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Top 100+ Design Patterns Interview Questions And Answers - May 29, 2020

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Top 100+ Design Patterns Interview Questions And Answers

Question 1. What Are Design Patterns?

Answer :

Design patterns represent the nice practices used by experienced item-orientated software program developers. Design styles are solutions to preferred troubles that software program developers confronted all through software program development. These solutions had been acquired through trial and error by numerous software builders over quite a extensive time period.

Question 2. What Is Gang Of Four (gof)?

Answer :

In 1994, 4 authors Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides published a e book titled Design Patterns - Elements of Reusable Object-Oriented Software which initiated the concept of Design Pattern in Software improvement. These authors are collectively known as Gang of Four (GOF).

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Question 3. Name Types Of Design Patterns?

Answer :

Design styles can be classified in three categories: Creational, Structural and Behavioral patterns.

Creational Patterns - These design patterns offer a way to create objects even as hiding the creation logic, in preference to instantiating gadgets without delay the usage of new opreator. This gives software greater flexibility in deciding which gadgets need to be created for a given use case.
Structural Patterns - These layout styles subject elegance and item composition. Concept of inheritance is used to compose interfaces and define approaches to compose gadgets to gain new functionalities.
Behavioral Patterns - These layout patterns are mainly involved with conversation among gadgets.
Question four. What Are J2ee Patterns?

Answer :

These layout styles are especially involved with the presentation tier. These styles are recognized with the aid of Sun Java Center.

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Question five. What Is Factory Pattern?

Answer :

Factory sample is one of most used design pattern in Java. This form of layout pattern comes beneath creational pattern as this sample affords one of the excellent methods to create an object.

In Factory pattern, we create item with out exposing the introduction common sense to the customer and discuss with newly created object using a common interface.

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Question 6. What Is Abstract Factory Pattern?

Answer :

Abstract Factory styles paintings around a exquisite-manufacturing unit which creates different factories. This manufacturing facility is also called as manufacturing facility of factories. This form of layout pattern comes under creational pattern as this pattern offers one of the fine ways to create an item.

In Abstract Factory sample an interface is accountable for growing a manufacturing facility of related gadgets without explicitly specifying their instructions. Each generated manufacturing unit can deliver the objects as in line with the Factory sample.

Question 7. What Is Singleton Pattern?

Answer :

Singleton pattern is one of the best design styles in Java. This kind of design sample comes under creational pattern as this sample presents one of the exceptional methods to create an object.

This pattern entails a single elegance that is accountable to create an object at the same time as making sure that simplest single object gets created. This elegance presents a manner to access its only item which may be accessed without delay with out want to instantiate the object of the class.

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Question 8. How Can You Create Singleton Class In Java?

Answer :

It is  step technique. First, make the constructor non-public in order that new operator can not be used to instantiate the magnificence. Return an item of the item if no longer null otherwise create the object and return the identical through a technique.

Question nine. What Are The Difference Between A Static Class And A Singleton Class?

Answer :

Following are the differences among a static class and a singleton class.

A static class can't be a top stage elegance and can't enforce interfaces where a singleton magnificence can.
All members of a static magnificence are static however for a Singleton class it isn't always a demand.
A static elegance get initialized while it is loaded so it can not be lazily loaded in which a singleton elegance can be lazily loaded.
A static class item is saved in stack whereas singlton class object is stored in heap memory space.
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Question 10. Can We Create A Clone Of A Singleton Object?

Answer :

Yes, we are able to create a a twin of a singleton item.

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Question eleven. How To Prevent Cloning Of A Singleton Object?

Answer :

Throw exception inside the frame of clone() technique.

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Question 12. Name Some Of The Design Patterns Which Are Used In Jdk Library?

Answer :

Following are a number of the layout styles which can be utilized in JDK library.

Decorator patttern is utilized by Wrapper lessons.
Singleton pattern is utilized by Runtime, Calendar lessons.
Factory pattern is used by Wrapper elegance like Integer.ValueOf.
Observer pattern is used by event dealing with frameworks like swing, awt.
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Question thirteen. What Is The Benefit Of Factory Pattern?

Answer :

Factory sample encapsulates the implementation details and underlying implementation can be changed without any effect on caller api.

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Question 14. What Is Builder Pattern?

Answer :

Builder pattern builds a complex object the usage of easy gadgets and using a grade by grade technique. This builder is unbiased of other objects.

Question 15. What Is Prototype Pattern?

Answer :

Prototype pattern refers to growing reproduction item while keeping performance in thoughts. This sample includes implementing a prototype interface which tells to create a just like the modern object.

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Question 16. When Prototype Pattern Is To Be Used?

Answer :

This sample is used whilst creation of object directly is pricey. For instance, an object is to be created after a pricey database operation. We can cache the object, returns its clone on subsequent request and replace the database as and whilst wished as a consequence reducing database calls.

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Question 17. What Is Adapter Pattern?

Answer :

Adapter sample works as a bridge between two incompatible interfaces. This pattern involves a unmarried magnificence which is accountable to enroll in functionalities of impartial or incompatible interfaces.

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Question 18. Give An Example Of Adapter Pattern?

Answer :

A real existence example could be a case of card reader which acts as an adapter among memory card and a laptop. You plugin the memory card into card reader and card reader into the computer in order that reminiscence card can be read via laptop.

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Question 19. What Is Bridge Pattern?

Answer :

Bridge is used whilst we want to decouple an abstraction from its implementation in order that the two can range independently. This type of layout sample comes under structural pattern as this sample decouples implementation magnificence and abstract elegance by using offering a bridge structure among them.

This pattern entails an interface which acts as a bridge which makes the capability of concrete training independent from interface implementer instructions. Both kinds of classes can be altered structurally without affecting every different.

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Question 20. What Is Filter Pattern?

Answer :

Filter sample or Criteria pattern is a layout pattern that permits builders to filter a set of gadgets using different standards and chaining them in a decoupled way via logical operations. This form of design pattern comes underneath structural sample as this sample combines more than one criteria to achieve single criteria.

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Question 21. What Is Composite Pattern?

Answer :

Composite sample is used in which we need to treat a set of items in similar way as a single item. Composite sample composes gadgets in term of a tree shape to symbolize component in addition to whole hierarchy. This type of layout sample comes underneath structural sample as this sample creates a tree shape of organization of objects.

This pattern creates a category that includes institution of its personal gadgets. This class gives approaches to regulate its institution of equal items.

Question 22. What Is Decorator Pattern?

Answer :

Decorator sample allows a consumer to add new capability to an existing item with out altering its structure. This sort of layout pattern comes below structural pattern as this sample acts as a wrapper to present magnificence.

This pattern creates a decorator elegance which wraps the original magnificence and presents extra capability preserving class techniques signature intact.

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Question 23. What Is Facade Pattern?

Answer :

Facade sample hides the complexities of the gadget and affords an interface to the consumer using which the patron can get right of entry to the machine. This type of layout sample comes beneath structural sample as this pattern adds an interface to current system to cover its complexities.

This sample entails a single magnificence which affords simplified methods required via purchaser and delegates calls to methods of current machine training.

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Question 24. What Is Flyweight Pattern?

Answer :

Flyweight pattern is normally used to lessen the range of objects created and to lower reminiscence footprint and increase overall performance. This sort of design sample comes underneath structural pattern as this sample affords approaches to lower item remember as a result enhancing the object shape of utility.

Flyweight sample tries to reuse already existing comparable kind objects via storing them and creates new object when no matching item is discovered.

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Question 25. What Is Proxy Pattern?

Answer :

In proxy pattern, a class represents capability of any other magnificence. This type of layout sample comes below structural pattern.

In proxy sample, we create object having authentic item to interface its capability to outer global.

Question 26. What Is Chain Of Responsibility Pattern?

Answer :

As the call suggests, the chain of duty sample creates a sequence of receiver objects for a request. This pattern decouples sender and receiver of a request based totally on sort of request. This sample comes under behavioral styles.

In this pattern, normally each receiver includes reference to some other receiver. If one item can not cope with the request then it passes the equal to the subsequent receiver and so on.

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Question 27. What Is Command Pattern?

Answer :

Command sample is a information driven layout pattern and falls below behavioral sample class. A request is wrapped under an item as command and passed to invoker item. Invoker object appears for the appropriate object which could cope with this command and passes the command to the corresponding item which executes the command.

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Question 28. What Is Interpreter Pattern?

Answer :

Interpreter pattern presents a way to assess language grammar or expression. This type of pattern comes below behavioral pattern. This pattern entails imposing an expression interface which tells to interpret a particular context.

Question 29. Give An Example Where Interpreter Pattern Is Used?

Answer :

This pattern is utilized in SQL parsing, image processing engine etc.

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Question 30. What Is Iterator Pattern?

Answer :

Iterator pattern could be very commonly used layout pattern in Java and .Net programming surroundings. This pattern is used to get a manner to get admission to the factors of a group object in sequential manner with none need to recognise its underlying illustration. Iterator pattern falls beneath behavioral pattern class.

Question 31. What Are The Entities Of Service Locator Pattern?

Answer :

Following are the entities of this type of design sample.

Service - Actual Service to be able to system the request. Reference of such carrier is to be seemed upon in JNDI server.
Context / Initial Context - JNDI Context carries the reference to provider used for research reason.
Service Locator - Service Locator is a single point of touch to get services by means of JNDI research caching the services.
Cache - Cache to store references of offerings to reuse them.
Client - Client is the item that invokes the offerings via ServiceLocator.
Question 32. What Is Mediator Pattern?

Answer :

Mediator sample is used to reduce communique complexity among more than one items or lessons. This sample affords a mediator class which typically handles all of the communications among exceptional classes and supports clean maintenance of the code through free coupling. Mediator sample falls below behavioral pattern class.

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Question 33. What Is Memento Pattern?

Answer :

Memento pattern is used to restore kingdom of an object to a preceding state. Memento pattern falls beneath behavioral sample category.

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Question 34. Name The Actor Classes Used In Memento Pattern?

Answer :

Memento sample uses three actor instructions. Memento incorporates state of an item to be restored. Originator creates and shops states in Memento items and Caretaker item is responsible to restore item country from Memento.

Question 35. What Is Observer Pattern?

Answer :

Observer pattern is used whilst there may be one-to-many dating between objects such as if one object is modified, its depenedent gadgets are to be notified automatically. Observer pattern falls below behavioral pattern class.

Question 36. Name The Actor Classes Used In Observer Pattern?

Answer :

Observer pattern makes use of 3 actor classes. Subject, Observer and Client. Subject is an item having techniques to connect and detach observers to a customer item. We have created an abstract magnificence Observer and a concrete elegance Subject that is extending class Observer.

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Question 37. What Is State Pattern?

Answer :

In State pattern a category behavior adjustments based totally on its country. This form of layout pattern comes under behavior sample. In State sample, we create objects which represent diverse states and a context item whose conduct varies as its country item changes.

Question 38. What Is Null Object Pattern?

Answer :

In Null Object sample, a null item replaces test of NULL item example. Instead of putting if take a look at for a null fee, Null Object displays a do nothing relationship. Such Null item can also be used to provide default behaviour in case facts isn't available.

In Null Object sample, we create an summary class specifying various operations to be done, concrete instructions extending this class and a null item elegance presenting do not anything implemention of this elegance and could be used seemlessly wherein we want to test null value.

Question 39. What Is Strategy Pattern?

Answer :

In Strategy pattern, a category behavior or its set of rules can be changed at run time. This form of layout pattern comes beneath conduct pattern.

In Strategy sample, we create objects which constitute various techniques and a context item whose behavior varies as in keeping with its approach object. The method item modifications the executing set of rules of the context object.

Question forty. What Is Template Pattern?

Answer :

In Template sample, an abstract class exposes described way(s)/template(s) to execute its methods. Its subclasses can override the method implementation as consistent with need however the invocation is to be within the identical manner as described by means of an summary magnificence. This pattern comes below behavior sample category.

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Question 41. What Is Visitor Pattern?

Answer :

In Visitor pattern, we use a traveler elegance which changes the executing set of rules of an element class. By this manner, execution algorithm of detail can vary as and while traveler varies. This sample comes underneath conduct pattern class. As in line with the pattern, detail object has to simply accept the vacationer item in order that visitor item handles the operation at the detail item.

Question 42. What Is Mvc Pattern?

Answer :

MVC Pattern stands for Model-View-Controller Pattern. This pattern is used to separate software's issues.

Model - Model represents an object or JAVA POJO carrying statistics. It can also have good judgment to update controller if its records modifications.
View - View represents the visualization of the facts that version incorporates.
Controller - Controller acts on each version and view. It controls the facts flow into model object and updates the view whenever statistics modifications. It continues view and model separate.
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Question forty three. What Is Business Delegate Pattern?

Answer :

Business Delegate Pattern is used to decouple presentation tier and commercial enterprise tier. It is basically use to reduce communication or remote research capability to commercial enterprise tier code in presentation tier code. In commercial enterprise tier we've following entities.

Client - Presentation tier code can be JSP, servlet or UI java code.
Business Delegate - A single access factor magnificence for customer entities to provide get right of entry to to Business Service strategies.
LookUp Service - Lookup provider object is accountable to get relative business implementation and offer enterprise item access to enterprise delegate object.
Business Service - Business Service interface. Concrete training enforce this business service to provide actual enterprise implementation good judgment.
Question 44. What Is Composite Entity Pattern?

Answer :

Composite Entity pattern is used in EJB patience mechanism. A Composite entity is an EJB entity bean which represents a graph of objects. When a composite entity is up to date, internally structured items beans get up to date mechanically as being managed by way of EJB entity bean. Following are the contributors in Composite Entity Bean.

Composite Entity - It is primary entity bean. It may be coarse grained or can include a coarse grained item to be used for patience purpose.
Coarse-Grained Object - This object contains structured items. It has its personal existence cycle and also manages lifestyles cycle of structured gadgets.
Dependent Object - Dependent object is an object which relies upon on coarse grained object for its patience lifecycle.
Strategies - Strategies represents the way to put in force a Composite Entity.
Question 45. What Is Data Access Object Pattern(dao) Pattern?

Answer :

Data Access Object Pattern or DAO pattern is used to split low degree information gaining access to API or operations from excessive level enterprise offerings. Following are the members in Data Access Object Pattern.

Data Access Object Interface - This interface defines the same old operations to be performed on a model object(s).
Data Access Object concrete class - This magnificence implements above interface. This class is accountable to get facts from a information source which can be database / xml or another garage mechanism.
Model Object or Value Object - This object is easy POJO containing get/set strategies to save statistics retrieved using DAO class.
Question forty six. What Is Front Controller Pattern?

Answer :

The the front controller design pattern is used to provide a centralized request handling mechanism so that every one requests could be dealt with by using a single handler. This handler can do the authentication/ authorization/ logging or monitoring of request after which pass the requests to corresponding handlers. Following are the entities of this type of design pattern.

Front Controller - Single handler for all styles of requests coming to the software (either net based totally/ laptop based).
Dispatcher - Front Controller may additionally use a dispatcher item that may dispatch the request to corresponding unique handler.
View - Views are the item for which the requests are made.
Question 47. What Is Intercepting Filter Pattern?

Answer :

The intercepting clear out layout pattern is used when we need to do some pre-processing / submit-processing with request or response of the utility. Filters are defined and applied on the request earlier than passing the request to actual target software. Filters can do the authentication/ authorization/ logging or monitoring of request after which pass the requests to corresponding handlers.

Question 48. What Are The Entities Of Intercepting Filter Pattern?

Answer :

Following are the entities of this type of design sample.

Filter - Filter so as to performs certain undertaking prior or after execution of request by using request handler.
Filter Chain - Filter Chain contains more than one filters and assist to execute them in defined order on course.
Target - Target item is the request handler.
Filter Manager - Filter Manager manages the filters and Filter Chain.
Client - Client is the item who sends request to the Target object.
Question 49. What Is Service Locator Pattern?

Answer :

The provider locator design pattern is used whilst we need to locate diverse offerings using JNDI lookup. Considering excessive price of looking up JNDI for a service, Service Locator sample uses caching technique. For the primary time a carrier is needed, Service Locator looks up in JNDI and caches the provider object. Further research or equal provider via Service Locator is executed in its cache which improves the performance of software to superb quantity.

Question 50. What Is Transfer Object Pattern?

Answer :

The Transfer Object pattern is used whilst we want to bypass data with a couple of attributes in a single shot from customer to server. Transfer item is also known as Value Object. Transfer Object is a simple POJO class having getter/setter methods and is serializable in order that it may be transferred over the community.

It does now not have any behavior. Server Side enterprise magnificence normally fetches information from the database and fills the POJO and send it to the patron or bypass it via cost. For patron, switch object is study-only. Client can create its personal switch item and bypass it to server to update values in database in a single shot. Following are the entities of this kind of layout sample.

Business Object - Business Service fills the Transfer Object with records.
Transfer Object - Simple POJO having techniques to set/get attributes simplest.
Client - Client both requests or sends the Transfer Object to Business Object.




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