如何制作对象的防御性副本？

您需要做的是

  MutableObject return_immutable_field() {    return immutable_field;  }

改成：

  MutableObject return_immutable_field() {    MutableObject tmp = new MutableObject();    tmp.mutable_field = immutable_field.mutable_field;    return tmp;  }

有关说明，请参见http://www.javapractices.com/topic/TopicAction.do?Id=15