Relations with Other Patterns

Question 1

What is the relation between Factory Method and Abstract Factory and Prototype?

Answer 1

Abstract Factory classes are often based on a set of Factory Methods, but you can also use Prototype to compose the methods on these classes.

Question 2

What is the diffrence between Factory Method and Prototype?

Answer 2

You don’t need a class hierarchy to implement a prototype. You can have a working prototype right in the base class, so no inheritance is really necessary (if you don’t have subclasses). The factory method doesn’t really make sense without a class hierarchy

Question 3

What is the relation between Factory Method and Iterator?

Answer 3

You can use Factory Method along with Iterator to let collection subclasses return different types of iterators that are compatible with the collections

Question 4

What is the relation between factory Method and Template Method?

Answer 4

Factory Method is a specialization of Template Method. At the same time, a Factory Method may serve as a step in a large Template Method.

Question 5

What is the diffrence between Abstarct Factory and Builder?

Answer 5

Builder focuses on constructing complex objects step by step. Abstract Factory specializes in creating families of related objects. Abstract Factory returns the product immediately, whereas Builder lets you run some additional construction steps before fetching the product.

Question 6

Why Abstract Factory may be an alternative for Facade?

Answer 6

Abstract Factory can serve as an alternative to Facade when you only want to hide the way the subsystem objects are created from the client code.

Question 7

How you can use Abstract Factory along with Bridge?

Answer 7

You can use Abstract Factory along with Bridge. This pairing is useful when some abstractions defined by Bridge can only work with specific implementations. In this case, Abstract Factory can encapsulate these relations and hide the complexity from the client code.

Question 8

What is the relation between Abstarct Factory and Singleton?

Answer 8

Abstract Factories can all be implemented as Singletons.

Question 9

What is the relation between Builder and Composite?

Answer 9

You can use Builder when creating complex Composite trees because you can program its construction steps to work recursively.

Question 10

What is the relation between Builder and Bridge?

Answer 10

You can combine Builder with Bridge: the director class plays the role of the abstraction, while different builders act as implementations.

Question 11

What is the relation between Builder and Singleton?

Answer 11

Builders can all be implemented as Singletons.

Question 12

How Prototype can help working with Command pattern?

Answer 12

Prototype can help when you need to save copies of Commands into history

Question 13

How Prototype can help with Composite and Decorator patterns?

Answer 13

Designs that make heavy use of Composite and Decorator can often benefit from using Prototype. Applying the pattern lets you clone complex structures instead of re-constructing them from scratch.

Question 14

What is the relation between Prototype and Memento?

Answer 14

Sometimes Prototype can be a simpler alternative to Memento. This works if the object, the state of which you want to store in the history, is fairly straightforward and doesn’t have links to external resources, or the links are easy to re-establish

Question 15

What is the relation between Facade and Singleton?

Answer 15

A Facade class can often be transformed into a Singleton since a single facade object is sufficient in most cases.

Question 16

What is the diffrence between Singleton and Flyweight?

Answer 16

Flyweight would resemble Singleton if you somehow managed to reduce all shared states of the objects to just one flyweight object. But there are two fundamental differences between these patterns:

1. There should be only one Singleton instance, whereas a Flyweight class can have multiple instances with different intrinsic states.
2. The Singleton object can be mutable. Flyweight objects are immutable.

Question 17

What is the diffrence between Adapter and Bridge?

Answer 17

Bridge is usually designed up-front, letting you develop parts of an application independently of each other. On the other hand, Adapter is commonly used with an existing app to make some otherwise-incompatible classes work together nicely.

Question 18

What is the diffrence between Adapter and Decorator?

Answer 18

Adapter provides a completely different interface for accessing an existing object. On the other hand, with the Decorator pattern the interface either stays the same or gets extended. In addition, Decorator supports recursive composition, which isn’t possible when you use Adapter.

Question 19

What is the diffrence between Adapter and Proxy?

Answer 19

With Adapter you access an existing object via different interface. With Proxy, the interface stays the same.

Question 20

What is the diffrence between Adapter and Facade?

Answer 20

Facade defines a new interface for existing objects, whereas Adapter tries to make the existing interface usable. Adapter usually wraps just one object, while Facade works with an entire subsystem of objects.

Question 21

What is the relation between Composite and CoR?

Answer 21

Chain of Responsibility is often used in conjunction with Composite. In this case, when a leaf component gets a request, it may pass it through the chain of all of the parent components down to the root of the object tree.

Question 22

What is the relation between Composite and Iterator?

Answer 22

You can use Iterators to traverse Composite trees.

Question 23

What is the relation between Composite and Visitor?

Answer 23

You can use Visitor to execute an operation over an entire Composite tree

Question 24

What is the relation between Composite and Flyweight?

Answer 24

You can implement shared leaf nodes of the Composite tree as Flyweights to save some RAM.

Question 25

What is the relation between Composote and Decorator?

Answer 25

A Decorator is like a Composite but only has one child component. There’s another significant difference: Decorator adds additional responsibilities to the wrapped object, while Composite just “sums up” its children’s results.

However, the patterns can also cooperate: you can use Decorator to extend the behavior of a specific object in the Composite tree.

Question 26

What is the diffrence between Decorator and CoR?

Answer 26

Chain of Responsibility and Decorator have very similar class structures. Both patterns rely on recursive composition to pass the execution through a series of objects. However, there are several crucial differences.

The CoR handlers can execute arbitrary operations independently of each other. They can also stop passing the request further at any point. On the other hand, various Decorators can extend the object’s behavior while keeping it consistent with the base interface. In addition, decorators aren’t allowed to break the flow of the request.

Question 27

What is the diffrence between Decorator and Strategy>?

Answer 27

Decorator lets you change the skin of an object, while Strategy lets you change the guts

Question 28

What is the diffrence between Decorator and Proxy?

Answer 28

Decorator and Proxy have similar structures, but very different intents. Both patterns are built on the composition principle, where one object is supposed to delegate some of the work to another. The difference is that a Proxy usually manages the life cycle of its service object on its own, whereas the composition of Decorators is always controlled by the client.

Question 29

What is the diffrence between Facade and Flyweight?

Answer 29

Flyweight shows how to make lots of little objects, whereas Facade shows how to make a single object that represents an entire subsystem.

Question 30

What is the diffrence between Facade and Mediator?

Answer 30

Facade and Mediator have similar jobs: they try to organize collaboration between lots of tightly coupled classes.

1. Facade defines a simplified interface to a subsystem of objects, but it doesn’t introduce any new functionality. The subsystem itself is unaware of the facade. Objects within the subsystem can communicate directly.
2. Mediator centralizes communication between components of the system. The components only know about the mediator object and don’t communicate directly.

Question 31

What is the diffrence between Facade and Proxy?

Answer 31

Facade is similar to Proxy in that both buffer a complex entity and initialize it on its own. Unlike Facade, Proxy has the same interface as its service object, which makes them interchangeable.

Question 32

What is the diffrence between CoR and Command, Mediator, Observer?

Answer 32

Chain of Responsibility passes a request sequentially along a dynamic chain of potential receivers until one of them handles it.

Command establishes unidirectional connections between senders and receivers.

Mediator eliminates direct connections between senders and receivers, forcing them to communicate indirectly via a mediator object.

Observer lets receivers dynamically subscribe to and unsubscribe from receiving requests.

Question 33

What is the relation between Command and Memento?

Answer 33

You can use Command and Memento together when implementing “undo”. In this case, commands are responsible for performing various operations over a target object, while mementos save the state of that object just before a command gets executed.

Question 34

What is the diffrence between Command and Strategy?

Answer 34

Command and Strategy may look similar because you can use both to parameterize an object with some action. However, they have very different intents.

You can use Command to convert any operation into an object. The operation’s parameters become fields of that object. The conversion lets you defer execution of the operation, queue it, store the history of commands, send commands to remote services, etc.

On the other hand, Strategy usually describes different ways of doing the same thing, letting you swap these algorithms within a single context class.

Question 35

What is the relation between Command and Visitor?

Answer 35

You can treat Visitor as a powerful version of the Command pattern. Its objects can execute operations over various objects of different classes.

Question 36

What is the relation between Iterator and Memento?

Answer 36

You can use Memento along with Iterator to capture the current iteration state and roll it back if necessary.

Question 37

What is the relation between Visitor and Iterator?

Answer 37

You can use Visitor along with Iterator to traverse a complex data structure and execute some operation over its elements, even if they all have different classes.

Question 38

What is teh diffrence between Mediator and Observer?

Answer 38

The difference between Mediator and Observer is often elusive. In most cases, you can implement either of these patterns; but sometimes you can apply both simultaneously. Let’s see how we can do that.

The primary goal of Mediator is to eliminate mutual dependencies among a set of system components. Instead, these components become dependent on a single mediator object. The goal of Observer is to establish dynamic one-way connections between objects, where some objects act as subordinates of others.

There’s a popular implementation of the Mediator pattern that relies on Observer. The mediator object plays the role of publisher, and the components act as subscribers which subscribe to and unsubscribe from the mediator’s events. When Mediator is implemented this way, it may look very similar to Observer.

When you’re confused, remember that you can implement the Mediator pattern in other ways. For example, you can permanently link all the components to the same mediator object. This implementation won’t resemble Observer but will still be an instance of the Mediator pattern.

Now imagine a program where all components have become publishers, allowing dynamic connections between each other. There won’t be a centralized mediator object, only a distributed set of observers.

Question 39

What is the relation between State and Strategy?

Answer 39

State can be considered as an extension of Strategy. Both patterns are based on composition: they change the behavior of the context by delegating some work to helper objects. Strategy makes these objects completely independent and unaware of each other. However, State doesn’t restrict dependencies between concrete states, letting them alter the state of the context at will.

Question 40

What is the diffrence between Stategy and Template Method?

Answer 40

Template Method is based on inheritance: it lets you alter parts of an algorithm by extending those parts in subclasses. Strategy is based on composition: you can alter parts of the object’s behavior by supplying it with different strategies that correspond to that behavior. Template Method works at the class level, so it’s static. Strategy works on the object level, letting you switch behaviors at runtime.