Count Documents

Overview

In this guide, you can learn how to count the number of documents in your MongoDB collections. There are two instance methods in the MongoCollection class that you can call to count the number of documents in a collection:

• countDocuments() returns the number of documents in the collection that match a specified query. If you specify an empty query filter, the method returns the total number of documents in the collection.

• estimatedDocumentCount() returns an estimation of the number of documents in the collection based on the collection metadata. You cannot specify a query when using this method.

The estimatedDocumentCount() method returns more quickly than the countDocuments() method because it uses the collection's metadata rather than scanning the entire collection. The countDocuments() method returns an accurate count of the number of documents and supports specifying a filter.

CountOptions opts = new CountOptions().hintString("_id_");
long numDocuments = collection.countDocuments(new BsonDocument(), opts);

When you call the countDocuments() method, you can optionally pass a query filter parameter. You cannot pass any parameters when you call estimatedDocumentCount().

You can also pass an optional parameter to either of these methods to specify the behavior of the call:

Both methods return the number of matching documents as a long primitive.

Count Documents Example: Full File

The following example estimates the number of documents in the movies collection in the sample_mflix database, and then returns an accurate count of the number of documents in the movies collection with Canada in the countries field.

/**
 * This file demonstrates how to open a change stream by using the Java driver.
 * It connects to a MongoDB deployment, accesses the "sample_mflix" database, and listens
 * to change events in the "movies" collection. The code uses a change stream with a pipeline
 * to only filter for "insert" and "update" events.
 */
package usage.examples;
import static com.mongodb.client.model.Filters.eq;
import org.bson.Document;
import org.bson.conversions.Bson;
import com.mongodb.MongoException;
import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
public class CountDocuments {
    public static void main(String[] args) {
        // Replace the uri string with your MongoDB deployment's connection string
        String uri = "<connection string uri>";
        try (MongoClient mongoClient = MongoClients.create(uri)) {
            MongoDatabase database = mongoClient.getDatabase("sample_mflix");
            MongoCollection<Document> collection = database.getCollection("movies");
            Bson query = eq("countries", "Spain");
            // Retrieves and prints the estimated number of documents in the collection
            long estimatedCount = collection.estimatedDocumentCount();
            System.out.println("Estimated number of documents in the movies collection: " + estimatedCount);
            // Retrieves and prints the number of documents with a "countries" value of "Spain"
            long matchingCount = collection.countDocuments(query);
            System.out.println("Number of movies from Spain: " + matchingCount);
        }
    }
}

If you run the preceding sample code, you should see output that looks something like this (exact numbers can vary depending on your data):

Estimated number of documents in the movies collection: 23541
Number of movies from Spain: 755

Additional Information