code: public MemorySize add(MemorySize other)
    {
      if(other == null)
        throw new NullPointerException();
      
      if(getSizeUnit() == other.getSizeUnit())
        return new MemorySize(getSize() + other.getSize(), getSizeUnit());
  
      return new MemorySize(getSizeInBytes() + other.getSizeInBytes(),
                          SizeUnit.BYTE);
    }
docs: Return a new instance of MemorySize that adds up the size of this object
  with that of the 'other'
  
  @param other
  @return a new instance of MemorySize that adds up the size of this object
  with the that of the 'other'