Java OOPs

Question 1

What is OOPs?

Answer 1

OOPs (Object Oriented Programming) is a type of programming which is based on objects rather than just functions and procedures.

Individual objects are grouped into classes

Question 2

What are the main features of OOPs?

Answer 2

• Inheritance
• Encapsulation
• Polymorphism
• Abstraction

Question 3

What is an object?

Answer 3

An object is a unit of OOPs and is an instance of a class

Question 4

What is a class?

Answer 4

A class is the blueprint for an object in Java, it defines the objects behavior/state

Question 5

What is Inheritance?

Answer 5

Inheritance is the process in which the Child class acquires the properties of a Parent class using the ‘extends’ keyword

Advantage: Code reusability

Question 6

Types of Inheritance

Answer 6

Single Inheritance: One Child class extends one Parent class

Multilevel Inheritance: Class A extends Class B and Class B extends Class C

Hierarchical Inheritance: Many child classes extends one Parent class

Hybrid Inheritance: Combination of Single and Multilevel Inheritance

Question 7

Why is multiple inheritance not supported in Java?

Answer 7

If one Child class extends 2 Parent classes and both parent classes contain the same method, then the Child class would run into problems when it inherits them

Question 8

What is Encapsulation?

Answer 8

Encapsulation is the process of binding data (Variables) and code (methods) into a single unit

Advantage: Security

Ex: Java Class (methods + variables) = capsule

Question 9

How do you achieve Encapsulation?

Answer 9

Encapsulation can be achieved by:
- Creating PRIVATE variables in a class
- Providing Getter and Setter methods to make variables only accessible through those methods

Question 10

What is Abstraction?

Answer 10

Abstraction is the process of hiding the implementation and providing only the necessary data at high level to the users

Advantage: Security

Ex: ATM machine (only the service is visible, not the implementation)

Question 11

How do you achieve Abstraction in Java?

Answer 11

Abstract Class (Partial Abstraction)
Interface (Pure Abstraction)

Question 12

What is an Abstract Class?

Answer 12

A class is made abstract when only partial implementation of the class is known

It can have abstract and non abstract methods, constructors and static methods

It can not be instantiated

Question 13

What is an Interface?

Answer 13

It is a completely abstract class that is used to group related methods with empty bodies. uses the “implements” keyword in a class, which must give the empty methods their bodies

Question 14

What are the differences between Abstract Class and Interface?

Answer 14

Abstract Class
- Abstract classes can have abstract and non abstract methods
- Does not support multiple Inheritance
- Uses the keyword ‘extends’
- Ex: public abstract class Shape{public abstract void draw();};

Inheritance
- Inheritance can only have abstract methods
- Supports multiple Inheritance
- Uses the keyword ‘implements’
- Ex: public interface Drawable{void draw();};

Question 15

What is Polymorphism?

Answer 15

Polymorphism is the process in which one object is used in many forms

It occurs when we have classes that are related to each other by inheritance

Question 16

What are the types of Polymorphism?

Answer 16

Static / Compile Time Polymorphism - Method Overloading

Dynamic / Run Time Polymorphism - Method Overriding

Question 17

What is Method Overloading?

Answer 17

Same method name, but different method arguments

Advantage: Makes code simpler

Ex: void add(int a, int b); void add(int a, int b, int c);

Question 18

What is Method Overriding?

Answer 18

Same method, same arguments. It is used in Parent-Child relationship/inheritance classes

Advantage: Code reusability

Question 19

Can we overload a static method in Java?

Answer 19

Yes, you can overload a static method in Java as they are with different method signatures, this no problem will occur

Question 20

Can we override a static method in Java?

Answer 20

No, you cannot override a static method because static methods belong to a class and resolved at compile time using the type of reference variable

Question 21

Can we override a private method in Java?

Answer 21

No, since the private method is only accessible and visible inside the class they are declared

Question 22

What are the rules for method overriding in Java?

Answer 22

If the Parent class throws checked exceptions, then the Child class should throw that exception or its subtype exception

But no restriction for unchecked exceptions as both classes are independent

Question 23

Is Java 100% Object Oriented?

Answer 23

No, because primitive data types are not considered objects, the same with static classes, as they can be used without an object

Question 24

What is the difference between String, StringBuilder, and StringBuffer?

Answer 24

• A string is an object representation of char values which is immutable, internally, strings are a special type of char array
• StringBuilder and StringBuffer both create mutable strings objects, but the difference between the two is that StringBuffer is thread safe, whereas StringBuilder is not

Question 25

Why are Strings immutable?

Answer 25

Strings are immutable because string objects are cached in the string pool and since all the strings in the pool are shared between multiple clients, there is a risk that a change would effect other clients

Question 26

Does finally always execute in Java?

Answer 26

Not is System.exit() is called or the system crashes

Question 27

What methods does the Object Class have?

Answer 27

All classes are rooted in the Object class, so all have access to
- protected Object clone() throws CloneNotSupportedException
Creates and returns a copy of this object
- public boolean equals(Object obj)
Indicates whether some other object is “equal to” this one
- protected void finalize() throws Throwable
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object
- public final Class getClass()
Returns the runtime class of an object
- public int hashCode()
Returns a hash code value for the object
- public String toString()
Returns a string representation of the object

Question 28

How can you make your class immutable?

Answer 28

• Declare the class as final so it cant be extended
• Make all the fields private so that direct access is not allowed
• Dont provide setter methods for variables
• Make all mutable fields final so that its value can be assigned only once
• Initialize all the fields via a constructor performing a deep copy
• Perform cloning of objects in the getter methods to return a copy rather than returning the actual object reference

Question 29

What is a singleton class in Java and how do we make a class singleton?

Answer 29

A Singleton class is a class where only one instance can be created at any time in one JVM

Ex:
public class Animal {
private static Animal single_instance = null;
private Animal(){};
public static Animal.getInstance(){
if(single_instance == null)
single_instance = new Animal();
return single_instance;
}
}