Java OOP

Question 1

Describe what ‘Public’ means in a user defined method

Answer 1

public - access modifier. It means the method can be accessed from anywhere.

Question 2

Describe what ‘Static’ means in a user defined method

Answer 2

Static means that the method can be accessed without any objects.

Question 3

Describe what ‘Void’ means in a user defined method

Answer 3

Void means that the method does not return any value

Question 4

Describe the complete syntax of a method

Answer 4

modifier static returnType nameOfMethod (parameters) {
    // method body
}

Question 5

What are some advantages of using methods?

Answer 5

The main advantage is code reusability. We can write a method once, and use it multiple times. We do not have to rewrite the entire code each time. Think of it as, “write once, reuse multiple times

Methods make code more readable and easier to debug

Question 6

What is a constructor?

Answer 6

In Java, every class has its constructor that is invoked automatically when an object of the class is created. A constructor is similar to a method but in actual, it is not a method.

Question 7

What are the three types of constructors in Java?

Answer 7

No-Arg Constructor
Default Constructor
Parameterized Constructor

Question 8

Describe a No-Arg Constructor

Answer 8

A Java constructor may or may not have any parameters (arguments). If a constructor does not accept any parameters, it is known as a no-arg constructor. For example,

private Constructor( ) {
   // body of constructor
}

Question 9

Describe a default constructor

Answer 9

If you do not create any constructors, the Java compiler will automatically create a no-argument constructor during run-time. This constructor is known as the default constructor. The default constructor initializes any uninitialized instance variables with default values

Question 10

Describe parametrized constructor

Answer 10

Similar to methods, we can pass parameters to a constructor. Such constructors are known as a parameterized constructor. For example,

private Constructor (arg1, arg2, ..., argn) {
    // constructor body
}

Question 11

What is constructor overloading?

Answer 11

In constructor overloading, there are two or more constructors with different parameters

Question 12

Important Notes about Constructors

Answer 12

Constructors are invoked implicitly when you instantiate objects.

The two rules for creating a constructor are:
The name of the constructor should be the same as that of class.
A Java constructor must not have a return type.

Question 13

What are access modifiers?

Answer 13

In Java, access modifiers are used to set the accessibility (visibility) of classes, interfaces, variables, methods, constructors, data members, and the setter methods. For example,

class Animal {
    public void method1() {...}

private void method2() {…}
}

Question 14

There are four access modifiers keywords in Java

Answer 14

Default
Private
Protected
Public

Question 15

What is a private access modifier

Answer 15

When variables and methods are declared private, they cannot be accessed outside of the class

Question 16

What is a protected access modifier

Answer 16

When methods and data members are declared protected, we can access them within the same package as well as from subclasses

Question 17

What is a public access modifier

Answer 17

When methods, variables, classes, and so on are declared public, then we can access them from anywhere. The public access modifier has no scope restriction

Question 18

Describe ‘this’ keyword

Answer 18

In Java, this keyword is used to refer to the current object inside a method or a constructor

Question 19

What is the final keyword used for in Java

Answer 19

In Java, the final keyword is used to denote constants. It can be used with variables, methods, and classes

Question 20

What is a recursive function?

Answer 20

In Java, a method that calls itself is known as a recursive method. And, this process is known as recursion

Question 21

What is inheritance?

Answer 21

Inheritance is one of the key features of OOP (Object-oriented Programming) that allows us to define a new class from an existing class

Question 22

Describe ‘is-a’ relationship

Answer 22

Inheritance is an is-a relationship. We use inheritance only if an is-a relationship is present between the two classes.

Here are some examples:

A car is a vehicle.
Orange is a fruit.
A surgeon is a doctor.
A dog is an animal.

Question 23

Types of inheritance

Answer 23

There are five types of inheritance.

Single inheritance - Class B extends from class A only.

Multilevel inheritance - Class B extends from class A; then class C extends from class B.

Hierarchical inheritance - Class A acts as the superclass for classes B, C, and D.

Multiple inheritance - Class C extends from interfaces A and B.

Hybrid inheritance - Mix of two or more types of inheritance.

Question 24

Describe abstraction

Answer 24

Abstraction is a process where you show only “relevant” data and “hide” unnecessary details of an object from the user

Think — a coffee machine. It does a lot of stuff and makes quirky noises under the hood. But all you have to do is put in coffee and press a button

Question 25

Describe Encapsulation

Answer 25

Encapsulation is achieved when each object keeps its state private, inside a class. Other objects don’t have direct access to this state. Instead, they can only call a list of public functions — called methods

Question 26

Describe Polymorphism

Answer 26

Polymorphism is a object oriented programming feature that allows us to perform a single action in different ways

Question 27

What is an interface?

Answer 27

An interface is a blueprint of a class, which can be declared by using interface keyword. Interfaces can contain only constants and abstract methods (methods with only signatures no body).Like abstract classes, Interfaces cannot be instantiated, they can only be implemented by classes or extended by other interfaces. Interface is a common way to achieve full abstraction in Java

Question 28

What is abstraction?

Answer 28

Hiding internal details and showing functionality is known as abstraction. For example phone call, we don’t know the internal processing

Question 29

What are the four principles of object oriented programming?

Answer 29

The four principles of OOP are

Encapsulation
Abstraction
Inheritance
Polymorphism

Question 30

In terms of inheritance, what is the effect of keeping a constructor private?

Answer 30

It has direct implications for inheritance, since a subclass calls its parents constructor. The class A can be inherited, but only by its own or its parents inner classes

Question 31

In Java, does the finally block get executed if we insert a return statement inside the try block of a try-catch-fianlly?

Answer 31

Yes, it will get executed. The finally block get executed when the try block exits.