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Core Java - Interview Questions and Answers for 'Oops concept' - 59 question(s) found - Order By Newest

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Frequently asked to fresh graduates and less experienced developers. Among the first few questions in many interviews.
  Q1. What are the difference between composition and inheritance in Java?Core Java
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Ans. Composition - has-a relationship between objects.
Inheritance - is-a relationship between classes.

Composition - Composing object holds a reference to composed objects and hence relationship is loosely bound.
Inheritance - Derived object carries the base class definition in itself and hence its tightly bound.

Composition - Used in Dependency Injection
Inheritance - Used in Runtime Polymorphism

Composition - Single class objects can be composed within multiple classes.
Inheritance - Single class can only inherit 1 Class.

Composition - Its the relationship between objects.
Inheritance - Its the relationship between classes.

  Sample Code for inheritance

  Sample Code for composition

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Very frequently asked to Fresh graduates.
  Q2. What is the difference between Encapsulation and Abstraction?Core Java
Anonymous
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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 Q3. What are points to consider in terms of access modifier when we are overriding any method?Core Java
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Ans. 1. Overriding method can not be more restrictive than the overridden method.

reason : in case of polymorphism , at object creation jvm look for actual runtime object. jvm does not look for reference type and while calling methods it look for overridden method.

If by means subclass were allowed to change the access modifier on the overriding method, then suddenly at runtime when the JVM invokes the true objects version of the method rather than the reference types version then it will be problematic

2. In case of subclass and superclass define in different package, we can override only those method which have public or protected access.

3. We can not override any private method because private methods can not be inherited and if method can not be inherited then method can not be overridden.

  Sample Code for Overriding

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Basic and Very Frequently asked.
  Q4. What is Polymorphism in Java ?Core Java2016-11-06 09:10:54

Ans. Polymorphism means the condition of occurring in several different forms.

Polymorphism in Java is achieved in two manners

1. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas

2. Dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding.

  Sample Code for overloading

  Sample Code for overriding

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 Q5. Why every object constructor automatically call super() in Object before its own constructors?Core Java
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Ans. Derived object carries the body of its class as well as the body of the parent class. Its body ( member elements ) is initialized using its own class constructor whereas the body ( member elements ) carried from the parent class are initialized using super class constructor. So In order to initialize the elements of the parent class before its own elements are even initialized, super is called.

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 Q6. Can you give a real world example of Encapsulation and Abstraction ?Core Java2016-05-29 13:47:31

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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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies.
  Q7. Difference between Overloading and Overriding ?Core Java
Anonymous
Ans. Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

  Sample Code for overloading

  Sample Code for overriding

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Frequently asked to fresh graduates and less experienced.
  Q8. Difference between Composition and Inheritance ?Core Java
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Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects.

In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects.

From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects.

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 Q9. How compiler handles the exceptions in overriding ?Core Java
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Ans. 1)The overriding methods can throw any runtime Exception , here in the case of runtime exception overriding method (subclass method) should not worry about exception being thrown by superclass method.

2)If superclass method does not throw any exception then while overriding, the subclass method can not throw any new checked exception but it can throw any runtime exception

3) Different exceptions in java follow some hierarchy tree(inheritance). In this case , if superclass method throws any checked exception , then while overriding the method in subclass we can not throw any new checked exception or any checked exception which are higher in hierarchy than the exception thrown in superclass method

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 Q10. Which of the following Java feature promotes access protection or Hiding ?

a. Inheritance
b. Encapsulation
c. Abstraction
d. Composition
Core Java
Anonymous
Ans. Encapsulation

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Frequently asked to fresh graduates and less experienced.
 Q11. Which of the following is tightly bound ? Inheritance or Composition ?Core Java
Anonymous
Ans. Inheritance.

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 Q12. What are the features of encapsulation ?Core Java
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Ans. Combine the data of our application and its manipulation at one place.

Encapsulation Allow the state of an object to be accessed and modified through behaviors.

Reduce the coupling of modules and increase the cohesion inside them.

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 Q13. How is Abstraction implemented in Java ?Core Java
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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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 Q14. If everything is an object , Cant we declare every object as

Object obj = new String();
Core Java2016-12-02 14:45:05

Ans. Yes, we can do that. Compiler wont complain. But using object reference we can only access methods which have been defined for object class i.e clone(), equals(), hashCode(), toString() etc.

We cannot access methods defined in String class or in any class in hierarchy between String and Object.

For example - we cannot do obj.append("abc") as it will now give compile time error.

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  Q15. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.
Core Java2017-01-09 11:45:34

Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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 Q16. Can we have multiple inheritance in java using abstract class ?Core Java2018-07-04 16:17:49

Ans. No

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 Q17. Does use of Final class enforces composition over Inheritance in Java ? Core Java2018-07-16 06:49:53

Ans. Yes, to a certain extent. But the objective for Final class could be beyond just enforcing composition as certain classes might have been created without inheritance or composition in mind.

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 Q18. What could be the reasons for restricting inheritance using final class ?Core Java2018-07-16 06:52:00

Ans. 1. Enforcing composition over inheritance
2. Restricting overriding of certain methods
3. Final methods are faster than regular instance methods
4. Enforcing Immutability

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 Q19. Difference between Encapsulation and Data Hiding ?
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Ans. Data Hiding is a broader concept. Encapsulation is a OOP's centri concept which is a way of data hiding in OOP's.

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 Q20. Difference between Abstraction and Implementation hiding ?
Anonymous
Ans. Implementation Hiding is a broader concept. Abstraction is a way of implementation hiding in OOP's.

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 Q21. What are the examples of Abstraction in Java ?
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Ans. function calling - hides implementation details
wrapper classes
new operator - Creates object in memory, calls constructor

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 Q22. Can we extend an Enum ?Core Java
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Ans. No. Enums are final by design.

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 Q23. Is this Polymorphism ?

Map<String, List<String>> inventoryManagerCountMap = new HashMap<String, ArrayList<String>>();
Core Java
Admin
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Ans. No. This will result in compilation error as Polymorphism cannot be performed on Object types.

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Must know at all levels. Among Top 10 frequently asked questions in Java. Very frequently asked to fresh graduates or less experienced professionals.
  Q24. What is Inheritance ?Core Java

Ans. Its a facility for code reuse and independent extension wherein a derived class inherits the properties of parent class.

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 Q25. How is static and dynamic polymorphism achieved in Java ?Core Java

Ans. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding.

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  Q26. How is multiple inheritance implemented in Java ?Core Java

Ans. There was no multiple inheritance in java before version 8 as implementing multiple interfaces cannot be termed as inheritance.

With Java 8 , came the concept of default methods wherein the class implementing the interface carries the default implementation from the interface and hence facilitating multiple inheritance.

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Very Frequently asked to fresh graduates and less experienced.
 Q27. What are the principle concepts of Object Oriented Programming ?Core Java2016-10-18 13:53:31

Ans. Abstraction Polymorphism Inheritance Encapsulation

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 Q28. What is multilevel inheritance ?Core Java2016-06-23 13:52:50

Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html

Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection.

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 Q29. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?Core Java2016-07-20 17:16:28

Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.

For example -

interface MyInterface {
public void default myMethod(){
}
}

interface MyInterface2 {
public void default myMethod(){
}
}

class MyClass implements MyInterface,MyInterface2 {
}

This code will compilation error "Duplicate Default Method"

if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.

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 Q30. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?Core Java2016-08-04 09:24:34

Ans. No, If both Parent and Derived are outer classes.

public class Vehicle {
private static String manufacturingDate = "2016";
}

public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}

Yes, If derived is the inner class of Parent.

public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}

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 Q31. Difference between multiple and multi level inheritance ?Core Java2016-08-13 07:51:02

Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java.

Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java.

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  Q32. What are the core OOPs concepts ?Core Java2017-02-06 16:15:55

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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 Q33. How are the concept of Association related to Composition and Inheritance ?Core Java2017-02-06 16:18:04

Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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 Q34. Which of the two - compile time and run time polymorphism - requires signature of the method to be different ?Core Java2017-05-02 08:31:29

Ans. runtime polymorphism or method overriding doesn't require method name and signature to be different whereas compile time polymorphism or method overloading requires method name to be same but the signature to be different.

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 Q35. How does java identifies which method to be called in method overriding or runtime polymorphism, when both methods share the same name and signature ? Core Java2017-05-02 08:33:52

Ans. Java identifies the method to be called at runtime by the object that is being referenced.

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 Q36. What is "dynamic method dispatch" in java ?Core Java2017-05-07 16:27:26

Ans. dynamic method dispatch is a process in which a call to an
overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java.

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 Q37. What is dynamic dispatch in Java ?Core Java2017-05-29 09:57:31

 This question is still unanswered. Can you please provide an answer.


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 Q38. What happens to private variables during inheritance ?Core Java2017-06-07 16:35:35

Ans. Private variables are not inherited. They are simply ignored and not made part of the derived object body.

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 Q39. How can we hide a class in Java ?Core Java2017-06-07 16:42:57

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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Very frequently asked. Usually among very first few questions.
 Q40. Define encapsulation in Java ?Core Java2017-06-12 07:55:47

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

  Sample Code for encapsulation

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