Does the solution shown below work for you?  It looks like the call to
headerBuffer.asCharBuffer(); was returning a zero capacity buffer.  The
loop was using the capacity of the other buffer as a counter.
Therefore, the loop was entered (capacity of the other buffer was 4)
and an attempt was made to add data to the zero capacity buffer.

What IDE are you using?  Eclipse has some nice debugging tools that
might have been helpful.

Hope this helps,
John

/**
*
*/
package com.mydomain.example;

import java.nio.ByteBuffer;
import java.nio.CharBuffer;

/**
* @author greshje
*
*/
public class Example {

    public static void main(String[] args) throws Exception {
        // Test fixed length header strings
        String zeroString = "0000";
        byte[] bytes = zeroString.getBytes();
        int messageHeaderSize = bytes.length;
        // int MessageHeaderSize = 4;
        System.out.println("Length in bytes of 0000 = "
                 + messageHeaderSize);
        ByteBuffer headerBuffer =
                 ByteBuffer.allocateDirect(messageHeaderSize);
        System.out.println("HeaderBuffer.capacity() = "
                + headerBuffer.capacity());
        headerBuffer.put(bytes);

        StringBuffer headerStringBuffer = new StringBuffer(zeroString);
        // CharBuffer headerCharBuffer = headerBuffer.asCharBuffer();
        CharBuffer headerCharBuffer = CharBuffer.allocate(4);
        for (int counter = 0;
                  counter < headerCharBuffer.capacity();
                  counter++) {
            System.out.println(counter);
            headerCharBuffer.put(counter, (char) 0);
        }
    }

}