用java来写mongo

好的，以下是使用Java连接MongoDB并实现你所提出的需求的代码示例：

首先需要引入MongoDB Java Driver的依赖：

<dependency>
    <groupId>org.mongodb</groupId>
    <artifactId>mongo-java-driver</artifactId>
    <version>3.12.9</version>
</dependency>

然后可以按照以下步骤实现操作：

1. 导入必要的包：

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

2. 连接MongoDB数据库：

MongoClient mongoClient = new MongoClient("localhost", 27017);
MongoDatabase database = mongoClient.getDatabase("test");

3. 获取grade集合：

MongoCollection<Document> collection = database.getCollection("grade");

4. 插入20条数据：

String[] students = {"Tom", "Jack", "Jie", "Juliy", "Anma"};
String[] courses = {"Java", "BigData", "Math", "Python"};

for (int i = 0; i < 20; i++) {
    String student = students[(int) (Math.random() * students.length)];
    String course = courses[(int) (Math.random() * courses.length)];
    int age = (int) (Math.random() * 7) + 15; // 15到21岁之间的随机年龄
    Document document = new Document("student", student)
            .append("course", course)
            .append("age", age);
    collection.insertOne(document);
}

5. 查询选修课程为“BigData”的学生信息：

Document query = new Document("course", "BigData");
MongoCursor<Document> cursor = collection.find(query).iterator();
while (cursor.hasNext()) {
    System.out.println(cursor.next().toJson());
}

完整代码如下：

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

public class Main {

    public static void main(String[] args) {
        MongoClient mongoClient = new MongoClient("localhost", 27017);
        MongoDatabase database = mongoClient.getDatabase("test");
        MongoCollection<Document> collection = database.getCollection("grade");

        String[] students = {"Tom", "Jack", "Jie", "Juliy", "Anma"};
        String[] courses = {"Java", "BigData", "Math", "Python"};

        for (int i = 0; i < 20; i++) {
            String student = students[(int) (Math.random() * students.length)];
            String course = courses[(int) (Math.random() * courses.length)];
            int age = (int) (Math.random() * 7) + 15; // 15到21岁之间的随机年龄
            Document document = new Document("student", student)
                    .append("course", course)
                    .append("age", age);
            collection.insertOne(document);
        }

        Document query = new Document("course", "BigData");
        MongoCursor<Document> cursor = collection.find(query).iterator();
        while (cursor.hasNext()) {
            System.out.println(cursor.next().toJson());
        }

        mongoClient.close();
    }
}