java mongodb分组查询并求count

以下是使用Java和MongoDB进行分组查询并求count的示例代码：

import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import org.bson.Document;

import java.util.Arrays;

public class GroupCountExample {
    public static void main(String[] args) {
        // 连接到MongoDB数据库
        MongoClient mongoClient = new MongoClient("localhost", 27017);
        MongoDatabase database = mongoClient.getDatabase("your_database_name");
        MongoCollection<Document> collection = database.getCollection("your_collection_name");

        // 构建分组查询条件
        Document groupFields = new Document("_id", "$c_sender");
        groupFields.put("count", new Document("$sum", 1));

        // 执行分组查询
        Document group = new Document("$group", groupFields);
        Iterable<Document> results = collection.aggregate(Arrays.asList(group));

        // 输出结果
        for (Document result : results) {
            String sender = result.getString("_id");
            int count = result.getInteger("count");
            System.out.println("Sender: " + sender + ", Count: " + count);
        }

        // 关闭连接
        mongoClient.close();
    }
}

请注意，你需要将代码中的`your_database_name`和`your_collection_name`替换为你实际使用的数据库和集合名称。