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Core Java - Interview Questions and Answers for 'Abstract' - 40 question(s) found - Order By Newest

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Very frequently asked to Fresh graduates.
  Q1. What is the difference between Encapsulation and Abstraction?Core Java
Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level

2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world.

3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something.

4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation.

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.
  Q2. Differences between abstract class and interface ?Core Java
Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

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 Q3. Can you give a real world example of Encapsulation and Abstraction ?Core Java
Ans. Car Engine is an example of encapsulation and abstraction. You ignite the car using an interface called starter and least bothered about how the tire actually moves (This is abstraction). The engine encapsulates the complete process to itself only and doesn't allow you to start the other components like the radiator etc ( this is excapsulation )

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 Q4. Difference between Abstract and Concrete Class ?Core Java
Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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 Q5. Difference between Factory and Abstract Factory Design Pattern ?Design
Ans. Factory Pattern deals with creation of objects delegated to a separate factory class whereas Abstract Factory patterns works around a super-factory which creates other factories.

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 Q6. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java
Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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 Q7. Can we declare an abstract method private ?
Ans. No Abstract methods can only be declared protected or public.

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 Q8. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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 Q9. How is Abstraction implemented in Java ?Core Java
Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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 Q10. Can we overload abstract methods ?Core Java
Ans. Yes

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 Q11. Why can't we create an instance of abstract class ? Why is it restricted by compiler ?Core Java
Ans. Because Abstract class is incomplete i.e there are abstract methods that needs to be defined by extending classes.

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 Q12. What is abstraction ?Design
Ans. Abstraction is a process of hiding the implementation details and describing only the functionality to the user.

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 Q13. Can we have multiple inheritance in java using abstract class ?Core Java
Ans. No

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 Q14. Difference between Base Class, Derived Class , Abstract Class and Concrete Class ?Core Java
Ans. Base Class is a relative term used to signify the Parent class in Parent - Child or Base - derived relationship.

Derived Class is a relative term used to signify the Child class in Parent - Child or Base - derived relationship.

Abstract Class is a class that is not allowed to be instantiated and hence can serve only a base class for it's usage.

Concrete Class is the class which is supposed to be instantiated and hence provide definition for all implementing interface methods and extending abstract methods.

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.
  Q15. What is an abstract class ?Core Java
Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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 Q16. Difference between Abstraction and Implementation hiding ?
Ans. Implementation Hiding is a broader concept. Abstraction is a way of implementation hiding in OOP's.

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 Q17. What are the examples of Abstraction in Java ?
Ans. function calling - hides implementation details
wrapper classes
new operator - Creates object in memory, calls constructor

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 Q18. what will be the output ?

class Animal {
public void eat() throws Exception {
}
}

class Dog2 extends Animal {
public void eat(){}
public static void main(){
Animal an = new Dog2();
an.eat();
}
}
Core Java
Ans. Compile Time Error: Unhandled exception type Exception

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 Q19. Will this code Work ? If not , Why ?

java.util.Calendar c = new java.util.Calendar();
Core Java
Ans. No. It gives the error "Cannot Instantiate the type Calendar". Calendar is an abstract class and hence Calendar object should be instantiated using Calendar.getInstance().

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 Q20. Is java.util.Date an abstract Class ? Is java.util.Calendar an abstract Class ?Core Java
Ans. Date is not a abstract class whereas Calendar is.

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 Q21. Will this code compile ?

public class BuggyBread1{
abstract public void test();
}
Core Java
Ans. No. It will give the compilation error saying "The abstract method test in type BuggyBread1 can only be defined by an abstract class".

We need to declare the class abstract for it to have any abstract method.

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 Q22. Will this Code compile ?

abstract public class BuggyBread1{
abstract public void test(){};
}
Core Java
Ans. No. This will give a compilation error saying "Abstract methods do not specify a body".

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 Q23. What is ADT or Abtstract Data Type ?Core Java
Ans. ADT is a container which holds different types of objects with specifications.

For example - Stack, Array, Liked list, Tree

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 Q24. Why can't we declare a class abstract as well as final ?Core Java
Ans. Abstract means that the class is only meant to be subclassed whereas final means that it cannot be subclassed so both concepts - abstract and final are actually mutually exclusive and hence not permitted together.

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 Q25. In the following code , how many methods needs to be implemented in Class B ?

public interface A{
   public void method1();
   public void method2();
   public void method3();
}

abstract class B implements A{
}
Core Java
Ans. As Class B has been declared abstract , we can either implement any of these methods and just declare rest of them abstract.

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 Q26. What should a class do if its extending an abstract class ?Core Java
Ans. It should either implement the abstract methods or re-declare them abstract.

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 Q27. Can we declare a main method as abstract ?
Ans. No. Static methods cannot be overridden and hence make no sense to be declared abstract.

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 Q28. When should we use Abstract classes ?Core Java
Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

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 Q29. Can we override abstract methods ?Core Java
Ans. Abstract methods are the methods that need to be overridden in the derived class ( either as implementing method or again abstract method ) so it's not only allowed but its required to override abstract method in the derived class.

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 Q30. Can we override an abstract method with abstract method in the derived class ?Core Java
Ans. Yes, but in that case the derived class itself should be abstract. We cannot have an object of a class without definition for the method which has been declared abstract in the parent.

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