Java NIO ByteBuffer: put and get strings

Question

I've the following test code. I'd like to know how I can put, and get a String using a Java NIO ByteBuffer. I've added two comments where I need help.

package testPipe;

import java.io.UnsupportedEncodingException;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;

public class TestMemBuff {

    static final String dataFile = "invoicedata";
    static final double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };
    static final int[] units = { 12, 8, 13, 29, 50 };
    static final String[] descs = { "Java T-shirt", "Java Mug",
            "Duke Juggling Dolls", "Java Pin", "Java Key Chain" };

    public static Charset charset = Charset.forName("UTF-8");
    public static CharsetEncoder encoder = charset.newEncoder();
    public static CharsetDecoder decoder = charset.newDecoder();

    public static void main(String[] args) throws UnsupportedEncodingException {

        double price;
        int unit;
        String desc;
        double total = 0.0;

        ByteBuffer buf = ByteBuffer.allocate(1024);


        for (int i = 0; i < prices.length; i++) {
            buf.putDouble(prices[i]);
            buf.putInt(units[i]);
            buf.asCharBuffer().put(descs[i]);           // Is it correct?
        }

        buf.flip();

        // INPUT
        while (buf.hasRemaining()) {

            price = buf.getDouble();
            unit = buf.getInt();
            desc =  buf.asCharBuffer().toString();       //This must be wrong!
            System.out.format("You ordered %d" + " units of %s at $%.2f%n",
                    unit, desc, price);
            total += unit * price;
        }
    }

}

This is the output when I execute it:

You ordered 12 units of ????
[...]

and so on.

Thanks for your attention

Does it really print ???? ? What does toString do?

Sotirios Delimanolis
– Sotirios Delimanolis

2014-07-08 14:31:48 +00:00
Commented Jul 8, 2014 at 14:31 — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jul 8, 2014 at 14:31
Yes. There must be something wrong with encoding/decoding.

sdicola
– sdicola

2014-07-08 14:36:40 +00:00
Commented Jul 8, 2014 at 14:36 — sdicola
– sdicola, Commented Jul 8, 2014 at 14:36

Ivar · Accepted Answer · 2017-11-04 14:24:12Z

7

Writing part:

for (int i = 0; i < prices.length; i++) {
    buf.putDouble(prices[i]);
    buf.putInt(units[i]);

    byte[] descsBytes = descs[i].getBytes();
    buf.putInt(descsBytes.length);
    buf.put(descsBytes);
}

Reading part:

while (buf.hasRemaining()) {

    price = buf.getDouble();
    unit = buf.getInt();

    int len = buf.getInt();
    byte[] bytes = new byte[len]; 
    buf.get(bytes);
    desc = new String(bytes);

    System.out.format("You ordered %d" + " units of %s at $%.2f%n",
                unit, desc, price);
    total += unit * price;
}

edited Nov 4, 2017 at 14:24

Ivar

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answered Jul 8, 2014 at 15:29

user3161880

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3 Comments

sdicola Over a year ago

It works! Thank you very much! Could you tell me how to use ByteBuffer.asCharBuffer() properly? What was wrong with my approach?

user3161880 Over a year ago

asCharBuffer will create only the view of the ByteBuffer, whose position value will be independent of the original buffer. So, while writing the buffer in 2nd iteration, string bytes were getting overwritten by the double value, then int and so on...Please ref: docs.oracle.com/javase/7/docs/api/java/nio/… BTW, if you are certain about the max length of the descs[i], you can pre-allocate byte[] bytes to avoid new operator in the read loop.

sdicola Over a year ago

Thanks for your time. Greeat answer :)

Robert Panzer · Accepted Answer · 2014-07-08 15:23:16Z

1

ByteBuffer.asCharBuffer() has a somewhat own behaviour. It creates a new buffer that shares the content with the ByteBuffer but has an own, independent position & limit. So put() and get() on the CharBuffer do not change the position on the ByteBuffer!

So I would refrain from using ByteBuffer.asCharBuffer() at all as long as this is not exactly what you need.

So the solution would be to write and read the bytes into the original ByteBuffer:

buf.putInt(descs[i].length);
buf.put(descs[i].getBytes());`

To read the string again read the length and allocate the byte[] yourself:

int stringLength = buf.getInt();
byte[] bytes = new byte[stringLength];
buf.get(bytes);
desc = new String(bytes);

edited Jul 8, 2014 at 15:23

answered Jul 8, 2014 at 14:47

Robert Panzer

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1 Comment

sdicola Over a year ago

Thanks, but I still get errors: You ordered 12 units of ???? Exception in thread "main" java.nio.BufferUnderflowException at java.nio.CharBuffer.get(Unknown Source) at java.nio.CharBuffer.get(Unknown Source) at testPipe.TestMemBuff.main(TestMemBuff.java:48)

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