20 Basic Core Java Interview Questions & Answers- TCS, Accenture, Cognizant, Infosys, Wipro, HCL

Summary

In this informative video, Nishant from CareerRight delves into essential Java interview questions tailored for freshers. He covers Java's origins, its versatility across platforms, and its robust community support. The video explains Java's execution process, from source code compilation to bytecode interpretation by the JVM. Nishant also discusses access specifiers, the significance of the main method, and the nuances of static methods, constructors, and inheritance. He touches on method overloading, the final keyword's role in preventing overrides, and the organization of classes within packages. The video is a comprehensive guide for beginners looking to solidify their Java foundations.

Takeaways

• 😀 Java is a high-level, general-purpose programming language created by James Gosling in 1995 and is now owned by Oracle.
• 📝 Java supports multiple paradigms such as object-oriented, functional, and imperative, and it has a vast community support.
• 📱 Java is used for developing mobile apps, desktop applications, web applications, web servers, games, database connections, and more.
• 💻 A Java program is executed by being compiled into bytecode, which is then translated into machine code by the Java Virtual Machine (JVM).
• 🌐 Java is platform-independent because it compiles source code into platform-independent bytecode that can run on any platform with a JVM.
• 🔑 Java has four access specifiers: public, private, protected, and default, which define the accessibility of classes, interfaces, and variables.
• 🛠️ A method in Java is a block of code that performs a specific task and can take input data in the form of parameters.
• 🚀 The main method in Java is the entry point of a Java program, and it must be declared with the 'public static void' signature.
• 🔗 String[] args is a parameter in the main method that stores command-line arguments supplied when executing a Java program.
• 📚 A static method can be called without creating an object of the class, as it belongs to the class itself and not to any object.
• ⚙️ Method overloading allows multiple methods with the same name but different parameters within the same class.
• 🏭 Constructors in Java are special methods used to initialize objects and must have the same name as the class with no return type.
• 📘 Inheritance in Java allows a class (subclass) to acquire attributes and methods of another class (superclass), promoting code reusability.
• 🔒 The 'final' keyword in Java is used to prevent overriding and modifying variables or methods.
• 📦 Packages in Java are used to group related classes and avoid name conflicts, consisting of built-in packages and user-defined packages.
• 📖 An abstract class in Java is a restricted class that cannot be instantiated and can have both regular and abstract methods.

Q & A

What is Java?

Java is a high-level, general-purpose programming language created in 1995 by James Gosling and is now owned by Oracle. It is popular for being secure, fast, and powerful, supporting multiple paradigms such as object-oriented, functional, and imperative. Java is used to develop mobile applications, desktop applications, web applications, web servers, games, and database connections, and it runs on over 3 billion devices worldwide.

How is a Java program executed?

A Java program is written in a Java file as source code, then compiled into bytecode by the Java compiler. This bytecode is not directly executable by the machine; it is translated into machine code by the Java Virtual Machine (JVM), which is platform-specific, allowing the program to run on any platform that has a JVM.

How is Java platform-independent?

Java's platform independence is achieved through the use of the Java compiler, which converts source code into bytecode. This bytecode is platform-independent and can be executed on any platform using the appropriate JVM. For example, bytecode compiled on Windows can be translated and executed on Linux or macOS through their respective JVMs.

What are the various access specifiers in Java?

In Java, access specifiers define the scope or accessibility of classes, interfaces, variables, etc. There are four access specifiers: public, private, protected, and default. Public allows access from anywhere, private is accessible only within the class, protected is accessible within the same package, subclass, and within the class, and default (no specifier) is accessible within the same package.

What is a method in Java?

A method in Java is a block of code that performs a specific task and can also be called a function. Methods can take input data in the form of parameters and must be declared within a class. They are defined by a name followed by parentheses. Methods can be predefined or user-defined.

What is the significance of the main method in a Java program?

The main method is the entry point of a Java program, where execution starts. It is one of the most important methods in Java. The Java compiler or JVM looks for the main method when executing a program.

What are string arguments in the Java main method?

String arguments in the Java main method are used to handle command-line arguments supplied when executing a Java program. They are stored as an array of String objects and are necessary for the main function to be treated as the entry point of the program.

What is a static method in Java?

A static method in Java is a method that can be accessed directly by specifying its name without creating an object of the class. It belongs to the class, not to the object, and does not require any object state to be accessed.

What is method overloading in Java?

Method overloading in Java allows multiple methods to have the same name but different parameters, achieved by changing the number or data type of arguments. Overloading the main method is also possible, and the JVM will recognize the entry point by checking for the string argument parameters.

What is a constructor in Java?

A constructor in Java is a special method used to initialize an object. It must have the same name as the class and cannot have a return type. The default constructor is called every time an object is created using the 'new' keyword.

How many types of constructors are there in Java?

There are three types of constructors in Java: no-argument constructor, parameterized constructor, and default constructor. No-argument constructors have no parameters, parameterized constructors contain arguments, and default constructors are created by the compiler when no other constructor is defined.

What is the difference between a Java constructor and a Java method?

A Java constructor is a special method used to initialize an object and must have the same name as the class, with no return type. It is invoked implicitly. In contrast, a method is a block of code that performs a task, can have a return type, and may or may not have the same name as the class. Methods are invoked explicitly.

What is inheritance in Java?

Inheritance in Java is a process where a class acquires attributes and methods of another class, promoting code reusability. A subclass inherits from a superclass, gaining access to its attributes and methods.

How is inheritance implemented in a Java program?

In Java, inheritance is implemented using the 'extends' keyword, allowing a class to inherit attributes and methods from another class.

What is a final variable in Java?

A final variable in Java is a variable that cannot be changed once it has been assigned a value. It is used to prevent overriding and modification. If a method is declared as final, it cannot be overridden by any subclasses.

What is a package in Java?

A package in Java is used to group related classes and is a collection of related classes. It helps avoid name conflicts and allows for better maintainable code.

What are the different types of packages in Java?

Packages in Java are divided into built-in packages, which are pre-written and stored in the Java API, and user-defined packages, which are created by the user.

What is an abstract class in Java?

An abstract class in Java is a restricted class from which objects cannot be created directly. It can have both regular methods and abstract methods without bodies. To access its members, it must be inherited by a subclass.

Outlines

00:00

📘 Java Basics and Execution

This paragraph introduces Java as a high-level, general-purpose programming language created by James Gosling in 1995 and owned by Oracle. It highlights Java's popularity, security, speed, and power, along with its support for multiple paradigms like object-oriented, functional, and imperative. Java is extensively used for developing mobile, desktop, web applications, web servers, games, and database connections, running on over 3 billion devices globally. The execution process of a Java program involves writing source code, compiling it into bytecode, and then translating this bytecode into machine code by the Java Virtual Machine (JVM), which is platform-specific. The paragraph also explains Java's platform independence, achieved through the JVM, allowing the same bytecode to run on any platform.

05:02

🔑 Access Specifiers and Java Main Method

This section discusses the concept of access specifiers in Java, which determine the scope or accessibility of classes, interfaces, and variables within a program. There are four access specifiers: public, private, protected, and default. Public allows access from anywhere, private is accessible only within the class, protected is accessible within the same package, subclass, and the class itself, and default allows access within the same package. The paragraph also explains the structure and importance of the main method in a Java program, which serves as the entry point and must be declared with the 'public static void' signature. Additionally, it touches upon the use of the 'String[] args' parameter in the main method for handling command-line arguments.

10:04

🔍 Understanding Java Methods and Constructors

This paragraph delves into the concept of methods in Java, which are blocks of code that perform specific tasks and can take input parameters. Methods must be declared within a class and can be predefined or user-defined. The main method is highlighted as the starting point of a Java program. The discussion then shifts to static methods, which can be called without creating an object of the class and belong to the class rather than an instance. Method overloading is introduced, allowing multiple methods with the same name but different parameters. The paragraph also covers constructors, which are special methods used to initialize objects, and their types: no-argument, parameterized, and default constructors created by the compiler when none are defined.

15:05

🚀 Inheritance, Final Variables, Packages, and Abstract Classes

This section covers inheritance in Java, where a class acquires attributes and methods from another class, promoting code reusability. It explains the concepts of subclasses and superclasses, with subclasses inheriting properties from superclasses. The use of the 'extends' keyword to implement inheritance is discussed, along with the idea of final variables and methods, which prevent modification or overriding. The paragraph introduces packages in Java as a way to group related classes and avoid name conflicts, distinguishing between built-in packages provided by the Java API and user-defined packages. Lastly, it touches upon abstract classes, which cannot be instantiated and can contain both regular and abstract methods, requiring inheritance to a subclass to access their members.

Keywords

💡Java

Java is a high-level, general-purpose programming language developed in 1995 by James Gosling and is now owned by Oracle. It is designed for versatility, running on over 3 billion devices globally. Java is central to the video as it forms the basis of the discussion on programming concepts, including its applications in mobile, web, and desktop development.

💡Java Virtual Machine (JVM)

The Java Virtual Machine (JVM) is a critical component that enables Java's platform independence. It translates bytecode (the compiled Java source code) into machine code that can run on any operating system. The JVM ensures that Java code can 'write once, run anywhere,' which is a key theme in the video when explaining Java's cross-platform capabilities.

💡Bytecode

Bytecode is the intermediate code generated after compiling Java source code. It is platform-independent and can be executed on any machine with the appropriate JVM. In the video, bytecode's role is explained as an essential step that allows Java programs to run on different operating systems without modification.

💡Access Specifiers

Access specifiers define the visibility and accessibility of classes, variables, and methods in Java. The video outlines four types—public, private, protected, and default—each determining how parts of the code can be accessed within a program or across different packages. Understanding access specifiers is key for managing data encapsulation and security in Java.

💡Method

A method in Java is a block of code that performs a specific task, similar to a function in other programming languages. The video explains that methods can take parameters, return values, and must be defined within a class. Java’s 'main' method is particularly important, as it serves as the entry point for program execution.

💡Static Method

A static method is a method that belongs to the class, rather than any particular object of the class. It can be called directly without creating an instance of the class. In the video, this is highlighted as a way to avoid creating unnecessary objects when a method does not rely on instance variables.

💡Method Overloading

Method overloading allows multiple methods to have the same name but differ in the number or type of parameters. In the video, it is explained as a feature that improves code readability and reusability, showing an example where two methods share the same name but have different parameters.

💡Constructor

A constructor is a special type of method used to initialize objects. It has the same name as the class and does not return any value. The video discusses different types of constructors (no-argument, parameterized, and default), emphasizing their role in object creation and how they differ from regular methods.

💡Inheritance

Inheritance is a fundamental object-oriented programming concept where a subclass inherits properties and methods from a superclass. The video explains how inheritance promotes code reusability by allowing subclasses to extend the functionality of existing classes, such as in the example of a 'vehicle' superclass being extended by 'bus' and 'car' subclasses.

💡Final Keyword

The final keyword in Java is used to restrict the modification of variables, methods, and classes. A final variable cannot be changed once assigned, a final method cannot be overridden, and a final class cannot be inherited. This concept is introduced in the video to explain how Java ensures immutability and prevents unwanted changes to code.

Highlights

Java is a high-level, general-purpose programming language created in 1995 by James Gosling and is now owned by Oracle.

Java supports multiple paradigms such as object-oriented, functional, and imperative programming, and has huge community support.

Java programs are compiled into bytecode, which is then translated into machine code by the Java Virtual Machine (JVM) for execution.

The JVM is platform-specific, enabling Java's 'write once, run anywhere' capability by running bytecode on any platform.

There are four access specifiers in Java: public, private, protected, and default, which control the visibility and scope of classes, variables, and methods.

A method in Java is a block of code that performs a specific task, and can accept parameters for flexibility.

The Java main method serves as the entry point for a Java program's execution.

Static methods in Java can be called without creating an object of the class, whereas non-static methods require an instance of the class.

Method overloading allows multiple methods with the same name but different parameters to coexist in Java, enhancing code flexibility.

A constructor in Java is a special method used to initialize objects. It must have the same name as the class and doesn't return a value.

Java supports three types of constructors: no-argument, parameterized, and default constructors.

Inheritance in Java allows a class (subclass) to acquire attributes and methods from another class (superclass), promoting code reusability.

Final variables in Java cannot be modified once assigned, and final methods cannot be overridden by subclasses.

A package in Java is used to group related classes, helping in avoiding naming conflicts and enhancing maintainability.

Abstract classes in Java cannot be instantiated directly; they must be inherited by a subclass to access their members.

Transcripts

hello everyone it's nishant from career

right here and in today's video i'm

gonna talk about new topic

and the topic is basic code java

interview questions and answers for

freshers

so without wasting time let's come to

the concrete stuff now

alright so let's begin with the first

one what is java java is high level

general purpose programming language

created in 1995 by james gosling and is

now owned by oracle

java is one of the most popular language

in the world

and is secured fast and powerful

it supports multi paradise such as

object oriented functional imperative

etc

and has a huge community support behind

it

it is used to develop mobile application

desktop applications web applications

web servers games database connection

and many more

and runs on more than 3 billion devices

around the world

now coming to the next one how is java

program executed

a java program is written and saved in a

java file known as a source code

this source code is then passed to the

compiler

where it compiles into byte code

now this bytecode cannot be directly

executed by the machine

the bytecode is translated into machine

code first by the java virtual machine

that is jbm

and then executed by the machine

now what is this jbm

jbm is platform specific which means

every platform has its own java virtual

machine

okay so moving on to the next one how is

java platform independent

the java programming language is

platform independent which means the

code can be written once and run

anywhere

and this is achieved because of java

compiler that converts source code into

byte code

and this byte code is platform

independent and can be executed on any

platform through java virtual machine

for example if a program is written and

compiled on a windows platform

the program's byte code is first be

translated into machine code

and then can be executed on any platform

such as windows linux mac os through

their java virtual machine

now coming to the next one what are the

various access specifiers in java

excess specifiers define the scope or

accessibility of the classes

interfaces variables etc

it defines how they can be accessed from

other parts of the program

and there are four access specifiers in

java

they are public private protected and

default

now coming to public

now when defined as public the data

items and functions are accessible from

anywhere in the program

next is private

when defined as private

they are only accessible from the class

where they are defined

now next is protected

when defined as protected they are

accessible from the classes that belong

to the same package

the subclass of the class and from

within the class where they are defined

now next is default

and if you do not mention any specifier

then by default the default access

specifier is active which means they are

accessible from all the classes that

belong to the same package

okay so coming to the next one what is a

method in java

a method is a block of code that

performs a specific task

methods are also called functions

methods can also take input of the data

in the form of parameters

methods must be declared within a class

and they are defined by the name of the

method followed by a parenthesis

there are methods which are already

predefined such as

system.out.printing

and we can also define our own methods

now here is an example of our own method

now in this example you can see forward

slashes which are comments

which are ignored by the compiler but it

is very useful for the user for better

reliability of the code

okay so let's try to understand what is

java main method

in a java program the main method is a

method where the execution starts

or it is the entry point of a java

program

hence it is one of the most important

methods of java

the java compiler or java virtual

machine looks for the main method

when it starts executing a java program

now let's have an example of main method

in a java program

okay so the next one is what is string

arguments

that always appear on the java main

method

string array is used to declare a simple

string array

and argument

is the name of the string array

argument stores command line arguments

that are supplied when executing a java

program

as an area of string objects

and if we run our program from the

command line and supply arguments such

as

then argument will contain string 1 and

2.

hence it is necessary parameter for the

main function

and these parameters are absolutely

necessary in main method

in order for it to be treated as entry

point of the java program

alright so moving on to the next one

what is a static method

a method which is declared as static can

be accessed directly by specifying its

name it can be called without creating

an object of class

whereas a method declared as public

without static

can be accessed only by the creation of

an object of the class

static method belongs to the class and

not to the object

a static method doesn't require any

object state

so when you do not want to access

instance variable you can call static

methods without instantiating the object

now let's have an example of static

method

so in this example we have a class

called test

and the test class we have two methods

one is static method and other is normal

method

now come to the main method

in the main method you can see we have

called static method without creating

object of the class test

but in order to call a normal method

we have created an object of class test

and then called

a normal method

alright so moving on to the next one

what is method overloading

in method overloading multiple methods

can have same name with different

parameters

method overloading can be achieved in

two ways by changing the number of

arguments and by changing

the data type of arguments

now let's have an example over here

so in this example you can see we have

two methods with same name

but different type of arguments

now the next question is

can we overload the main method

yes we can overload the main method

we can have any number of main method

now the question arises how does the

compiler know which is the entry point

into the program if there are multiple

main methods

the compiler will check for the

parameters string argument in the main

method and will recognize it as the

entry point into the program

jbm always calls the original main

method it doesn't call the overloaded

main method

alright so coming to the next one what

is a constructor in java

a constructor in java is a special

method that is used to initialize an

object

a constructor must have the same name as

that of the class

and doesn't have a return type

and every time an object is created

using the new keyword the default

constructor is called

now let's have an example of a

constructor

so in this example you can see the

moment we create an object

it will automatically call the

constructor

now coming to the next one how many

types of constructors are used in java

now based on arguments accepted by the

constructor there are three types of

constructors

and the first one is no argument

constructor

a no argument constructor is a

constructor that doesn't have any

arguments or parameters the values are

defined within the constructor itself

next is parameterized constructor

a parameterized constructor is a

constructor that contains arguments or

parameters

next one is default constructor

a default constructor is a constructor

that is created by the compiler

when we do not define any constructor in

the program

a default constructor must not be

confused with a no argument constructor

they are not the same in java

okay so the next question is explain the

difference between java constructor and

java method

a java constructor is a special method

used to initialize an object

whereas a method is a block of code that

performs a certain task

a constructor must not have a return

type whereas a method can have a return

type

the constructor name must be same as

that of class

whereas the method name may or may not

be same as that of class

the constructor is invoked implicitly

whereas a method is invoked explicitly

now coming to the next one what is

inheritance in java

inheritance is a process where a class

acquires attributes and methods of

another class

when we inherit the attributes and

methods of an existing class we can

access all the attributes and methods of

that class in the program

and this promotes code reasonability

now the next question is what is

subclass and superclass

subclass is a class that inherits from

another class

and is also known as a child

it is derived from super class and also

inherits the properties of super class

whereas superclass is a class that is

being inherited from

and it is also known as a paret

it is a class from which many subclasses

can be created

the subclasses also inherit the

properties of a super class

let's have an example of these classes

in this example bus car truck

are all subclasses of the superclass

vehicle

now the next question is how is

inheritance implemented in a java

program

in java to inherit from a class we use

the extend keyword

let's have an example of inheritance

now in this example the class main can

use all attributes and methods of the

arithmetic class as if they were their

own

okay so coming to the next one what is a

final variable

a variable declared with the final

keyword is a final variable and is used

to prevent from overriding and modifying

so we can say a final variable once

assigned a value can never be changed

so if we initialize a variable with the

final keyword then we cannot modify its

value

and if we declare a method as final

then it cannot be overridden by any

subclasses

now the next one is what is a package in

java

a package in java is used to group

related classes or it is a collection of

related classes

packages are used to avoid name

conflicts between classes

and it allows to write a better

maintainable code

the next one is what are the different

types of packages

packages are divided into two categories

built-in packages and user-defined

packages

now built-in packages are pre-written

and are stored in the java api that are

free to use

they consist of packages for managing

input output data programming and many

more

and here are some of the commonly used

built-in packages

next is user defined packages and these

packages are created by the user

now coming to the last question in this

series what are abstract class an

abstract class is a restricted class

where we cannot create objects of the

class

to access the members of the abstract

class

it must be inherited to a subclass where

we can then access the members of the

abstract class through the object of the

subclass

an abstract class can have both regular

methods and abstract methods without its

body

you