code: public void
  setActive(int len) {
  	if (len > byteBuffer.capacity() - byteBuffer.position()) {
  		throw new IllegalArgumentException("cannot set active " +
  						   "region past end of input");
  	}
  	byteBuffer.limit(byteBuffer.position() + len);
  }
docs: Marks the following bytes in the stream as active.
  @param len The number of bytes in the active region.
  @throws IllegalArgumentException The number of bytes in the active region
  is longer than the remainder of the input.