Design Patterns

Question 1

What is the Strategy Pattern?

Answer 1

Encapsulating algorithms into objects so that they can be swapped out. E.g. interchangable “fly” behavior objects on different types of duck (rubber, wooden, real)

One possible use of this pattern would be loading data from different sources.

Question 2

What is the Object Oriented Concept of Abstraction?

Answer 2

Unnecessary implementation details should be hidden, so that someone using a piece of code only has to consider relevant information about a thing.

Question 3

What is the Object Oriented Concept of Encapsulation?

Answer 3

Using methods to modify state, rather than just exposing state externally.

Question 4

What is the Object Oriented Concept of Polymorphism?

Answer 4

The concept that different classes can be used with the same interface. That different classes can provide their own unique implementations of the same interface.

Question 5

What is the Object Oriented Concept of Inheritance?

Answer 5

Deriving a class from a parent class. You can use this to extend or modify a parent class.

Question 6

Why should you encapsulate what varies?

Answer 6

If you hide the implementation details that may change, you keep freedom for both the “using code” and the encapsulated class to evolve independently.

Question 7

Given two class types, TypeA and TypeB, when should you favor composition over inheritance?

Answer 7

When TypeB wants only some/part of the behavior exposed by TypeA.

Question 8

Why should you program to interfaces, and not implementations?

Answer 8

So that client code is not concerned with how the implementation works. This means the interface completely states what a type can do, and makes the code more maintainable.

Question 9

Given two class types, TypeA and TypeB, when should you favor inheritance over composition?

Answer 9

When TypeB wants to expose the complete interface (all public methods, no less) of TypeA such that TypeB can be used where TypeA is expected.

Question 10

What is the push version of the Observer Pattern?

Answer 10

You have two kinds of objects, Subjects and Observers. Subjects have a list of observers that can be updated via register/remove, and can send updates to the whole list. Observers have an update method that does something with the data passed in by the Subject.

Question 11

What is the pull version of the Observer Pattern?

Answer 11

You have two kinds of objects, Subjects and Observers. Subjects have a list of observers that can be updated via register/remove, and can send updates to the whole list. Observers then pull data from the subject when their update method is called.

Question 12

In the observer pattern, can you assume that the Observers are notified in a specific order?

Answer 12

No

Question 13

What is “coupling”?

Answer 13

The degree of knowledge that one object has about the other object it interacts with.

Question 14

Why is loose coupling better?

Answer 14

It increases flexibility and ability for code to handle changes, by reducing the dependency between multiple objects.

Question 15

What is the open-closed principle?

Answer 15

Classes should be open for extension, but closed to modification

Question 16

What is the decorator pattern?

Answer 16

Where decorator objects have the same supertype as the object they decorate (wrap around). You can use one or more decorators to wrap an object. Since they have the same supertype, they can be passed around in place of the original object. The decorator adds its own behavior either before/after delegating to the object it decorates, to do the rest of the job. This provides a flexible alternative to subclassing for extending functionality.

Question 17

What is the factory method pattern?

Answer 17

The Factory Method Pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

https://en.wikipedia.org/wiki/Factory_method_pattern

Question 18

What is the dependency inversion principle?

Answer 18

Depend upon abstractions. Do not depend on concrete classes.

Question 19

What is the abstract factory pattern?

Answer 19

An AbstractFactory defines the interface that all concrete factories must implement. This is useful for having multiple concrete factories for different “kinds” of related objects without specifying their concrete classes.

Question 20

What is a Singleton?

Answer 20

A class that can only have one instantiated object of its type.

Question 21

In Java, what could cause a Singleton to be instantiated multiple times?

Answer 21

When the classes that use the singleton have different class loaders. https://stackoverflow.com/questions/39533902/java-singleton-usage-with-multiple-class-loaders

Question 22

What is the Command pattern?

Answer 22

Encapsulating a request as an object, letting you parameterize clients with different requests, queue, or log requests, or supporting undoable operations.

Question 23

What method does a Command object have?

Answer 23

execute(). it holds the code that actually does things.

Question 24

What is the Adapter pattern for?

Answer 24

Changing one interface into another interface you want.

Question 25

How is a class adapter different from an object adapter?

Answer 25

A class adapter extends the class to be adapted, and implements the target interface. An object adapter implements the target interface, but contains an instance of the class to be adapted.

Question 26

What is the Facade pattern for?

Answer 26

Providing a simpler interface for using a more complex object.

Question 27

What is the Template Method Pattern?

Answer 27

The Template Method Pattern defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.

Question 28

When you’re creating a template method, how do you know when to use abstract methods vs hooks (default implementations that can be overridden) ?

Answer 28

Use abstract methods when your subclass MUST provide an implementation of the method or step in the algorithm. Use hooks when that part of the algorithm is optional. With hooks, a subclass may choose to implement that hook, but it doesn’t have to.

Question 29

What is the Hollywood Principle?

Answer 29

High level components should call low level components, but not the other way around (Don’t call us, we’ll call you)

Question 30

Setting custom compare behavior for an object, and then a Java Array calling that compare method to sort an array, is an example of using what design pattern?

Answer 30

Template Method Pattern

Question 31

The Factory Method is a specialization of what method?

Answer 31

The Template Method

Question 32

What two methods does the Iterator pattern have?

Answer 32

hasNext(), which returns true if there is another item and next() which returns the next item

Question 33

What is the Iterator pattern?

Answer 33

The Iterator pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation

Question 34

What is the single responsibility principle?

Answer 34

A class should only have one reason to change (more responsibilities means more reasons to change)

Question 35

What is the Composite Pattern?

Answer 35

The composite pattern allows you to compose objects into tree structures to represent part/whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Question 36

What is the State pattern?

Answer 36

Have a State class with all of the actions you want to be able to take in any given state, then subclass it for the different possible states. This allows an object to alter its behavior when the internal state changes, by changing its class.

Question 37

What is the Proxy pattern?

Answer 37

The Proxy Pattern creates a representative object that controls access to another object, which may be remote, expensive to create, or need securing.

Question 38

What patterns is MVC made of?

Answer 38

Observer, Strategy, and Composite

Question 39

How does MVC use the observer pattern?

Answer 39

The model uses it to keep its observers updated but stay decoupled from them

Question 40

How does MVC use the strategy pattern?

Answer 40

The controller is the strategy for the view.

Question 41

How does MVC use the composite pattern?

Answer 41

The view uses the composite pattern to implement the user interface, which usually consists of nested components like frames, panels, and buttons