It is one of the most popular programming languages
today, with its role in the development of web applications, mobile apps, software,
gaming systems, as well as server-side technologies.
Java programming has been here for over 20 years now and
has been nothing less than a boon for programmers and developers. Famous as an object-oriented programming language, Java is not only multi-purpose, but also a secure,
high-performing, and trusted coding
language. This is the reason behind its usage in even enterprise-grade software development.
C and C++ programming are the base of the Java syntax.
Among the list of Java interview questions, you can be
asked why Java is called platform independent. You must be ready with the correct
answer.
The meaning of platform-independent is that you can
write a Java program on one machine and
execute it on other machines or platforms. It has become possible because of
the bytecode, and VMs can handle this
code accordingly. So, there will be no issues related to the hardware when it comes
to running the code.
Java was developed in the year 1991.
Java was developed by James Gosling. He was a computer scientist based in Canada, and is famous as the founder of Java programming. When he invented Java, he was working at Sun Microsystems, which Oracle later acquired.
Write once, run anywhere, or WORA in Java means that it
is a coding language where you write
a program for some purpose only once and then use it or run it across multiple operating systems. For instance, you can
write a program and run it on Windows, macOS,
Android, Linux, etc.
“Java’s write once, run anywhere” term was first
initiated by Sun Microsystems, where the founder of this language
used to work. This characteristic makes Java a portable programming language.
There is a
wide range of use cases of Java programming language. Below are its main applications:
● Mobile App Development
Despite the
introduction of Kotlin, Java is still used as a reliable programming language
for Android app development.
This coding language has the software development kits (SDKs) and libraries that are required to develop mobile apps.
● Chatbot
Development
Another use
of Java is in chatbot development. Smart chatbots that use natural language processing (NLP) can be built using this programming language.
● Development
of Games
One of the most important applications of Java is in building games or gaming apps. Some of the world-famous games like
Minecraft, Spiral Knights, SimCity, Saints Row 2, Asphalt 3, FIFA 11, Wakfu, Tokyo City Nights and many more are built on Java.
● Cloud
Computing
The write
once, use anywhere characteristic of Java makes it a highly applicable language for cloud applications as well.
Plenty of cloud platforms rely on this programming for a decentralized experience.
● Big
Data
Big data platforms or tools heavily depend on Java, and it is considered a language on which the future of big data relies. This
is because of its features that enable faster processing of large sets of data.
● Enterprise-grade Web Apps
Enterprise-level
apps that are mission-critical are developed using Java programming. Even top brands like Wipro, Google,
Infosys, and HCL use it to develop enterprise apps. It is because of its high performance
it enables and supports a wide range of server-side technologies.
Some of the most popular web apps built on Java include LinkedIn, IRCTC, and AliExpress.
● Internet
of Things (IoT)
In IoT
technology, sensors and hardware devices process the data. These things are
mostly run using programs written in Java programming language.
● Artificial
Intelligence (AI)
As one of the
most suitable programming languages for artificial intelligence (AI) projects, Java can be used to develop
intelligent solutions. For instance, it is great for building search algorithms, neural networks, ML-based services, deep learning applications, etc.
JVM in Java, as the name suggests, is a virtual machine that plays a crucial part in the execution of source code. It works as an abstraction layer between the runtime environment and the hardware.
JRE in Java is simply an environment that allows developers or programmers to run Java-based apps on operating
systems. You can say that it facilitates the interaction between
OS and the program.
Java JRE provides several resources to programmers, such as libraries, JVM, Java Plug-in, Web Start, etc. It is available to download on Windows, Linux, macOS, and Oracle Solaris.
Java Standard Edition, abbreviated as Java SE, is a computing platform on which programmers and developers build and deploy Java-based projects. This platform comes with plenty of Java libraries and APIs, including java.util, java.net, java.math, java.io, and many more.
Java operators are simply the symbols used to perform a
wide range of different operations. Every operator
has its specific operation or functionality.
For instance, you can use the + operator for
the addition of two values, the - operator for subtraction, the * operator for multiplication, and the /
operator for division.
You can classify the operators in Java into five categories, as mentioned below:
● Arithmetic
operators
These are used for mathematical calculations, or arithmetic operations, to be precise.
|
+ |
Addition |
|
- |
Subtraction |
|
* |
Multiplication |
|
/ |
Division |
|
% |
Modulo |
● Unary
operators
These
operators play a vital role in performing operations related to increasing or
decreasing the values.
|
Unary minus (-) |
To make a value negative |
|
Unary plus (+) |
Generally not used as values are positive by default |
|
Increment (++) |
To increase the value by 1 |
|
Decrement (--) |
To decrease the value by 1 |
|
Inverting (!) |
To inverse the value |
● Assignment
operators
These
operators, as the name suggests, are used to assign values to variables in a
Java program.
|
Operator |
Presentation |
Meaning |
|
= |
X = Y |
X = Y |
|
+= |
X += Y |
X = X + Y |
|
-= |
X -= Y |
X = X = Y |
|
*= |
X *= Y |
X = X * Y |
|
%= |
X %= |
X = X % Y |
● Relational
or comparison operators
If you want
to see the relations between values, then the relational operators in Java are used. For instance, you can check
whether the given values are equal to each other, greater/lesser than each other, greater than equal to, or lesser than equal to each other.
If the boolean values don’t meet the relational operator criteria, then accordingly, it returns
true or false.
|
Operator |
Meaning |
|
== |
Is equal to |
|
!= |
Is not equal to |
|
> |
Is greater than |
|
< |
Is lesser than |
|
>= |
Is greater than or equal to |
|
<= |
Is lesser than or equal to |
● Logical
operators
The digital electronics
field heavily depends on AND and OR gate operations. The same applies in Java for decision-making based on logical AND and OR operators.
|
Operator |
Meaning |
|
&& |
Logical AND |
|
|| |
Logical OR |
|
! |
Logical NOT |
Java JIT compiler or Just-In-Time compiler is one of the key parts of Java Runtime Environment (JRE). Its role is the compilation of bytecode to native machine code when it's in runtime. As a result, the overall performance of the Java apps is optimized
A class in Java is a template
based on some logic using
which we can create multiple
objects that follow the same logic as the class. The role of a Java
class is to define the data types and methods of the objects.
In simple terms,
you can call the class
the main category, which includes several
items called objects.
Here, the objects
under a class will have similar properties or characteristics.
For example, if you visit Amazon and browse the “Mobiles” category, it will
show you smartphones of all brands
and types. But all these smartphones will have similar properties like a camera,
some RAM, the ability to make calls,
download apps, send messages, etc.
So, here, the ‘Mobiles’ is a class,
and all the smartphones are objects.
A package in Java is like a folder that holds classes,
interfaces, and sub-packages. Here, the point to be
noted is that everything in this folder or Java package has some similarity or relation
based on their functionality.
The aim of using a Java package is to better organize
the workloads, avoid conflicts in names, and control access.
In Java, there are two types of packages:
a.
Built-in packages
b. User-defined packages
The built-in packages include java.lang, java.util,
java.io, and java.net, which can be used from
Java API. Apart from these, you can also build your own packages, which are
called user-defined packages.
Keywords in Java programming are actually some predefined words in syntax that a programmer can’t use in the form of classes, methods, identifiers, or variables. These are also known as reserved words in Java.
There are over 50 keywords
in Java. Here is the full list in alphabetical order:
1.
abstract
2. assert
3. boolean
4. break
5. byte
6. case
7. catch
8. char
9. class
10. continue
11. const
12. default
13. do
14. double
15. else
16. enum
17. exports
18. extends
19.
final
20. finally
21. float
22. for
23. goto
24. if
25. implements
26. import
27.
instanceof
28. int
29. interface
30. long
31.
module
32. native
33. new
34. package
35. private
36. protected
37. public
38. requires
39. return
40. short
41. static
42. strictfp
43. super
44. switch
45. synchronized
46. this
47. throw
48. throws
49. transient
50.
try
51. var
52. void
53. volatile
54. while
It is one of the basic Java
interview questions for freshers, and sometimes for experienced professionals as well.
Here are the top 10 features
of Java that you must know:
●
Simple, Clean, Easy to Learn
One of the best things about Java
is that it is easy to learn and understand, even for beginners. Its syntax is simple as it is based on basic
languages like C++. The code written in Java is also clean and easy to run.
●
Object-oriented Programming Language
Java is completely based on
objects, hence it is called an object-oriented programming language.
●
Java is Both Compiled and Interpreted
A programming language is
generally compiled or it is interpreted. Only rare languages exhibit
both things. Java is one such coding language that has the features
of both compilation and interpretation.
●
Java is Platform Independent
This is one of the top features of
Java programming. The meaning of platform-independent here is that you can write a Java program on one machine and
execute it on other machines or platforms. It has become
possible because of the BYTE code.
●
Portability
The portability in Java is the
result of having features like platform independence and architecture neutrality. Programmers can run Java code on a wide
range of virtual machines and hardware
because its bytecode
can be converted accordingly.
●
Robust Programming Language
The abilities of Java like garbage
collection and exception
handling make it a solid programming language.
●
Highly Secure
Security plays a crucial part
whether you are developing a basic app or a business-critical solution. On that front, Java is
considered as the most secure language as it helps in writing code that is free from viruses and other
security threats. That is the reason behind its application in enterprise-grade app development.
●
Java Multithreading Features
The multithreading feature helps in writing
code that can perform multiple
tasks simultaneously. Moreover,
the thread tasks
consume less processing power and memory.
●
Easy Interpretation
Regardless of the computer
architecture, Java programs can be run and interpreted on any type of machine.
You can call it architecture-neutral language.
●
High Performance
Instead of being an interpreted language, Java offers faster performance because of its just-in-time compiler.
As you might already know that Java is all about classes
and objects. But what is an object? This
is a crucial topic that you should include in your list of Java interview
questions and answers for freshers. Let’s know its meaning below.
An object is an instance of a class in Java. In fact, it
is created from a class only using the ‘new’
keyword. Every object has its identity, behavior, and state, the way things in
the real world also have these
three things.
There are several
differences between Java and JavaScript. Whether you are a fresher or an experienced professional, this is among
the top Java interview questions for you. Below, we have curated a tabular comparison of Java vs JavaScript so
that it becomes easier for you to understand the main differences.
|
Java |
JavaScript |
|
Object-oriented programming language |
Object-based scripting language |
|
Can be used for complicated tasks and processes |
Can’t be used for
complicated tasks |
|
Needs code
compilation |
Text-based code |
|
Independent language |
Needs to be used
with HTML |
|
Strongly typed programming language. Need to declare variables before using them in the
program. |
Loosely
typed language. No issues whether data types
are declared or not |
|
It’s statically-type |
It’s dynamically-typed |
|
High memory
consumption |
Low memory
consumption |
|
Saved as byte code |
Saved as source code |
|
For concurrency, it uses threads |
For concurrency, it uses events |
|
.java extension used to save
programs |
.js extension used to save
programs |
|
Supports multithreading |
Doesn’t support multithreading |
|
Objects are based on class |
Objects are based on prototype |
|
Need JDK or Java
Development Kit to run the code |
Need text editor to run the code |
|
Primarily used
for backend development |
Can use
for both front-end and back-end |
The object class is considered the superclass of all the remaining classes.
No. It’s not possible.
Another important Java interview questions for freshers
and experienced developers can be about the differences between
Java and C++ programming languages. To help you understand it easily, we have created
the following comparison between the both:
|
Java |
C++ |
|
Platform independent |
Platform dependent |
|
Uses compiler and interpreter both |
Compiler only |
|
Garbage controller to automate memory
management |
Manual memory
management |
|
Support for comments |
Doesn’t support comments |
|
Doesn’t support goto statement |
Supports goto
statement |
|
Developed by James Gosling |
Developed by Bjarne Stroustrup |
|
Used for various
types of development purposes, like web
apps, Windows apps, etc. |
System programming is the primary use. |
|
Supports procedural and object-oriented programming both |
Supports object-oriented programming only |
|
Limited number
of libraries |
Large number
of libraries |
|
Write once,
run anywhere |
Write once,
compile anywhere |
Java interview questions about the difference between
Java JDK, JRE, and JRM are very common. Here is the tabular comparison to help you find the right answer.
|
JDK |
JRE |
JVM |
|||
|
Java Development Kit |
Java Runtime Environment |
Java Virtual Machine |
|||
|
An SDK required to build |
A software package
that |
A virtual machine |
that |
||
|
Java-based apps. It comes |
comes with class libraries. |
makes Java |
a |
||
|
with several tools like |
Used for running Java |
platform-independent |
|
||
|
debugger, compiler, and |
projects. |
language. |
|
||
|
more. |
|
|
|
||
|
Platform-dependent. |
Platform-dependent. |
Platform-independent. |
|||
|
Mostly used |
in |
code |
Used for providing an |
Used to define
the execution |
|
|
execution |
|
during |
environment where
the code |
and supporting JRE. |
|
|
development. |
|
|
execution can
be done. |
|
|
|
JDK = JRE + Development |
JRE = JVM + Libraries |
JVM = Support JRE to load, |
|||
|
Tools |
|
verify, and
execute code |
|
|
Comes with |
various tools |
Comes with class libraries |
Doesn’t include any tools or |
|
related to |
debugging, |
and supporting files. |
a library. |
|
monitoring, |
and overall |
|
|
|
development. |
|
|
|
Not. It’s not possible.
The printIn() and print() methods
are used to print a text in Java.
●
printIn() example
public class Main {
public static void main(String[] args) { System.out.println("Hi There!");
System.out.println("Welcome to IGG
Learning!");
System.out.println("Let’s know top Java Interview Questions
and Answers!");
}
}
Output:
Hi There!
Welcome to IGG
Learning!
Let’s know the top Java Interview Questions and Answers!
●
print() example
public class Main {
public static void main(String[] args) { System.out.print("Hi There!
");
System.out.print("I can now learn the core interview questions
on Java.");
}
}
Output:
Hi There!
I can now learn the core interview questions on Java.
When a Java program is divided into multiple small
parts, and these parts are executed parallelly and run simultaneously, this process is called multithreading.
The role of Java multithreading is to create lightweight programs
or threads so that processing power can be used in an optimum
manner.
The Java Thread class is used to execute multithreading. It enables the creation of small threads which can run in a concurrent manner.
An applet in Java is used for setting up dynamic content
on a web page. You can call it a program
that is added to the page, which then runs in a browser and shows the dynamic content
to the end user.
Some of the primary benefits of a Java applet include
lower response time at the front end, secure
code, and the applet program working great on popular operating systems, like Windows,
Linux, and macOS.
It is an important concept that you must know while appearing
for the interview. This is one of the most asked Java garbage collection interview questions.
The meaning of garbage collection in Java programming is
that it automates the memory management for programs running
on the Java virtual machine.
So, when you create and run programs,
heaps of memory are allocated
for memory consumption. In the long
run, there will be several objects of the Java program that won’t be required.
What the garbage
collection does is remove the unused objects
automatically. As a result, it optimizes memory.
In case a program contains keywords as a variable, class, etc., then it will show a compile-time error.
If you have been searching for Java OOPS interview
questions and answers, then this is going
to be a top question. It is because inheritance is a crucial concept in Java
related to object-oriented programming (OOP).
In simple terms, Java inheritance means creating classes
that inherit features of some other classes.
So, if you want to build a class that has some relationship with any other
classes, then you need to use the inheritance method.
As a result, the new class that you create will have the
features of the inherited class. It is a good mechanism
to use to set up a hierarchy
between classes.
This is yet another important concept in the list of top
Java interview questions and answers related to OOPS.
The general meaning of polymorphism is “the condition of
occurring in several different forms.”
So, polymorphism in Java can be defined as the ability of a class to take
different forms or to provide
different functionalities.
For example, you can use this method to show a single
message in multiple forms, based on the set parameters.
If you have been in the field of programming for some
time now, then one of the core Java interview
questions for 3 years experience will be about encapsulation. Let’s understand
its meaning here.
Java encapsulation is a process that allows you to
integrate the data variables and code and store them as a single thing. It is like two different capsules of medicine
are mixed together
to create a new single
capsule. This is the logic behind calling
it encapsulation.
An important thing to know here
is that after the encapsulation of variables of a class, It won’t be possible
for other classes
to access these variables.
Java interview questions related to serialization are so
common to be asked when you appear for a job interview.
When there is a need for transferring an object code in
Java from one JVM to another, then the serialization process is used. Here, what it does is convert
the object code to a stream so that it can be transferred to another JVM over the internet or network.
Once the stream is received on another JVM, it then goes
through the deserialization process so that it can be converted back to the object code and brought
into use.
JDBC stands for Java Database
Connectivity.
It is an API to establish and manage a connection to the
database right from within the Java program.
Using JDBC, the developers or programmers can connect to not only one but multiple
databases.
You can say that it works like a communication channel
between the program and the database. Developers can use it to connect to any
database if the relevant drivers are in use.
Since it is not a basic topic, it is an important
concept for people looking for core Java interview questions
for experienced.
Enum in Java stands for enumeration. It is a data type
that comes with a set of pre-defined constant values. These values are separated
by a comma. The enum concept was brought to this programming as part of Java 5. For declaring
the enums, the enum keyword is used.
Another concept to keep in your list of Java interview questions and answers for experienced developers is constructor overloading.
The role of Java constructors is to define the state of
an object. When there are various constructors
of a single class to be defined, it is called constructor overloading. As a
result, a class becomes
capable of possessing multiple constructors.
It is a constructor that is used when you need to make a
copy of an object of an existing object
of the same class. Here, the new copy of the object wouldn’t impact the
existing object.
We should use the extends keyword for this purpose.
Following are the main benefits
of Java inheritance:
●
Code reusability
●
Method overriding
●
Ability to achieve runtime
polymorphism
●
Optimize duplicate code
●
Improve the redundancy of the app
●
Code flexibility so that it can be changed
easily
There are five types of memory areas that a JVM can allocate:
●
Class(Method) Area
●
Heap memory
●
Stack memory
●
Program Counter Register
●
Native Method Stack
As a programming enthusiast, you should know about the
access modifiers while preparing for the Java interview
questions and answers.
As the name suggests, the access modifiers
in Java are used to manage the access level for classes, variables, methods, constructors,
etc. The access can be changed or specified using these access modifiers.
Access modifiers are of four types:
●
Public
●
Private
●
Default
●
Protected
There are five types
of Java inheritance:
●
Single-level inheritance
●
Multi-level Inheritance
●
Hierarchical Inheritance
●
Multiple Inheritance
●
Hybrid Inheritance
Yes. It is possible if we declare the member or object
private. In such a case, the subclass can’t access the private
members directly.
First, we need to apply the stream so those duplicate
elements will be found and then make a new collection by applying Collections.toSet() method.
Two types of memory are used in the Java Virtual Machine
(JVM). One is heap memory,
and another is stack memory.
The primary difference between the two is that heap
memory’s role is to store objects, whereas stack memory stores local variables and
the order of method execution. The following tabular
comparison shows all the key differences between
the both. While preparing for Java interview questions and answers,
ensure to understand this concept well.
|
Heap Memory |
Stack Memory |
|
Used to save JRE classes and objects |
Used to save methods, variables, and reference variables |
|
Memory size
is larger |
Memory size
is small |
|
It takes more time to access
or allocate heap
memory |
It takes
less time to access or allocate stack
memory |
|
No fixed
format or order |
LIFO (Last
In First Out) order |
|
Allows changes
to the allocated memory |
Doesn’t allow
changes to the allocated memory |
|
-Xmx and -Xms are used to increase/decrease memory
size |
-Xss is used to increase memory
size |
|
Memory allocation or deallocation is done manually |
Memory allocation or deallocation is done using
compiler |
|
Shared memory
for all threads |
Dedicated memory
for every object |
|
Higher cost |
Lower cost |
If you are an experienced developer, then you must know
about the top Java compilers. Because
this is going to be one of the top interview questions on Java for experienced professionals.
Here is the list of best compilers
for Java programming:
●
Eclipse
● NetBeans
●
Xcode
●
AndroidStudio
●
Tabnine
●
Codota
●
Codenvy
●
JDeveloper
●
jGrasp
●
IntelliJ IDEA
●
BlueJ
●
MyEclipse
●
Slickedit
●
JBoss Forge
●
JEdit
There are a number of key differences between the equals
method and the equality operator in Java. The primary
difference is that one is a method,
and another is an operator.
Such tricky concepts
are usually asked
when you have some experience in this field.
So, you need to study the core Java interview
questions and answers
for experienced professionals really well.
For this question,
we have created a tabular comparison to help you understand
the differences between
the equals method and equality
operator in Java.
|
Equals() Method |
Equality Operator (==) |
|
It is a method |
It is an operator |
|
Its role
is for comparing the content of an object |
Its role
is for comparing the reference values and objects |
|
It can be overridden |
Can’t be overridden |
|
Can’t be used with
primitives |
Can be used with
primitives |
No. It can’t be done.
No. It can’t be overridden.
You must be well-prepared for such Java programming interview
questions. There are three ways to declare
an infinite loop in Java.
1. While loop
Syntax
while(condition){
//code
}
2.
For Loop
Syntax
for(initialization;condition;updation){
//code
}
3. Do-While Loop
Syntax
do{
//code
}while(condition);
There are 50+ keywords in Java, and three
similar-sounding keywords from them are final,
finally, and finalize.
Let’s understand the differences between
them with the following comparison. You must know it because it
can be one of those core Java interview questions for experienced.
|
final |
finally |
finalize |
|
Its role is to execute
limitations or restrictions on classes, methods, or variables |
Its role is in exception handling. finally keyword
runs the crucial
code no matter
whether the exception occurs or not. |
Used for processing clean up during
garbage collection. |
|
Can be applied to classes, methods, and variables |
Can be applied to exception handling cases |
Can be applied to objects |
|
After declaring the final keyword, it can’t be updated. |
Whether an exception occurs or it does not, the finally keyword will run the crucial
code. |
The cleaning of objects during garbage collection is done using the finalize keyword. |
|
This keyword is applied
only |
It gets applied once the |
It applies at |
the |
time |
of |
|
when it is called. |
execution of the try-catch |
object cleaning. |
|
|
|
|
|
block is done. |
|
|
|
|
The role of the super keyword in Java is to refer to the adjacent parent class object. It is generally used with variables, methods, and constructors. In addition to being a reference keyword for parent class objects, it can also be used to trigger the parent class methods and constructors.
A Java ClassLoader is used to load the classes in JRE in
a dynamic manner. It is an important
component in the Runtime Environment that loads the class into the memory part of the JRE.
It is because of ClassLoaders that the JRE doesn’t have to have information about the files loaded to it.
There are three ClassLoader types in Java,
as defined below:
1. BootStrap ClassLoader
Used for loading
the important classes
and internal classes
of Java Development Kit (JDK).
This ClassLoader runs only when it is called by the Java Virtual Machine
(JVM).
2. Extension ClassLoader
Used for loading
classes from the extensions directory
of the JDK. It is a child
of the BootStrap ClassLoader.
3. System ClassLoader
Also called Application ClassLoader, it is used for loading the classes from the environment variable CLASSPATH. It is a child of the Extension ClassLoader.
No. It is not possible to access the subclass members.
Only superclass members
will be accessible.
We can use the @Functionalinterface annotation in the Java 8 to define a functional interface.
We can call the functional interface a large platform
that comes with numerous expressions. The
lambda expressions are one such part of this interface. This is the
interrelation between the two.
Generally, the static method and default method are used
when a number is defined in a functional interface.
Programmers need to follow
these guidelines:
●
Only a single method
should be used to define
the interface
●
You can’t define multiple
abstracts
●
Utilize @Functionalinterface annotation in order to define a functional interface
●
To define a number,
you can use whatever method
you want to
●
In case you override
the method of java.lang.object class, it won’t be counted as an abstract method.
These are the main types of functional interface:
●
Consumer
●
Predicate
●
Supplier
●
Function (Unary Operator and Binary Operator)
The primary difference between the two is that Map wraps
the return value in the ordinal type, whereas
FlatMap doesn’t do it.
There is one big reason to go for Meta space instead of PermGen.
This reason is that the size
of PermGen is fixed. As a result, it can’t increase in a dynamic manner. On the
other hand, the Meta space does not
have any limitations in terms of size. Its size can increase dynamically.
Both composition and aggregation are associations in
Java. The former is considered a strong
association, while the latter is considered a weak association. Let’s
understand the differences between
them with the below tabular
comparison:
|
Aggregation |
Composition |
|
Weak |
Strong |
|
There is a relationship between classes |
A class belongs to another class |
|
Interrelated classes can be independent |
Classes are
dependent on each
other. |
|
As the classes can be independent, it is great
for reusing the code |
As the classes are not independent, code reusability becomes
difficult |