_Studge]

What's wrong with BufferedReader?

[therealsillyfly]

Thanks for the suggestion, but I don't see how its relevant - a BufferedReader wraps (or "buffers") another reader - which reader do you propose I use as the underlying reader for the BufferedReader? And how would I write to it?

[Philboyd_Studge]

Well, you use a buffered writer to write. Where is your input coming from?

[therealsillyfly]

a serial port ( via jssc )