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Q31. How to make sure that only one instance is created in Singleton Pattern ?

Ans. public class SingleTon {
private SingleTon() {
if (singleTon != null) {
throw new RuntimeException("cant not create the object");
}
}
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException("can not be create");
}

static private volatile SingleTon singleTon;
public static SingleTon getInstance() {
SingleTon singleTon = this.sample;
if (singleTon == null) {
synchronized (this) {
singleTon = this.singleTon;
if (singleTon == null) {
singleTon = this.singleton = new SingleTon();
}
}
}
return singleTon;
}
}

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Q32. What is a factory design pattern ?

Ans. Factory Design Patterns is the pattern that recommends creation of separate Factory Object for creation of other object.

So its like saying - If you want to create an object of ClassA, Talk to FactoryObject ( which is an object of FactoryClass ). FactoryObject in itself encapsulates the inputs and logic required to make the decision regarding the creation and construction of object.

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Q33. Difference between Singleton and Factory Design Pattern ?

Ans. Both are creational design patterns but singleton facilitates in creation and reuse of single object whereas Factory deals with creation of multiple objects.

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Q34. Name few creational design patterns ?

Ans. Factory,Abstract Factory,Singleton,Prototype and Builder

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Q35. Name few structural Design Patterns ?

Ans. Adapter,Bridge,Composite,Decorator,Facade,Flyweight,Proxy

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Q36. Name few Behavioral Design Patterns ?

Ans. Interpreter,Chain of Responsibility,Command,Iterator,Observer,Mediator,Memento

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Q37. Which is your favorite Design pattern, each in Creational , Structural and Behavioral and Why ?

Ans. [Open Ended Answer]

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Q38. Which are the most commonly used Design Patterns ?

Ans. Builder ( While Writing Unit Tests )
Prototype ( Cloning )
Adapter ( asList , toString )
Chain Of Responsibility ( Logging )
Singleton
Factory ( Action Mapping )
Proxy
Observer ( Event Listener )
MVC ( Web frameworks )
Filter ( Criteria )

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Q39. Explain Interceptor Design Pattern ?

Ans. It is used for intercepting the request. It's primary use is to implement security policy. All or specific request types can be intercepted and hence forwarded to authentication / authorization module so as to facilitate authorized requests to application.

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Q40. What are the disadvantages of Adapter Design Pattern ?

Ans. It introduces a layer of adaptations before it reaches the final and desired interface.

Moreover sometimes all requests are forwarded to adapter class. Some of such requests doesn't even require any sort of adaptions as they are qualified to call the final interface directly and introducing overheads.

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Q41. What could be the real world example of Adapter Design Pattern ?

Ans. Voltage converters / Power Adapters

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Q42. What are the trade offs between public constructor and static final method ?

Ans. Public constructor is simple and easy as it's the default way of object creation. So there are no additional coding overheads as compiler provides the default constructor if none is provided by coder.

With static final methods, it facilitates loose coupling by segregating the responsibility of object creation to a separate method. Validation can be done on the constructor arguments before calling it. Moreover if any adaption on the arguments is required that can achieved easily with factory method.On the flip side, there is coding overhead and additional method call.

Sample Code for constructor

Sample Code for factory

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Q43. How one dao Implementation object (singleton) can handle multiple requests?

^{This question was recently asked at 'Capgemini'.This question is still unanswered. Can you please provide an answer.}

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Q44. Explain Facade design pattern

Ans. Single gateway of the entire application is called Facade Design Pattern.

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Q45. What is the difference between Observer and Subscriber ?

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Q46. Design a Program to clone an object and all it's children.

^{This question was recently asked at 'Bloomberg'.This question is still unanswered. Can you please provide an answer.}

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Q47. Draw uml for singleton pattern.

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Q48. What is MVC in GUI components ?

Ans. The Model/View/Controller pattern, a strategy for dividing responsibility in a GUI component.

The model is the data for the component.

The view is the visual presentation of the component on the screen.

The controller is responsible for reacting to events by changing the model.

According to the MVC pattern, these responsibilities should be handled by different objects.

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Q49. Design pattern used in struts.

Ans. Struts controller uses the Command design pattern and the action classes use the Adapter design pattern. The process() method of the RequestProcessor uses the Template method design pattern.

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Q50. Listeners are example of ..

a. Factory design Pattern

b. Abstract Factory Design Pattern

c. Singleton Design Pattern

d. Observer Design Pattern

Ans.d. Observer Design Pattern

Q51. Which of the following Design pattern has been used with Hibernate Criteria ?

a. Filter

b. Prototype

c. Builder

d. Observer

Ans.a. Filter

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