We are advancing through our journey of building various real-world applications. We have explored the Command, Observer, and Strategy patterns in previous units. In this unit, we will integrate all three behavioral patterns into different scenarios to solve real-world problems.

In this unit, we will use different combinations of behavioral patterns to solve real-world problems. Let's have a quick recap of what each pattern does:

• Command Pattern: Encapsulates a request as an object, allowing clients to parameterize and queue requests.
• Observer Pattern: Defines a one-to-many dependency between objects, ensuring that when one object changes state, all its dependents are notified and updated automatically.
• Strategy Pattern: Defines a family of algorithms, encapsulates each one, and makes them interchangeable. Clients can choose the algorithm to use at runtime.

In this unit, we will apply different combinations of behavioral patterns to solve real-world problems. We will build the following applications:

1. Chat Application: We'll build a chat application that uses the Command pattern to send messages and the Observer pattern to notify users when new messages are received. The application involves creating a command to send messages to a chat room, which is then observed by users who receive notifications.

2. Game Application: We'll create a game application that uses the Strategy pattern for different attack strategies and the Command pattern to execute battle commands. Different strategies like aggressive or defensive can be encapsulated and executed using command objects.

3. Weather Monitoring Application: We'll develop a weather monitoring application that uses the Observer pattern to notify users when the weather changes and the Command pattern to set the weather updates. The central weather station acts as a subject that notifies registered observers about any change in weather.

4. Chat and Order Management System: Finally, we will build a combined chat application and order management system that uses all three patterns together. This will provide an advanced scenario where we see how these patterns can be layered and integrated for complex real-world applications. Observers can be notified about order status updates, commands can be used to process orders, and strategies can determine delivery methods. This example involves a more complex implementation and is designed for deeper mastery of the patterns.