Unit 9 - Architecture, patterns and reuse Flashcards

Question

The book, Design Patterns: Elements | of Reusable Object-Oriented Software, describes patterns in to which three categories?

Answer 1

. creational patterns, which deal with ways of creating objects independently of the clients that will use them . structural patterns, which deal with relationships among classes and objects . behavioural patterns, which deal with how objects communicate and interact.

Answer 2

. Adapter . Observer (touched on in the discussion of architectural styles), which is a behavioural pattern . Factory, which is a creational pattern . Singleton, another creational pattern often associated with Factory. . MVC (Model View Controller)

Answer 3

``` . Name . Purpose (also called intent) . What problem does it aim to solve? . How it works – a description of the solution . When to use it . Example of use. ```

Answer 4

. Name. Adapter. . Purpose. Allows a client to use a class that has a different interface from the one the client is expecting. . How it works. An Adapter class is introduced that provides the client with the interface it is expecting but forwards client requests to an object of the class with the incompatible interface (the Adaptee). The interface the client is expecting is called the Target. . When to use it. When you want to use a class with a client that is expecting a different interface from the one the class provides. . Example. Legacy software may need to be integrated with a newer system that uses a different interface.

Answer 5

. Name. Model-view-controller (MVC). . Purpose. Splits user interface interaction into three distinct roles: the model of the domain, the view representing that domain, and the controller of changes to the domain. . How it works. It identifies three roles. ◦ The first is the model, corresponding to an object with some information about the domain. The model contains data and behaviour and is not directly accessible to the user. If we consider MVC as a layered architecture the model resides in the application domain (or business) layer. ◦ The view is the representation of the model in the user interface: it displays the output from the system and is what the user perceives of the model’s state. Both the viewer and the controller reside in the presentation layer, the layer responsible for user dialogue aspects. ◦ The controller handles all user inputs that affect the model. The controller also resides in the presentation layer. ◦ User inputs to the controller cause changes to the model’s state, which in turn are reflected in the view ◦ Although the view and the controller are distinct roles, it is important to understand that they are not always represented by different objects. For example, a tick box shows the status of some setting, making it part of the view, but it also lets the user change the setting, making it also part of the controller. ◦ This tight integration is typical of many frameworks used for building user interfaces. . When to use it. When you have a user interface that you want kept separate from the model. The advantages of this include the following. ◦ Separation of concerns. When designing a model you focus on business objects and processes. When designing an interface you are concerned with user interaction. ◦ Facilitating testing. User interfaces are notoriously hard to test because users can perform so many different sequences of actions, and such testing is usually done manually. Code implementing business logic is easier to test and tests can be automated. Keeping the model separate means that you can take advantage of the relative ease of testing. ◦ Flexibility. Multiple interfaces may be developed for the same model, allowing the design to cater for different types of users and different contexts of use.

Answer 6

These are the main advantages: . The user interface is not affected by changes in the implementation of the business logic. . The same domain logic can be used with different user interfaces. . The business logic can be tested separately from the interface logic.

Answer 7

. Name. Observer (also sometimes known as publish–subscribe like the notification architectural style which it resembles). . Purpose. When partitioning a system into individual classes you want the coupling between them to be loose so you have the flexibility to vary them independently. But a mechanism is needed to ensure that when the state of an object changes related objects are updated to keep them in step. . How it works. ◦ One object has the role of the subject (or publisher) and one or more other objects the role of observers (or subscribers). The observers register themselves with the subject and if the state of the subject changes the observers are notified and can then update themselves. ◦ There are two extreme variants of this pattern. In the push model the subject sends the observers detailed information about the change that has occurred. In the pull model the subject simply notifies the observers that there has been a change and it is the responsibility of the observers to find out the details they need to update themselves. ◦ The Subject and the Observer are interfaces that are implemented by the corresponding concrete classes. ◦ When the state of the subject changes the observer is sent an update() message. The implementation of the update() operation in the ConcreteObserver is then responsible for taking appropriate action to update the Observer. . When to use it. When different parts of a system have to be kept in step with one another without being too tightly coupled. . Example. As noted, the relationship between the view and the model in an MVC design can be realised by applying the observer pattern. The view registers with the model and is notified every time the model’s state changes, allowing it to update itself to reflect the change.

Answer 8

(a) The model is the monitor that keeps track of the free spaces. The view is the display. The controller is the software at the entry and exit that sends signals to the monitor. (b) There are two examples of the Observer pattern. The software at the entry and exit sends notifications to the monitor, which updates its count of free spaces. When the count changes the display must be notified and will then need to ask the monitor for the new count so that it can update itself. (c) From the question a product line has already been identified. The next steps are: ◦ examine existing systems – talk to developers and domain experts and analyse what the general user will need, generate a domain model ◦ from the domain model produce one or more reference architectures ◦ generate and document a repository of components that can be used with the reference architectures.

Answer 9

The Singleton pattern is used where there should only be one instance of a class. An example would be the strategy of implementing use cases by having all messages from the user interface sent to a single object that is an instance of a central class. Having more than one instance of this class at once might cause problems, for example they might interfere with one another. It’s similar to the idea that a company should only have one chief executive. . Name. Singleton. . Purpose. In many cases only a single instance of a class is required and allowing creation of more than one instance would compromise the design of the system. . How it works. ◦ The Singleton class provides no public operation for creating instances. Instead it defines a public operation getInstance() that lets clients access the unique instance of the class. ◦ One way to implement this is shown in Figure 34. The «singleton» stereotype denotes a Singleton class. Singleton is responsible for creating its own unique instance and no other class should be able to create an instance, so the create() operation is private. . When to use it. When there must be only one instance of a class. Often this is associated with some global resource that other classes need access to. . Example. The application of the singleton pattern to the design of a media manager in a multimedia application. There should be only one instance of the manager, which is created by the MediaManager class itself the first time a client accesses the manager. This strategy of creation on demand is called lazy instantiation.

Answer 10

We could use a singleton instance of HotelChain as a system object that all messages from the user interface are sent to.

Answer 11

A factory is a specialised object for creating and initialising objects needed by clients. The name is taken from real-life factories, which are facilities dedicated to manufacturing products required by their clients. In the design pattern the object the factory produces is called the product.

Answer 12

. Name. Factory. ``` . Purpose. If the creation and initialisation of an object is complex and liable to change, making clients responsible for the task introduces an undesirable level of coupling. Encapsulating the creation in a dedicated factory class hides the details from the client and reduces the coupling. ``` . How it works. ``` ◦ The client has a dependency, shown by a dashed line, on the factory for the creation of the product, and the factory depends on the Product class to create one of its instances. The UML stereotype «create» indicates that the dependency between Factory and Product is at object creation. This expresses the fact that the factory must know what class to instantiate. ``` ◦ The factory is often a singleton. Having a single instance and making it available through a system-wide access point makes it possible for all clients that require the product to use the same factory and avoid duplicated code. . When to use it. Whenever object creation and initialisation is complex or depends on information that clients may not know or is likely to change. . Example. Many applications need to use a database management system (DBMS) for storage. A DBMS is an independent program that applications communicate with using a particular protocol. A suitable object can handle the communications but needs to be created and configured correctly for the DBMS concerned. A connection factory can provide the required object without the client application needing to know any details.

Answer 13

The ones we thought of were maintainability and portability (and flexibility if it is counted as distinct from maintainability). You may have come up with others.

Answer 14

. Name. Factory method. . Purpose. Sometimes a client requires a factory for a product without knowing the actual class of the product, only its interface. ``` . How it works. ◦ The client is not aware of the actual class of the factory or the product, and interacts with them only via the interfaces. At run time the appropriate class for the factory will be decided in some way, for example from a configuration file or from a system setting, and the concrete factory instantiated. The factory will then create a product of the required class, without the client needing to know what that class is. ``` . When to use it. When the decision about what concrete product to create needs to be deferred. . Example. Consider a framework for document generation, in which applications can define application-specific documents that can be manipulated by an editor.

Answer 15

Structural

Answer 16

Behavioural

Answer 17

Creational