Typical java interview questions sorted by experience


Junior
* Name some of the characteristics of OO programming languages
* What are the access modifiers you know? What does each one do?
* What is the difference between overriding and overloading a method in Java?
* What’s the difference between an Interface and an abstract class?
* Can an Interface extend another Interface?
* What does the static word mean in Java?
* Can a static method be overridden in Java?
* What is Polymorphism? What about Inheritance?
* Can a constructor be inherited?
* Do objects get passed by reference or value in Java? Elaborate on that.
* What’s the difference between using == and .equals on a string?
* What is the hashCode() and equals() used for?
* What does the interface Serializable do? What about Parcelable in Android?
* Why are Array and ArrayList different? When would you use each?
* What’s the difference between an Integer and int?
* What is a ThreadPool? Is it better than using several “simple” threads?
* What the difference between local, instance and class variables?

Mid
* What is reflection?
* What is dependency injection? Can you name a few libraries? (Have you used any?)
* What are strong, soft and weak references in Java?
* What does the keyword synchronized mean?
* Can you have “memory leaks” on Java?
* Do you need to set references to null on Java/Android?
* What does it means to say that a String is immutable?
* What are transient and volatile modifiers?
* What is the finalize() method?
* How does the try{} finally{} works?
* What is the difference between instantiation and initialisation of an object?
* When is a static block run?
* Why are Generics are used in Java?
* Can you mention the design patterns you know? Which of those do you normally use?
* Can you mention some types of testing you know?

Senior
* How does Integer.parseInt() works?
* Do you know what is the “double check locking” problem?
* Do you know the difference between StringBuffer and StringBuilder?
* How is a StringBuilder implemented to avoid the immutable string allocation problem?
* What does Class.forName method do?
* What is Autoboxing and Unboxing?
* What’s the difference between an Enumeration and an Iterator?
* What is the difference between fail-fast and fail safe in Java?
* What is PermGen in Java?
* What is a Java priority queue?
* *s performance influenced by using the same number in different types: Int, Double and Float?
* What is the Java Heap?
* What is daemon thread?
* Can a dead thread be restarted?

Source: medium.

Java for Everything: ☕

Java + Spring = Enterprise Applications

Java + Hibernate = Object-Relational Mapping

Java + Android = Mobile App Development

Java + Swing = Desktop GUI Applications

Java + JavaFX = Modern GUI Applications

Java + JUnit = Unit Testing

Java + Maven = Project Management

Java + Jenkins = Continuous Integration

Java + Apache Kafka = Stream Processing

Java + Apache Hadoop = Big Data Processing

Java + Microservices = Scalable Services

Best Programming Resources: https://topmate.io/coding/886839

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Top 10 String Programs for Java Interview

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🔺HTML-> w3schools
🔺CSS -> freeCodeCamp
🔺Javascript -> @javascript_courses
🔺NodeJS -> freeCodeCamp
🔺ExpressJS -> MDN
🔺MongoDB -> MDB University
🔺REST APIs -> Postman
🔺Git -> learnGitBranching
🔺Deployment -> Firebase

Websites to learn Java

Java is a popular programming language that is widely used for developing various types of applications, including web applications, mobile apps, desktop applications, and enterprise systems. Here are some key concepts to understand the basics of Java:

1. Object-Oriented Programming (OOP): Java is an object-oriented programming language, which means it focuses on creating objects that contain both data and methods to operate on that data. Key principles of OOP in Java include encapsulation, inheritance, and polymorphism.

2. Classes and Objects: In Java, a class is a blueprint for creating objects. An object is an instance of a class that represents a real-world entity. Classes define the properties (attributes) and behaviors (methods) of objects.

3. Variables and Data Types: Java supports various data types, including primitive data types (e.g., int, double, boolean) and reference data types (e.g., String, arrays). Variables are used to store data values in memory.

4. Methods: Methods in Java are functions defined within a class to perform specific tasks. They encapsulate behavior and can accept parameters and return values.

5. Control Flow Statements: Java provides control flow statements such as if-else, switch-case, loops (for, while, do-while), and break/continue statements to control the flow of program execution.

6. Inheritance: Inheritance is a key feature of OOP that allows a class (subclass) to inherit properties and behaviors from another class (superclass). It promotes code reusability and establishes an "is-a" relationship between classes.

7. Polymorphism: Polymorphism allows objects of different classes to be treated as objects of a common superclass. It enables methods to be overridden in subclasses to provide different implementations.

8. Abstraction: Abstraction involves hiding the complex implementation details and showing only the essential features of an object. Abstract classes and interfaces are used to achieve abstraction in Java.

9. Encapsulation: Encapsulation is the process of bundling data (attributes) and methods that operate on that data within a class. It helps in data hiding and protects the internal state of an object.

10. Exception Handling: Java provides mechanisms for handling exceptions that occur during program execution. The try-catch-finally blocks are used to handle exceptions gracefully and prevent program crashes.

Understanding these basic concepts of Java will help you get started with programming in Java. Practice writing Java programs, exploring different features of the language, and building small projects to strengthen your Java skills.

Java projects 👇

Why Learn Java?

1 - Beginner-Friendly :
Easy to learn and understand for coding newcomers.

2 - Versatile :
Used for web, mobile, desktop apps, and even games.

3 - Platform-Independent :
“Write once, run anywhere” with Java Virtual Machine (JVM).

4 - Highly Secure :
Perfect for banking and enterprise-level applications.

5 - Strong Job Market :
Java developers are always in demand worldwide.

6 - Powerful Community :
Tons of resources, libraries, and frameworks available.

7 - Scalable :
Ideal for building apps that grow with user demand.

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Learn Java in 50 Days

2 Month Roadmap to learn Java and Spring from basics to advanced

Week 1-2: Core Java

- Basic Syntax: Data types, operators, loops (for, while, do-while)
- OOP Concepts: Classes, objects, inheritance, polymorphism, encapsulation, abstraction
- Collections Framework: List, Set, Map, Queue, Iterator, etc.
- Exception Handling: Try-catch, custom exceptions, multi-catch
- File I/O: Reading/writing files using java.io and java.nio
- Java 8+ Features: Lambdas, Streams, Optional, Functional Interfaces
- Multithreading: Threads, Runnable, ExecutorService, Future, and basic synchronization

Week 3-4: Advanced Java & JDBC

- JVM Internals: ClassLoader, JIT, memory management, garbage collection
- Generics: Usage and wildcards
- Design Patterns: Singleton, Factory, Strategy, Observer, Dependency Injection (Intro)
- JDBC: Connecting with databases (CRUD operations), PreparedStatement, Connection pooling

Week 5-6: Spring Framework (Basics)

- Spring Core
- Dependency Injection (DI) and Inversion of Control (IoC)
- Beans, Scopes, and Bean Life Cycle
- Autowiring and Spring Annotations
- Spring AOP: Aspect-Oriented Programming fundamentals
- Spring Data JPA: Basic CRUD operations with JPA
- Entities, Repositories, and Custom Queries

Week 7: Spring Boot (Basics)

- Spring Boot Fundamentals: Understanding the Spring Boot architecture
- REST API creation
- Spring Boot Annotations (@RestController, @RequestMapping, etc.)
- Running a Spring Boot application
- Spring Boot Auto Configuration: Application properties and profiles
- Spring Boot with Database: Integrate Spring Boot with MySQL using Spring Data JPA

Week 8: Spring Boot (Advanced)

- Security: Introduction to Spring Security (Basic Authentication)
- Advanced Spring Boot Features: Exception handling (@ControllerAdvice)
- Logging with Spring Boot
- Pagination and Sorting
- Testing: Write unit tests using JUnit and Mockito in Spring Boot

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