Newbie to java

import java.lang.Thread;

public class HelloWorld extends Thread {

    public void run()
    {
        System.out.println("Hello World");
    }
    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Thread t = new HelloWorld();
        CharacterBuffer cb;
        cb.addChar('c'); <<<<<<<<<<<<<<< error
        t.start();
    }

}

=============================

public class CharacterBuffer
{
    private byte[] data_ = null;
    private int len_ = 0;

    synchronized public void addChar(byte c)
    {
        if (data_ == null || len_ == data_.length)
        {
            byte[] newData = new byte[len_+128];
            if (data_ != null)
                System.arraycopy(data_, 0, newData,0, len_);
            data_ = newData;
        }
        data_[len_++] = c;
    }
    synchronized public void writeBuffer()
    {
        System.out.write (data_, 0, len_);
        System.out.flush();
        len_ = 0;
    }
}
======================================
besides I'd like to make the thread print "abc123", how do I make it
work like that?
Thanks
Jack