在本指南中，我们将了解如何将 LocalDate 转换为 Date。在查看转换代码之前，让我们先看看 Date 和 LocalDate 之间有什么区别。

java.util.Date - date + time + timezone
java.time.LocalDate - only date

因此，要将 LocalDate 转换为 Date，我们必须在日期后附加时间和时区信息。考虑到这一点，转换步骤如下：

1. 获取默认时区，以便在日期中附加时区信息
2. 调用 atStartOfDay()，以便将时间与日期附加在一起
3. LocalDate to Date - local date + atStartOfDay() + 默认时区(default time zone) + toInstant() = Date

- 阅读剩余部分 -

As nobody answered, So I asked this on MongoDB-User Group

Say cars is your outputcollection (where you want to store output of map-reduce job).

• REPLACE: If your current MR is successful, all of the documents in cars will be deleted (regardless of their _id) and replaced by the current result.

• MERGE: No document in cars will be deleted. Instead, each document in your current result will replace an already existing document in cars with the same _id. If there is not any document in cars with that _id, it will be just inserted. You can see this like an upsert:

  db.cars.update({_id: newDocument._id}, {value: newDocument.value}, {upsert: true})

• REDUCE: This is very similar to MERGE. But instead of just replacing the existing document, both documents will be the input of your reduce function (i.e. reduce([oldDocument, newDocument])) and the resulting document will replace the existing one.
• INLINE: Returns your result as a variable in the same fashion a function does. Nothing is stored in MongoDB, so this does not impact any collection.

The complete answer can be found here.

https://stackoverflow.com/questions/30508245/what-does-mapreducecommand-outputtype-reduce-do-in-mapreduce-command-in-mongodb