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Q31. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?

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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Q32. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?

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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Q33. Difference between multiple and multi level inheritance ?

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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Q34. What are the core OOPs concepts ?

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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Q35. How are the concept of Association related to Composition and Inheritance ?

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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Q36. Which of the two - compile time and run time polymorphism - requires signature of the method to be different ?

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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Q37. What is "dynamic method dispatch" in java ?

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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Q38. What is dynamic dispatch in Java ?

Ans. Dynamic dispatch in java is also known as runtime polymorphism.It is a process in which a call to an overriden method is resolved at runtime.

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Q39. What happens to private variables during inheritance ?

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

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Q40. How can we hide a class in Java ?

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.

Q41. Define encapsulation in Java ?

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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Q42. Give an example of how Object Oriented Programming Concepts can be implemented.

Ans. You can implement encapsulation in Java by keeping the fields (class variables) private and providing public getter and setter methods to each of them. Java Beans are examples of fully encapsulated classes. Encapsulation in Java: Restricts direct access to data members (fields) of a class.

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Q43. Explain Inheritance and Polymorphism.

Ans. Getting parent class object properties into child class is called Inheritance.

The process of representing one form into multiple forms is called polymorphism.

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Q44. What are the uses of Abstraction ?

Ans. Loose Coupling
Facilitates designing complex applications by making them modular.

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Q45. Is Runtime Polymorphism possible without Inheritance ?

Ans. Yes, Runtime Polymprohism requires either Class inheritance or Interface implementation.

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Q46. how does abstraction help your application ?

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

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Q47. Explain OOps concepts.

Ans. There are four main OOP concepts in Java. These are:

Abstraction. Abstraction means using simple things to represent complexity. We all know how to turn the TV on, but we don?t need to know how it works in order to enjoy it. In Java, abstraction means simple things like objects, classes, and variables represent more complex underlying code and data. This is important because it lets avoid repeating the same work multiple times.

Encapsulation. This is the practice of keeping fields within a class private, then providing access to them via public methods. It?s a protective barrier that keeps the data and code safe within the class itself. This way, we can re-use objects like code components or variables without allowing open access to the data system-wide.

Inheritance. This is a special feature of Object Oriented Programming in Java. It lets programmers create new classes that share some of the attributes of existing classes. This lets us build on previous work without reinventing the wheel.

Polymorphism. This Java OOP concept lets programmers use the same word to mean different things in different contexts. One form of polymorphism in Java is method overloading. That?s when different meanings are implied by the code itself. The other form is method overriding. That?s when the different meanings are implied by the values of the supplied variables. See more on this below.

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Q48. What is composition ?

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Sample Code for composition

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Q49. What is Overriding ?

Ans. Overriding refers to the Methods with same name and parameters, such that the later over rides the former method.

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Q50. What are the characteristics of the object ?

Ans. 1. State

Instance variable or member elements forms the object state.

2. Behavior

Member functions forms the object behavior

3. Identity

Every object has a unique identity within JVM through a unique Id.

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Q51. Which of the OOP's feature - encapsulation or abstraction - is more important ?

Ans. Encapsulation facilitates security by hiding data and logic whereas Abstraction simplifies organization of data and related logic.

As applications scale, both concepts are required for easy management and maintenance. Encapsulation for security and criss cross communication between objects / modules will make it vulnerable. and Abstraction for better organization that enables better understanding of application code and easy maintainability.

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Q52. Which of the polymorphism type - static or runtime - is more important ?

Ans. Method overloading / static polymorphism compared to method overriding / runtime polymorphism has very limited usage as it just opens up an alternate way of defining a different method with the same name.

Method Overriding on other hand opens up many other features like contracting , interfacing , pluging development and hence development of libraries and frameworks.

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Q53. Which of the OOP's feature is most important ?

Ans. Encapsulation facilitate hiding and restricted access and hence more of a security feature. Encapsulation is definitely a great feature as when applications expand criss cross communication between objects / modules could lead to blunders.

Inheritance facilitates code reuse.

Polymorphism comprise of method overloaded ( which to me is negligible usage ) and method overriding. Method overriding is of great usage as it facilitates concept of interfaces and plugin development.

So it’s Security / Organization vs

Code Reuse / Support for other features like overriding vs

Contracts / Plugin Development facilitating the creation of frameworks / libraries.

Which is more important may vary from application to application , its scale , its use , its sensitivity etc.

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Q54. What is virtual table with respect to method overriding in Java ?

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

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Q55. How is the virtual method table implemented in Java?

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Q56. How many VTables are there for each class ?

Ans. There is one VTable for each class.

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Q57. Can a class extend itself in Java ?

Ans. No

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Q58. What is the relationship between Vehicle and Engine in this example ?

public class Vehicle {
   Enginer engine;
   public void move(){
      engine = new Engine();
      engine.start();
   }
}

a. Composition ( Vehicle has a Engine )

b. Composition ( Engine has a Vehicle )

c. Inheritance ( Vehicle is a Engine )

d. Inheritance ( Engine is a Vehicle )

Ans.a. Composition ( Vehicle has a Engine )

Q59. What is the relationship between Car and Vehicle in the following code ?

public class Car extends Vehicle{
   Engine engine;

   public static void main(String[] args){
      Vehicle vehicle = new Car();
      car.move();
   }

   public void move(){
      engine = new Engine();
      engine.start();
   }
}

a. Composition ( Vehicle has a Car )

b. Composition ( Car has a Vehicle )

c. Inheritance ( Vehicle is a Car )

d. Inheritance ( Car is a Vehicle )

Ans.d. Inheritance ( Car is a Vehicle )

Q60. What is the problem with the following code ?

public class Car extends Vehicle{
   Vehicle vehicle;

   Car(){
      super();
      this.vehicle = new Vehicle();
   }
}

a. There is an Inheritance as well as Composition relationship between Vehicle and Car which is not permitted

b. We cannot initialize the parent class instance within the constructor

c. Call to super is illegal

d. There is no problem

Ans.d. There is no problem

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