public void append(byte[] bytes,int offset,int len){
  ensureCapacity(length + len);
  System.arraycopy(bytes,offset,buffer,length,len);
  length+=len;
}

public void append ( byte [ ] bytes , int offset , int len ) { ensureCapacity ( length + len ) ; System . arraycopy ( bytes , offset , buffer , length , len ) ; length += len ; }
appends a byte array or part of to the buffer .
