理解引用类型变量的存储方式

想通过另一个List来复制现有list，然后使用新list进行操作，保证原list不变，如下代码并不能实现

public BasicInfo getBasicInfoByNeed(String sid, List<String> needList){
List<String> need = needList;
need.add("subject");
need.add("school_type");
if(need.contains("rethink"))
need.add("knowledge");
DBObject dbObject = new BasicDBObject();
dbObject.put("_id", sid);
BasicDBObject fieldsObject=new BasicDBObject();
for(String field:need)
fieldsObject.put(field, true);
Query query = new BasicQuery(dbObject, fieldsObject);
return mongoTemplate.findOne(query, BasicInfo.class);
}

正确的方式为

public BasicInfo getBasicInfoByNeed(String sid, List<String> needList){
List<String> need = new ArrayList<>().addAll(needList)
needList.add("subject");
needList.add("school_type");
if(needList.contains("rethink"))
needList.add("knowledge");
DBObject dbObject = new BasicDBObject();
dbObject.put("_id", sid);
BasicDBObject fieldsObject=new BasicDBObject();
for(String field:needList)
fieldsObject.put(field, true);
Query query = new BasicQuery(dbObject, fieldsObject);
return mongoTemplate.findOne(query, BasicInfo.class);
}