Can I peek on a BufferedReader?

Question

Is there a way to check if in BufferedReader object is something to read? Something like C++ cin.peek(). Thanks.

Answer 1

You can use a PushbackReader. Using that you can read a character, then unread it. This essentially allows you to push it back.

PushbackReader pr = new PushbackReader(reader);
char c = (char)pr.read();
// do something to look at c
pr.unread((int)c); //pushes the character back into the buffer

Answer 2

You can try the "boolean ready()" method. From the Java 6 API doc: "A buffered character stream is ready if the buffer is not empty, or if the underlying character stream is ready."

BufferedReader r = new BufferedReader(reader);
if(r.ready())
{
   r.read();
}

Answer 3

The following code will look at the first byte in the Stream. Should act as a peek for you.

BufferedReader bReader = new BufferedReader(inputStream);
bReader.mark(1);
int byte1 = bReader.read();
bReader.reset();

Answer 4

The normal idiom is to check in a loop if BufferedReader#readLine() doesn't return null. If end of stream is reached (e.g. end of file, socket closed, etc), then it returns null.

E.g.

BufferedReader reader = new BufferedReader(someReaderSource);
String line = null;
while ((line = reader.readLine()) != null) {
    // ...
}

If you don't want to read in lines (which is by the way the major reason a BufferedReader is been chosen), then use BufferedReader#ready() instead:

BufferedReader reader = new BufferedReader(someReaderSource);
while (reader.ready()) {
    int data = reader.read();
    // ...
}

Answer 5

BufferedReader br = new BufferedReader(reader);
br.mark(1);
int firstByte = br.read();
br.reset();

Answer 6

You could use a PushBackReader to read a character, and then "push it back". That way you know for sure that something was there, without affecting its overall state - a "peek".

Answer 7

The answer from pgmura (relying on the ready() method) is simple and works. But bear in mind that it's because Sun's implementation of the method; which does not really agree with the documentation. I would not rely on that, if this behaviour is critical. See here http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4090471 I'd rather go with the PushbackReader option.

Answer 8

my solution was.. extending BufferedReader and use queue as buf, then you can use peek method in queue.

public class PeekBufferedReader extends BufferedReader{

    private Queue<String>       buf;
    private int                 bufSize;

    public PeekBufferedReader(Reader reader, int bufSize) throws IOException {
        super(reader);
        this.bufSize = bufSize;
        buf = Queues.newArrayBlockingQueue(bufSize);
    }

    /**
     * readAheadLimit is set to 1048576. Line which has length over readAheadLimit 
     * will cause IOException.
     * @throws IOException 
     **/
    //public String peekLine() throws IOException {
    //  super.mark(1048576);
    //  String peekedLine = super.readLine();
    //  super.reset();
    //  return peekedLine;
    //}

    /**
     * This method can be implemented by mark and reset methods. But performance of 
     * this implementation is better ( about 2times) than using mark and reset  
     **/
    public String peekLine() throws IOException {
        if (buf.isEmpty()) {
            while (buf.size() < bufSize) {
                String readLine = super.readLine();
                if (readLine == null) {
                    break;
                } else {
                    buf.add(readLine);
                }
            }
        } else {
            return buf.peek();
        }
        if (buf.isEmpty()) {
            return null;
        } else {
            return buf.peek();
        }
    }

    public String readLine() throws IOException {
        if (buf.isEmpty()) {
            while (buf.size() < bufSize) {
                String readLine = super.readLine();
                if (readLine == null) {
                    break;
                } else {
                    buf.add(readLine);
                }
            }
        } else {
            return buf.poll();
        }
        if (buf.isEmpty()) {
            return null;
        } else {
            return buf.poll();
        }
    }
    public boolean isEmpty() throws IOException {
        if (buf.isEmpty()) {
            while (buf.size() < bufSize) {
                String readLine = super.readLine();
                if (readLine == null) {
                    break;
                } else {
                    buf.add(readLine);
                }
            }
        } else {
            return false;
        }
        if (buf.isEmpty()) {
            return true;
        } else {
            return false;
        }
    }
}