Top 50 most asked Java programming interview questions

List of Top Java Interview Questions

Here, We are listing Java interview question that is frequently asked in any job interviews as a Java developer or Java-based jobs in IT companies all over the world.

Q1 What is Java?

Java is a platform-independent and high-level, most secure, and object-oriented programming language. It was developed by Sun Microsystems.

Q2 What is the difference between JDK, JRE, and JVM?

• JDK - JDK stands for Java Development Kit.
• JRE - JRE stands for Java Runtime Environment.
• JVM - JVM stands for Java Virtual Machine that enables the computer to run the Java program.

Q3 Why Java is platform-independent?

Java is platform-independent because of its byte codes which can run on any system irrespective of its underlying operating system.

Q4 What is the principle of Oops concepts in Java?

There are four basic principles used in Java Oops concepts which are mentioned below list:

1. Inheritance
2. Encapsulation
3. Polymorphism
4. Abstraction

Q5 What is the JIT compiler?

JIT stands for Just-In-Time compiler, Just-In-Time compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.

Q6 What is this keyword in java?

"THIS" Keyword is a reference to the current object, it is used to refer current class instance variable, and also it is used to invoke the current class method.

Q7 How many features are in Java?

There are the following features listed in the Java programming language such as:

• Oops concepts
• Platform Independent
• High-Performance JIT Compiler
• Multi-threaded

Q8 What are constructors in Java?

The constructor in Java is used to refer to a block of code that is used to initialize an object. Basically, two types of constructors use in Java that mentioned below:

1. Default Constructor - Default constructors are the no-argument constructors
2. Parameterized Constructor - Initializing the instance variables with the provided values

Q9 What are the various access specifiers in Java?

• Public
• Protected
• Default
• Private

Q10 What is an object in Java?

An Object is the real-time entity, it having behavior and state. In Java, an object of a class is created by using the new keyword.

Q11 Does the constructor return any value?

The constructor implicitly returns the current instance of the class.

Q12 Why Java is considered dynamic?

Java is considered dynamic because it is designed to adapt to an evolving environment. Java programs can carry an extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.

Q13 Is the constructor inherited?

No, The constructor is not inherited.

Q14 What is an Encapsulation?

Encapsulation is a principle of Oops concepts and a technique used for hiding the properties and behaviors of an object. It allows outside access only as appropriate.

Q15 What is the number of bits used to represent ASCII, Unicode, the UTF-16, and the UTF-8 characters?

• ASCII: Used 7 bits as well as 8 bits digit patterns.
• Unicode: Used 16 bits digit patterns.
• UTF-8: Used 8, 16, and 18 bits digit patterns.
• UTF-16: Used 16 bits OR larger digital patterns.

Q16 What is a Class?

A class is the collection of variables and methods. Actually, All Java codes are written within a class.

Q17  What is Inheritance?

Inheritance means that one class can extend to another class. It is applicable to public and protected members only.

Q18 What is the difference between path and classpath variables?

• Path - It is used to find out binary files location by command prompt
• Classpath - It is used by the compiler and JVM to find out the location of library files

Q19 What is implicit casting?

Implicit casting is a process of converting lower range data type variables to higher range datatype variables.

Q20 What is Classloader?

Classloader is a subsystem of JVM which is used to load class files. Basically, Three Classloaders are built in Java.

• Bootstrap Classloader - Bootstrap Classloader is the first classloader which is the superclass of Extension classloader.
• Extension Classloader - Extension Classloader is the child classloader of Bootstrap and parent classloader of System classloader.
• System/Application Classloader - System/Application Classloader is the child classloader of Extension classloader.

Q21 What is explicit casting?

Explicit casting is a process of converting higher range data type variable to lower range datatype variable with the help of a cast operator.

Q22 What is an exception in Java?

Exceptions are unexpected events that occur at run time Java codes, basically, the exception is one that is “checked” at compile time. The exception indicates a serious problem in the code.

Q23 What are the Java IDE's?

IDE's are used to codes, mostly, NetBeans and Eclipse are two IDE's used of Java code.

Q24 What is the difference between 'system.out', 'System.err', and 'System.in'?

• 'system.out': It is used to display normal messages and results.
• System.err: It is used to represent error messages.
• System.in: It is used to display InputStream object which by default represents standard input device.

Q25 Difference between final, finally, and finalize() in Java

• The final keyword is a variable in Java. It acts as a constant, a final class is immutable, and a final method cannot be overridden in a child class.
• The 'finally' keyword is used with the try-catch block for handling exceptions & the 'finally code block' is always executed after the try or catch block is completed. It is optional in the try-catch block.
• finalize() function is the function of Object class. It is used to invoke before an object is discarded by the garbage collector. This method is called finalize().

Q26 What is Multi-threading in Java?

Multiple threads are executed simultaneously in Java & each thread starts its own stack based on the flow of the threads.

Q27 What is polymorphism in Java?

The Java polymorphism has the ability to define a function in multiple forms. There are two types of polymorphism used in Java which are:

1. Compile Time Polymorphism (Method Overloading)
2. Run Time Polymorphism (Method Overriding)

Method Overloading

When several methods have the same names but different numbers or types of parameters is called Method Overloading.

Method Overriding

A child class that implements the method with the same signature as a method in a parent class is called Method Overriding.

Q28 What are the features of the Interface?

An interface is a collection of abstract methods & here mentioned some features of Interface that are present in Java:

• An Interface cannot be instantiated.
• The interface does not contain any constructors.
• All of the methods in an interface are abstract.

Q29 Can we override the private methods?

The scope of private methods is limited to the class and we cannot access them outside of the class. So, we cannot override the private methods into other methods.