How to delete all documents in mongodb collection in java

If you want to remove all documents in collection then used below code :

 db.getCollection("mainCollection").remove(new BasicDBObject());

Or If you want to drop whole collection then used this :

db.getCollection("mainCollection").drop();

Using API >= 3.0:

MongoClient mongoClient = new MongoClient("127.0.0.1" , 27017);
MongoDatabase db = mongoClient.getDatabase("maindb");
db.getCollection("mainCollection").deleteMany(new Document());

To drop the collection (documents and indexes) you still can use:

db.getCollection("mainCollection").drop();

see https://docs.mongodb.org/getting-started/java/remove/#remove-all-documents

To remove all documents use the BasicDBObject or DBCursor as follows:

MongoClient client = new MongoClient("10.0.2.113" , 27017);
MongoDatabase db = client.getDatabase("maindb");
MongoCollection collection = db.getCollection("mainCollection")

BasicDBObject document = new BasicDBObject();

// Delete All documents from collection Using blank BasicDBObject
collection.deleteMany(document);

// Delete All documents from collection using DBCursor
DBCursor cursor = collection.find();
while (cursor.hasNext()) {
    collection.remove(cursor.next());
}