java - 如何在MongoDB Java中从键值对中分离值，并将值相加

Question

下面的代码只投影一个带有键值对的字段，问题是如何分离值并将它们相加并求平均值。

private static void selectAllRecordByRecordNumber(DBCollection collection) {
    BasicDBObject whereQuery = new BasicDBObject();
    BasicDBObject whereQuery1 = new BasicDBObject();
    BasicDBObject whereQuery2 = new BasicDBObject();
    BasicDBObject fields = new BasicDBObject("pr", 1);
    fields.put("_id", 0);
    int TotalPseudorange=0;
    whereQuery.put("st", 17);
    whereQuery2.put("pr",whereQuery);
    whereQuery1.put("pr",new BasicDBObject("st",17));
    DBCursor cursor = collection.find(whereQuery,fields);
    TotalPseudorange = TotalPseudorange + cursor.count();
    int count=cursor.count();
    int index_count=0;
    while(cursor.hasNext()) {
      System.out.println(cursor.next());
    }
    System.out.println("The calculation is complete & Done");
    System.out.println(count);
}

Answer 1

我认为您正在迭代对象中寻找 .get() 方法:

while(cursor.hasNext()) {
  System.out.println(cursor.next().get("pr"));
}

然后您可以将上面的内容替换为以下内容以获得 java8-fashion 中的平均值。

    StreamSupport.stream(Spliterators.spliteratorUnknownSize(cursor,
            Spliterator.ORDERED), false)
            .mapToDouble(item -> ((Number)item.get("pr")).doubleValue() )
            .summaryStatistics()
            .getAverage();

注意:我不太确定 get 的返回类型是什么，但我认为直接转换为 double 也可能是安全的。