Top 80+ Java Interview Questions and Answers

Java is an Object-Oriented Programming language that has gained substantial traction in the marketplace due to its ability to operate across a variety of computing platforms. It aims to be beginner-friendly (easy to learn, write and understand) whilst at the same time being capable of handling large-scale projects. It also has the benefit of “write once, run anywhere” which is bytecode, that can run on any platform there is a Java Virtual Machine (JVM).

A functional interface in Java is an interface that has only one abstract method, declaring the task to be performed by the class that uses the interface. A functional interface is usually termed an Single Abstract Method (SAM) and is a target for lambda expressions, which allow short anonymous functions to be used in place of old function methods. In other words, it is a contract for one job, and the implementing classes give the details of how to do that job.

Here are some key features of Java programming language: 

a) Object-Oriented: Java promotes code reusability and modularity by treating everything as an object, encapsulating data and behaviours

b) Platform independent: Java's bytecode enables the "write once, run anywhere" philosophy, allowing code to run on any platform with a JVM

c) Secure: Strong memory management and garbage collection minimise errors like buffer overflows

d) Simple and Readable: Java's clear syntax, similar to C++, makes it easy to learn and maintain.

e) Robust: Error prevention is prioritised through compile-time and runtime checks, enhancing code reliability

f) Multithreaded: Java supports concurrent task execution using threads, efficiently handling multiple processes.

g) Rich API: A comprehensive set of libraries for networking, file handling, and graphics streamlines development.
 

The main() method in Java is predefined in JVM (Java Virtual Machine). It is the starting point of Java program. Without the main() method, JVM will not execute the program.

Syntax:

Public static void main(String args[])

Java Virtual Machine (JVM): JVM is a specification that provides a runtime environment. JVM is also called an interpreter because it is used to execute the line-by-line Java program.  

Java Run-time Environment (JRE): JRE is an installation package that gives an environment to run the java program on your machine. JRE is only used by the end-users of the system.  

Java Development Kit (JDK): JDK provides an environment to write and execute the java program. JDK contains Development toolkit and JRE. 
 

Java is not a pure Object-Oriented Programming language, because it supports primitive data types. Primitive data types contain Boolean, int, char, float, double, short, byte.

Java has five significant types of memory allocations.

1) Class Memory

2) Heap Memory

3) Stack Memory

4) Program Counter-Memory

5)  Native Method Stack Memory
 

Yes, because in Java there is no specific rule for the order of specifiers.

Neither the Local Variables nor any primitives and Object references have any default value stored in them.

In software engineering and design, an association refers to a relationship between two or more classes or objects. It describes how classes or objects are connected or interact with each other. Associations can have various characteristics such as directionality, multiplicity, and navigability.

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Aggregation is a type of association in Object-Oriented Programming where one class represents a "whole" or "container" object that contains other "part" or "component" objects. It's a form of relationship between classes that represents a "has-a" or "is-part-of" relationship. In aggregation, the contained objects can exist independently of the container object.

Aggregation is a type of association in Object-Oriented Programming where one class represents a "whole" or "container" object that contains other "part" or "component" objects. It's a form of relationship between classes that represents a "has-a" or "is-part-of" relationship. In aggregation, the contained objects can exist independently of the container object.

A Copy Constructor in Java is a constructor that initialises an object through another object of the same class.

An empty interface in Java is referred to as a Marker interface. Serialisable and Cloneable are some famous examples of Marker Interface.

Object cloning involves creating a duplicate of an existing object. It's achieved using the Cloneable interface and the clone() method.

Wrapper classes in Java are classes that encapsulate primitive data types into objects. They provide a way to treat primitive data types as objects, enabling them to be used in scenarios where objects are required.

A class that can possess only one object at a time is called a singleton class.

By following these methods a singleton class can be implemented:

1) Make sure that the class has only one object

2) Give global access to that object
 

Packages encompass classes, interfaces, and required libraries and JAR files. Their utilisation fosters code reusability by organising and grouping related components.

Instance variables live within objects, persist throughout the object's lifetime, and can be accessed by all the object's methods. Local variables exist within methods or code blocks, only live during method execution, and are specific to that method.

The Java String Pool is a special area in memory where string literals are stored. When a string literal is created, Java checks the pool. If a string with the same value exists, the new string references the existing one, promoting memory efficiency by avoiding duplicate string storage.

An exception in Java is an event that disrupts the normal flow of a program's execution due to errors or unexpected conditions.

In Java, the ‘final’ keyword is used to restrict the further modification of classes, methods, and variables. When applied to a class, it prevents inheritance. When applied to a method, it prevents overriding. When applied to a variable, it makes its value immutable once initialised. The Final Keyword in Java plays a crucial role in ensuring the integrity and immutability of these elements in your code.

The main method in Java is declared static to serve as the entry point for the program. Being static allows it to be invoked by the JVM without creating an instance of the class, ensuring consistency and enabling program execution without object instantiation.

Constructors initialise objects during creation, invoked implicitly. They lack return types and are used for object setup. 
Methods are regular functions invoked explicitly, performing specific tasks. They can have return types and define object behavior. Both can be overloaded, but only methods can be overridden for polymorphism.
 

The Java thread lifecycle consists of several states through which a thread transitions during its lifetime:

a) New: Thread is created but not yet running.

b) Runnable: Thread is ready to run and waits for resources (like CPU).

c) Running: Thread is actively executing code.

d) Blocked: Thread is waiting for an event (like I/O) and cannot proceed.

e) Terminated: Thread has finished execution or encountered an error.
 

The outer braces of the double-brace initialisation construct an anonymous class that is descended from the provided class and gives an initialiser block for that class (the inner braces).

A memory leak occurs in a computer program when it allocates memory but fails to release it after it's no longer needed, leading to a gradual depletion of available memory. Memory leaks can cause performance degradation and eventual program failure due to insufficient memory.

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‘Super’ keyword is a method used to call and access the superclass constructor. It refers to the parent object. The main purpose of this keyword is to avoid the confusion between superclass and subclass.

Java is far from being a purely dynamic programming language. It is strictly typed, meaning the types of variables are defined at compile time. Dynamic languages, like Python, determine their types at runtime, making them truly dynamic.

It is a background service thread (low priority thread) which runs in the background to perform some specific tasks. It provides services to the user thread. JVM automatically ends this thread, when all the user threads die.  

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Java Data Base Connectivity (JDBC) is an Application Programming Interface (API) for Java. It allows Java applications to connect with databases. It is used to send queries and update statements to the database,  

”OutOfMemoryError” in Java is a runtime error. It occurs when JVM is unable to allot an object due to poor space in Java heap. Also, when the Garbage Collector is unable to free up the space, the “OutofMemoryError” issue can occur. It occurs when memory is insufficient.  

“Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at OutOfMemoryErrorExample.main(OutOfMemoryErrorExample.java:8)” 
 

Java is “Pass-by-Value".

“Pass-by-Value": It means, a function (method) is called by passing a value. It makes a copy in memory of the parameter’s value. 

ClassLoader is an abstract class in Java. It is a sub system of JVM (Java Virtual Machine). It is used to load class files when a program is executing. ClassLoader performs the first process in loading the executable file. It belongs to java.lang package. ClassLoader works based on three principles: Delegation, Visibility, and Uniqueness.

Constructor is a block of code same as like methods. A constructor used to start an object. It is automatically invoked when a new object is created.

JDK stands for Java Development Kit. It provides the tools and libraries needed to develop Java applications. There's no official variant of the JDK, but Oracle offers a commercially supported version and OpenJDK is the open-source alternative.

A Just-In-Time (JIT) compiler is a performance enhancer in Java. It translates bytecode (intermediate code of the Java program) to machine code at runtime, specifically for the most frequently executed segments of the program. The compiled machine code executes faster than the repeated interpretation of bytecode.

In Java, the ‘this’ keyword is used to denote the current object instance. It is used to differentiate between instance variables and parameters, call the current instance methods and variables, and pass the current object as a parameter.

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The process of creating multiple method signatures using one method name is called Method Overloading in Java. Two ways to achieve method overloading are:

1) Varying the number of arguments

2) Changing the return type of the Method
 

Yes, there are static methods that can be overloaded in Java as well. Method overloading is a technique where multiple methods can share the same name but different parameter lists in a class

Late binding, or dynamic binding, is a programming concept where method invocation is determined at runtime based on the object's actual type. It enables polymorphic behavior, allowing different objects to respond differently to the same method call.

Dynamic method dispatch is a feature in Object-Oriented Programming where method calls are resolved at runtime based on the actual type of the object. It enables polymorphic behavior by allowing subclass methods to override superclass methods, providing flexibility and runtime flexibility.

An applet's life cycle in Java follows these key stages:

1) Initialisation: The applet is initialised, memory is allocated, and initial setup occurs when the applet is first loaded

2) Start: The applet starts, initiating animations or processes when the user enters the webpage

3) Run: The applet runs, interacting with the user and performing its core functions

4) Stop: The applet stops, pausing animations or processes when the user leaves the webpage

5) Destroy: The applet is destroyed, releasing resources and freeing memory when the user leaves the webpage or closes the browser

Generics in Java takes care of the type safety issue at the compile time, preventing the use of mismatching types at runtime. They allow one or more methods/classes to be specified indicating with which arguments these methods/classes are available. This also improves readability of code and reusability, thus making castings less likely while preventing runtime errors.

The Externalisable interface in Java is used for custom serialisation and deserialisation of objects. Classes that implement this interface must define methods for writing and reading their state to and from an external source, such as a file or network stream.

In Java, an enumeration, or enum, is a special data type used to define a set of named constants. Enums are declared using the enum keyword and are typically used to represent a fixed number of possible values for a variable, providing type safety and clarity in code.

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Servlet Session Management is used to store session data in a Web container. In this mechanism, user’s data is managed by the session tracking method. Some other methods used to manage sessions in Java, are Cookies, HTTP Session API, URL writing, Hidden Fields, etc. 

A stream is a series of objects that support different methods that can be pipelined to produce the desired result. Stream pipelining is the concept of changing operations together. It is an API used to process the set of objects.

1) Stream and data structure are not same, but the process of stream is same as data structure. Stream takes data from collections, Array and input and output channels

2) Stream provides the result based on the pipelined methods

3) Every intermediate operation is slowly executed and returns a stream as a result

JCA: Java EE Connector Architecture and J2EE Connector Architecture. It is used to connect  application servers by connecting an enterprise’s data system. 

JPA: Java Persistence API. It is the standard API for persistence and object/relational mapping for the Java EE platform. It is crucial to Java Developers for data binding purposes.  

Output:  

HashMap before sorting, random order 

Java SE 7 ==> Dolphin 

J2SE 1.2 ==> Playground 

Java SE 6 ==> Mustang 

J2SE 5.0 ==> Tiger 

J2SE 1.3 ==> Kestrel 

J2SE 1.4 ==> Merlin 

JDK 1.1.4 ==> Sparkler 

HashMap after sorting by keys in ascending order 

J2SE 1.2 ==> Playground 

J2SE 1.3 ==> Kestrel 

J2SE 1.4 ==> Merlin 

J2SE 5.0 ==> Tiger 

JDK 1.1.4 ==> Sparkler 

Java SE 6 ==> Mustang 

Java SE 7 ==> Dolphin 

HashMap after sorting entries by values 

Java SE 7 ==> Dolphin 

J2SE 1.3 ==> Kestrel 

J2SE 1.4 ==> Merlin 

Java SE 6 ==> Mustang 

J2SE 1.2 ==> Playground

 JDK 1.1.4 ==> Sparkler 

J2SE 5.0 ==> Tiger
 

Removing duplicates in an Array List using LinkedHashSet

Output: 

Console 

[1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8] 

[1, 2, 3, 4, 5, 6, 7, 8] 
 

Reversing a string using a loop in Java 

Output:

Original string: FavTutor
 
Reversed string: rotuTvaF

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JSP is an abbreviation for Java Servlet Page. The JSP page consists of two types of text.

1) Static Data 

JSP elements

Yes, the main method in Java can be overloaded. Overloading allows defining multiple methods with the same name but different parameter lists. However, only the specific signature public static void main(String[] args) serves as the entry point for the application.

The super keyword is used to access the parent class. When there exist the same fields in both parent and child classes, then a use of a super keyword is possible to access data members of a parent class.

In the case of a shallow copy, primitive data types are copied, whereas in the case of a deep copy along with primitive data types of the object references are also copied.

An abstract class can have a combination of both abstract and non-abstract methods, whereas an interface has only abstract methods in it.

Thread usage in Java includes extending the Thread class or implementing the Runnable interface.

The 'throw' keyword in Java is used to explicitly throw an exception within a method or block of code, whereas the 'throws' keyword is used in method signatures to declare the exceptions that a method may throw, delegating the responsibility of handling those exceptions to the caller.

An infinite loop can be declared in Java by breaking the logic in the instruction block. For example:
 

The main goal of using garbage collection is to free the heap memory by eliminating unnecessary objects.

Yes, all non-static methods in Java are implicitly virtual, allowing them to be overridden by subclasses.

Covariant return type in Java allows overriding a superclass method in a subclass with a return type that is a subtype of the return type declared in the superclass, providing more flexibility in method overriding.

A final variable in Java is a variable that cannot be changed once it is initialised.

A final method in Java is a method that cannot be overridden in a subclass.  

A final class in Java is a class that cannot be subclassed.

The final blank variable in Java is one not initialised when declared.

A final method in Java cannot be overridden by subclasses, providing a fixed implementation. An abstract method is declared in an abstract class or interface without implementation, requiring subclasses to provide their own implementation.