This guide dmeonstrates basic CRUD operations with MongoDB using Java. Requires mongo-java-driver dependency (version 3.2.2 used here).

// Insert operation
public void insertDocument() {
    try(MongoClient mongo = new MongoClient("localhost", 27017)) {
        DB database = mongo.getDB("sampleDB");
        DBCollection collection = database.getCollection("users");
        
        DBObject document = new BasicDBObject();
        document.put("username", "testUser");
        document.put("score", 85);
        
        collection.insert(document);
        System.out.println("Document inserted successfully");
    }
}

// Delete operation
public void removeDocument() {
    try(MongoClient mongo = new MongoClient("localhost", 27017)) {
        DB database = mongo.getDB("sampleDB");
        DBCollection collection = database.getCollection("users");
        
        DBObject query = new BasicDBObject("username", "testUser");
        collection.remove(query);
        
        System.out.println("Document removed successfully");
    }
}

// Update operation
public void modifyDocument() {
    try(MongoClient mongo = new MongoClient("localhost", 27017)) {
        DB database = mongo.getDB("sampleDB");
        DBCollection collection = database.getCollection("users");
        
        DBObject query = new BasicDBObject("username", "testUser");
        DBObject update = new BasicDBObject("$set", 
            new BasicDBObject("score", 90));
            
        collection.update(query, update);
        System.out.println("Document updated successfully");
    }
}

// Query operation
public void findDocuments() {
    try(MongoClient mongo = new MongoClient("localhost", 27017)) {
        DB database = mongo.getDB("sampleDB");
        DBCollection collection = database.getCollection("users");
        
        DBObject query = new BasicDBObject("score", 
            new BasicDBObject("$gt", 80));
            
        try(DBCursor cursor = collection.find(query)) {
            while(cursor.hasNext()) {
                System.out.println(cursor.next());
            }
        }
        System.out.println("Query completed");
    }
}