Top 21 Java Interview Questions To Ace Your Interview In 2023
Java is getting popular day by day, and Java developers are in high demand. As a result, Java interview questions are a common occurrence for Java developers. We have provided a list of 21+ Java interview questions & answers in this article.
According to a recent survey, the most common Java interview questions are about:-
- a) Object Oriented Programming (OOP)
- b) Collections and data structures
- c) Algorithms
- d) Exception handling
- e) Multithreading
The average Java developer spends about 2 hours per week preparing for Java interviews.
The most effective way to prepare for Java interviews is to practice answering common Java interview questions.
We have created a list of 21+ Java interview questions to help you prepare for an interview.
What is Java?
It’s a programming language that’s known for being high level & Object Oriented. What makes it really special is its platform independence.
When you write a Java program, it is turned into something called “Bytecode”. This bytecode can run on any device or “Operating System” that has a “Java Virtual Machine (JVM)”. It’s like having a universal translator for your programs!
This flexibility makes Java a popular choice for developers because they can write code once and run it anywhere.
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Java Inheritance Interview Questions
Q: What is inheritance in Java?
Inheritance in Java lets a class take over the traits and methods of another class. It enables code reuse and supports the concept of hierarchy in object-oriented programming. Classes A and B are considered parents and children, respectively, if class B inherits the methods and attributes of class A.
Q: Explain the concept of single inheritance in Java.
In single inheritance, a child class can have only one parent class. In other words, a Java class can extend only one other class.
Q: Can you give an example of using inheritance in Java?
Think of it this way: there’s a basic class named “Animal” that captures the shared traits and actions of all animals.
We can now create “Dog” and “Cat” classes that extend the “Animal” base class.
Additional attributes and behaviors, beyond those shared with the base class, will be added to each derived class.
Interview Questions on Constructor in Java:
Q: What is a constructor in Java?
A constructor in Java is a special method that is used to initialize objects of a class. It is called automatically when an object is created.
Q: Can a constructor have a return type?
No, a constructor in Java does not have a return type. Its purpose is to create and initialize an object, so it does not return any value explicitly.
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Q: Explain the concept of constructor chaining in Java.
Constructor chaining in Java refers to the process of calling one constructor from another constructor within the same class or from the superclass. This allows for reusing code and avoiding duplicate initialization logic. By using the “this()” keyword, constructors can be called with different arguments to handle various initialization scenarios.
JDK, JRE, JVM Interview Questions
Q: What are JDK, JVM, and JRE?
Let’s go one by one.
- The first one is JDK, which is the Java development kit software package that includes the JVM & JRE. It contains both. JDK basically provides the standard edition of the Java application programming interface.
- The second is JVM, which is the Java Virtual Machine. It basically converts the program into bytecode. Which means it converts a user-friendly syntax to machine language that the Java machine and the interpreter can understand.
- The third one would be JRE, which is the Java runtime environment. It provides the libraries and resources for JDK to execute Java programs.
Q: Does Java include pointers?
No, Java does not include pointers.
The first of the main benefits of pointers would be for memory allocation purposes. Java already has a JVM. The JVM automatically handles memory allocation, so there is no real-time use case.
Regarding side benefits or pointers, like clearing memory, Java also has this garbage collector because it just points to a specific memory for clearing memory.
So, all in all, there is no particular use case for pointers in most instances. Because Java already has some functions or features to take care of most cases,
Q: List some Java 8 date and time APIs
- LocalDate: Represents a date (year, month, day) without time or timezone information.
- LocalTime:- Represents a time (hour, minute, second, nanosecond) without date or timezone info.
- LocalDateTime: Represents a date & time without timezone information.
- ZonedDateTime: Represents a date, time, and timezone information.
- Instant: Represents an instant in time, typically used for timestamping.
- Duration: Represents a duration between two time points, such as the difference between two instants.
- Period:- Represents a duration in terms of years, months, and days.
- DateTimeFormatter: Provides formatting and parsing functionality for date and time objects.
- ChronoUnit: Enum that represents units of time, such as days, hours, minutes, etc.
- ChronoField: Enum that represents fields of date and time, such as year, month, day, etc.
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Q: what is the default method and when does it get used?
The default method actually involves an implementation, which is found in an interface. The method adds new functionalities to an interface while preserving backward compatibility with the classes that have already implemented that particular interface. That’s the main purpose of the default method in general.
Q: What are the main classes implementing the List interface in Java?
- First, the most important will be the linked list. Which are linked elements with no index.
- The second is an ArrayList, which can be indexed.
- The third is a vector, similar to an array in some aspects, and can be indexed.
- Fourth would be a stack, which follows the last-in-first-out methodology of the data structure.
Q: What is an array in Java?
An array can be referred to as a collection of items that are stored in continuous memory locations. It must contain objects of the same data type when it organizes data into a related set of values that can easily be sorted, searched, manipulated, etc.
It helps create a collection of similarly defined variables and values in a single array variable. We cannot store any other data types in an array, and all elements must be the same data type; there cannot be any mix-and-match.
Q: What is a linked list? Mention linked lists supported by Java?
A linked list is similar to an array, but it refers to a linear data structure in which the elements are not necessarily stored continuously. It is a list of nodes, where each node tells you where the next node is.
That’s how it forms a chain-like structure and how it’s different from an array.
Java supports two types of linked lists:- singly linked lists and doubly linked lists.
- Single-linked lists are those where each node stores the data and references the next node.
- A doubly linked list points to two nodes: the one before and the one after.
The only two that Java supports are these.
Q: What is recursion?
Recursion refers to a function calling itself under a particular terminating condition. It follows the last in, first out methodology and uses the stack data structure.
Q: What is the difference between system.out system.err and system.in?
“system.out” and “system.err” represent the monitor by default, which can be used to send data or results to the monitor.
- “system.out” is specifically used to display normal messages and results. It’s basically the print function of the Java family.
- “system.err” is used to display the error messages.
- “system.in” refers to the input string object. By default, it represents a standard input device, which is a keyboard in almost 99 % of cases.
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Java Garbage Collection Interview Questions:
Q: What is garbage collection in Java?
Garbage collection is a great feature in Java. It automatically frees up memory used by unused objects. No manual memory deallocation needed. It simplifies memory management and improves program performance.
Q: How does garbage collection work in Java?
A: The garbage collector in Java locates objects that the program can no longer access and releases the memory those objects were using. It uses different algorithms such as mark-and-sweep, copying, and generational collection to manage memory efficiently.
Static Keyword in Java Interview Questions:
Q: What is the significance of the “static” keyword in Java?
A: In Java, we have the “static” keyword. It’s pretty handy as it allows us to define class-level entities. These entities are shared among all instances of a class.
You can apply the “static” keyword to variables, methods, and even nested classes. It’s a way to have something that belongs to the class itself, rather than individual objects.
Q: Can you explain the difference between a static method and an instance method?
A: In Java, there are two types of methods:- static & instance methods.
- Static methods belong to the class itself. It can be called without creating any object.
- Instance methods are tied to specific versions of a class. They can only be called after an object has been made.
- Static methods provide class-level functionality, while instance methods operate on individual objects.
Understanding this distinction is crucial when designing and using classes in Java.
Abstraction in Java Interview Questions:
Q: What is abstraction in Java?
A: Abstraction in Java is a concept of hiding the implementation details and exposing only the essential features of an object or class. It allows us to create abstract classes and interfaces with abstract methods.
Q: How is abstraction different from encapsulation?
A: Abstraction and encapsulation are related concepts but serve different purposes:
- Abstraction focuses on hiding internal details and providing a simplified interface. It is achieved using abstract classes & interfaces.
- Encapsulation focuses on bundling data and methods together and restricting access to the data. It is achieved using access modifiers like private, protected, and public.
To excel in Java interviews, prepare for common topics like object-oriented programming, collections, algorithms, exception handling, and multithreading. Set aside time each week to practice and enhance your understanding of these concepts. By mastering Java interview questions, you’ll increase your chances of landing your dream job as a Java developer.
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