Core Java - Interview Questions and Answers for 'Object Oriented Programming' | Search Interview Question

Core Java - Interview Questions and Answers for 'Object Oriented Programming' - 44 question(s) found - Order By Newest

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

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java java concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts inheritance object oriented programming (oops) inheritance vs composition object oriented programming (oops) oops concepts Asked in 29 Companies basic frequent}

Try 5 Question(s) Test

Very frequently asked to Fresh graduates.

Q2. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops oops concepts encapsulation object oriented programming (oops) oops concepts abstraction basic interview question encapsulation vs abstraction Asked in 10 Companies basic frequent}

Try 2 Question(s) Test

Q3. What are points to consider in terms of access modifier when we are overriding any method?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java overriding access specifier inheritence oops polymorphism object oriented programming (oops) oops concepts runtime polymorphism object oriented programming (oops) oops concepts Asked in 4 Companies}

Try 2 Question(s) Test

Basic and Very Frequently asked.

Q4. What is Polymorphism in Java ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve polymorphism object oriented programming (oops) oops concepts oops concepts Asked in 108 Companies Basic frequent}

Try 2 Question(s) Test

Q5. 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 )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve encapsulation object oriented programming (oops) oops concepts abstraction oops oops features java features Asked in 13 Companies}

Q6. Why every object constructor automatically call super() in Object before its own constructors?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops constructor super inheritance object oriented programming (oops) oops concepts inheritence Asked in 1 Companies intermediate}

Q7. How compiler handles the exceptions in overriding ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java overriding exceptions inheritence inheritance object oriented programming (oops) oops concepts oops}

Frequently asked to fresh graduates and less experienced.

Q8. Difference between Composition and Inheritance ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops oops concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts difference between basic interview question Asked in 12 Companies basic frequent}

Try 2 Question(s) Test

Q9. Why Java is not considered pure OOP's language ?

Core Java

Ans. There are 2 reasons for it.

1. Usage of Primitive types - Though Java provides classes for the primitive data types but as the usage of primitives is permissible, its considered unpure OOP's language.

2. Usage of Static members - Static members belong to the class and not objects and hence not considered fit for pure OOP's programming.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve oops object oriented programming object oriented language primitive data types Asked in 2 Companies}

Frequently asked to fresh graduates and less experienced.

Q10. Which of the following is tightly bound ? Inheritance or Composition ?

Core Java

Ans. Inheritance.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve inheritance object oriented programming (oops) oops concepts java oops composition object oriented programming (oops) oops concepts Asked in 1 Companies basic frequent}

Q11. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve multiple inheritance object oriented programming (oops) oops concepts diamond problem Asked in 20 Companies basic frequent}

Q12. What are the features of encapsulation ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops oops concepts encapsulation object oriented programming (oops) oops concepts basic interview question Asked in 2 Companies basic frequent}

Q13. 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 )

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java oops concepts abstraction interfaces abstract class encapsulation object oriented programming (oops) oops concepts Asked in 3 Companies basic frequent}

Q14. If everything is an object , Cant we declare every object as

Object obj = new String();

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve object initialization object declaration runtime polymorphism object oriented programming (oops) oops concepts}

Q15. 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 Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve polymorphism object oriented programming (oops) oops concepts overloading overriding basic}

Q16. Can we have multiple inheritance in java using abstract class ?

Core Java

Ans. No

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve abstract class multiple inheritance object oriented programming (oops) oops concepts Asked in 2 Companies}

Q17. Does use of Final class enforces composition over Inheritance in Java ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve final class composition object oriented programming (oops) oops concepts inheritance object oriented programming (oops) oops concepts composition object oriented programming (oops) oops concepts vs inheritance object oriented programming (oops) oops concepts expert}

Q18. What could be the reasons for restricting inheritance using final class ?

Core Java

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

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve final class reasons for final class restricting inheritance object oriented programming (oops) oops concepts expert}

Q19. Can we extend an Enum ?

Core Java

Ans. No. Enums are final by design.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java enums inheritance object oriented programming (oops) oops concepts yes-no Asked in 1 Companies intermediate}

Q20. Is this Polymorphism ?

Map<String, List<String>> inventoryManagerCountMap = new HashMap<String, ArrayList<String>>();

Core Java

Ans. No. This will result in compilation error as Polymorphism cannot be performed on Object types.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java polymorphism object oriented programming (oops) oops concepts}

Q21. What are stateless objects ? How are they different from immutable objects ? Which of these two is thread safe ?

Object Oriented Programming

Ans. Stateless objects are the objects without instance fields (instance variables). The class may have compile time constants i.e static final fields.Immutable objects are the objects which have state but the state cannot be changed after initialization. Both are Thread safe.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java stateless objects immutable immutability objects objects singleton scope}

Q22. What are advantages of stateless and / or immutable objects ?

Object Oriented Programming

Ans. They can be shared across multiple threads as they are thread safe.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve java stateless objects immutable immutability objects objects}

Must know at all levels. Among Top 10 frequently asked questions in Java. Very frequently asked to fresh graduates or less experienced professionals.

Q23. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve inheritance object oriented programming (oops) oops concepts oops concepts java concepts code reuse code re-use classes derived classes Asked in 14 Companies basic frequent}

Q24. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve static polymorphism object oriented programming (oops) oops concepts dynamic polymorphism object oriented programming (oops) oops concepts polymorphism object oriented programming (oops) oops concepts overloading overriding broadridg Asked in 1 Companies basic frequent}

Q25. 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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve multiple inheritance object oriented programming (oops) oops concepts inheritance object oriented programming (oops) oops concepts oops concept Asked in 10 Companies frequent}

Q26. What is multilevel inheritance ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve multilevel inheritance object oriented programming (oops) oops concepts inheritance object oriented programming (oops) oops concepts oops concepts Asked in 3 Companies basic}

Try 1 Question(s) Test

Q27. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve default methods java 8 multiple inheritance object oriented programming (oops) oops concepts diamond problem interfaces}

Q28. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?

Core Java

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
}
}
}

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve private members private methods private variables inheritance object oriented programming (oops) oops concepts members visibility inner classes nexted classes basic frequent}

Q29. Difference between multiple and multi level inheritance ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve inheritance object oriented programming (oops) oops concepts multiple inheritance object oriented programming (oops) oops concepts multi level inheritance object oriented programming (oops) oops concepts Basic frequent}

Q30. How are the concept of Association related to Composition and Inheritance ?

Core Java

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.

Help us improve. Please let us know the company, where you were asked this question :

_{Like Discuss Correct / Improve composition object oriented programming (oops) oops concepts inheritance object oriented programming (oops) oops concepts association relationship between objects}

Search Interview Questions

Help us and Others Improve. Please let us know the questions asked in any of your previous interview.