Being among the most used programming languages today, Java is applicable in the development of applications, web applications, and business software. Since it was proposed by James Gosling at Sun Microsystems in 1995, it has grown into a flexible language that is not constrained by any particular platform. Java is famous for WORA: Write Once, Run Anywhere capabilities, meaning Java applications can be executed on all systems with a Java Virtual Machine (JVM) installed.
As an interview fresher, preparing for your first job interview requires you to learn important core Java interview questions, basic Java interview questions, and Java technical interview questions that will assist you in landing your first job.
Preparing for a Java interview? For both freshers and experienced practitioners, knowing Java interview questions and answers is important. This guide compiles the most common Java interview questions of all levels, including core Java ones, OOPs, multithreading, collections, exception handling, etc. These Java technical interview questions will prepare you for your following interview for a Java developer position.
In this blog, we have consolidated 15 of the most asked Java interviews and compiled them with their answers, from basic Java concepts to different coding tasks.
Java Interview Questions for Beginners
1. What do you know about Java? Why do we consider it a platform-independent language?
Java is a high-level, object-oriented programming language, created by James Gosling in 1995 at Sun Microsystems. It is platform independent because Java programs can run on any system using Java Virtual Machine (JVM). This feature makes it possible to adhere to the WORA (Write Once, Run Anywhere) principle. Therefore, Java code can be written once and executed on multiple systems without the need for changes or edits, which increases the flexibility of Java for software engineering.
2. Expand on the meaning of JDK, JRE, and JVM.
- JDK (Java Development Kit): is a comprehensive development tool that contains JRE, a set of compilers and a set of Java application development debugging tools.
- JRE (Java Runtime Environment): A set of libraries and environment for executing Java applications, which comes with the required, but not the optional, development tools.
- JVM (Java Virtual Machine): Machine that aids a particular operating system to run Java programs. It is a major constituent that converts Java bytecode into machine code, it allows Java programs to work on any operating system supporting JVM.
3. What are the core aspects of Java?
- Java applications are able to function on different operating systems without the need for modifications to be implemented. Everything in Java is based on objects and classes. Objects and classes enables flexibility through reusability and modularity functionalities.
- Well organized and safe: Java features security components, memory allocation, and exception handling, leading to stable applications.
- Multithreaded: Java enhances performance by allowing multiple simultaneous tasks or processes to be executed.
- Automatic memory management: Java implements garbage collection to recover unused memory, thus lowering the chances of memory leaks along with the need for manual supervision.
4. What is the difference between Java and C++?
- C++ is platform dependent and needs to be compiled separately for different systems unlike Java which is platform independent due to its JVM.
- In C++ memory space has to be manually allocated and, whereas, in Java it is automatically done by garbage collection.
- Java does not support multiple class inheritance, but does for interfaces. C++ enables this with no limitation.
5. What are primitive and non-primitive data types in Java?
- Primitive types: These are basic data types like int, float, char, and boolean that store actual values directly in memory.
- Non-primitive types: These include String, Arrays, Classes, and Objects, which store references rather than actual values, allowing dynamic memory allocation.
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Core Java Interview Questions for Freshers
6. What are the four main principles of OOP in Java?
- Encapsulation– limiting the access of certain elements of an object by wrapping the data and methods together.
- Inheritance- enabling a child class to utilize the attributes and functions of its parent class.
- Polymorphism – provides flexibility through overloading and overriding a method in a class.
- Abstraction- restricting access to certain details of an object’s functionality through the use of interfaces and abstract classes.
7. What is the difference between a class and an object?
- Class: A blueprint or template that defines attributes (variables) and behaviors (methods) of objects.
- Object: An instance of a class that has its own values for attributes and can perform defined behaviors.
- Example: Car is a class with attributes like color and speed, while myCar is an object of that class with specific values.
8. Explain the difference between an interface and an abstract class.
- Abstract Class: Can contain both abstract methods (without body) and concrete methods (with implementation). It allows partial implementation.
- Interface: Contains only abstract methods (Java 8+ allows default and static methods). It supports multiple inheritance and defines a contract for classes to implement.
- Use case: Use an abstract class when methods need default behavior; use an interface for defining a common contract without implementation.
9. What is method overloading and method overriding?
- Method Overloading: Defining multiple methods with the same name but different parameters within the same class to improve code readability.
- Method Overriding: Redefining a method in a subclass that exists in the parent class to provide a specific implementation.
- Example: A printDetails() method can be overloaded with different parameter lists, while a toString() method in a subclass overrides the parent class’s toString() method.
10. What are constructors in Java, and how do they work?
A constructor is a special method used to initialize objects. It has the same name as the class and does not have a return type. Constructors in Java can be:
- Default constructor: No arguments; assigns default values.
- Parameterized constructor: Accepts arguments to initialize variables with specific values.
Advanced Java Interview Questions
11. What is the difference between HashMap and HashTable?
- HashMap: Not synchronized, allows null keys and values, and is faster in a single-threaded environment.
- HashTable: Synchronized, does not allow null keys/values, and is thread-safe but slower.
12. What is the difference between ArrayList and LinkedList?
- ArrayList: Uses dynamic arrays; faster for index-based access but slow for insertions and deletions.
- LinkedList: Uses a doubly linked list; faster for insertions/deletions but slower for indexed access.
13. What is Java Stream API?
Introduced in Java 8, the Stream API allows functional-style operations on collections. It enables operations like filtering, mapping, and reducing without modifying the original data.
14. What is a Lambda Expression in Java?
Lambda expressions (introduced in Java 8) provide a concise way to express instances of functional interfaces. They reduce boilerplate code for anonymous classes.
15. What is the Optional class in Java?
The Optional class (introduced in Java 8) is used to handle null values safely, preventing NullPointerException by providing methods like isPresent(), orElse(), and ifPresent().
Conclusion
Preparing for Java interviews requires a strong grasp of core Java concepts, coding skills, and problem-solving abilities. This blog covers 20 Java interview questions and answers for freshers, helping you ace your technical interview.
