Use MongoDB createIndex in Java

private MongoCollection<Document> createCollection(String collectionName) {
    String indexFieldName1 = "itemId";
    String indexFieldName2 = "time";
    MongoCollection<Document> mongoCollection = this.database.getCollection(collectionName);

    if (mongoCollection == null) {
        this.database.createCollection(collectionName);
        mongoCollection = this.database.getCollection(collectionName);
    }

    IndexOptions indexOptions1 = new IndexOptions().unique(false)
            .background(false).name(indexFieldName1);
    Bson keys1 = new Document(indexFieldName1, Integer.valueOf(1));
    mongoCollection.createIndex(keys1, indexOptions1);

    IndexOptions indexOptions2 = new IndexOptions().unique(false)
            .background(false).name(indexFieldName2);
    Bson keys2 = new Document(indexFieldName2, Integer.valueOf(1));
    mongoCollection.createIndex(keys2, indexOptions2);

    return mongoCollection;
}

References
https://www.programcreek.com/java-api-examples/?class=com.mongodb.client.MongoCollection&method=createIndex