Java Developer Interview ❤
It'll gonna be super helpful for YOU

𝗧𝗼𝗽𝗶𝗰 𝟭: 𝗣𝗿𝗼𝗷𝗲𝗰𝘁 𝗳𝗹𝗼𝘄 𝗮𝗻𝗱 𝗮𝗿𝗰𝗵𝗶𝘁𝗲𝗰𝘁𝘂𝗿𝗲
- Please tell me about your project and its architecture, Challenges faced?
- What was your role in the project? Tech Stack of project? why this stack?
- Problem you solved during the project? How collaboration within the team?
- What lessons did you learn from working on this project?
- If you could go back, what would you do differently in this project?

𝗧𝗼𝗽𝗶𝗰 𝟮: 𝗖𝗼𝗿𝗲 𝗝𝗮𝘃𝗮
- String Concepts/Hashcode- Equal Methods
- Immutability
- OOPS concepts
- Serialization
- Collection Framework
- Exception Handling
- Multithreading
- Java Memory Model
- Garbage collection

Tech Community
👉

𝗧𝗼𝗽𝗶𝗰 𝟯: 𝗝𝗮𝘃𝗮-𝟴/𝗝𝗮𝘃𝗮-𝟭𝟭/𝗝𝗮𝘃𝗮𝟭𝟳
- Java 8 features
- Default/Static methods
- Lambda expression
- Functional interfaces
- Optional API
- Stream API
- Pattern matching
- Text block
- Modules

𝗧𝗼𝗽𝗶𝗰 𝟰: 𝗦𝗽𝗿𝗶𝗻𝗴 𝗙𝗿𝗮𝗺𝗲𝘄𝗼𝗿𝗸, 𝗦𝗽𝗿𝗶𝗻𝗴-𝗕𝗼𝗼𝘁, 𝗠𝗶𝗰𝗿𝗼𝘀𝗲𝗿𝘃𝗶𝗰𝗲, 𝗮𝗻𝗱 𝗥𝗲𝘀𝘁 𝗔𝗣𝗜
- Dependency Injection/IOC, Spring MVC
- Configuration, Annotations, CRUD
- Bean, Scopes, Profiles, Bean lifecycle
- App context/Bean context
- AOP, Exception Handler, Control Advice
- Security (JWT, Oauth)
- Actuators
- WebFlux and Mono Framework
- HTTP methods
- JPA
- Microservice concepts
- Spring Cloud

𝗧𝗼𝗽𝗶𝗰 𝟱: 𝗛𝗶𝗯𝗲𝗿𝗻𝗮𝘁𝗲/𝗦𝗽𝗿𝗶𝗻𝗴-𝗱𝗮𝘁𝗮 𝗝𝗽𝗮/𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲 (𝗦𝗤𝗟 𝗼𝗿 𝗡𝗼𝗦𝗤𝗟)
- JPA Repositories
- Relationship with Entities
- SQL queries on Employee department
- Queries, Highest Nth salary queries
- Relational and No-Relational DB concepts
- CRUD operations in DB
- Joins, indexing, procs, function

𝗧𝗼𝗽𝗶𝗰 𝟲: 𝗖𝗼𝗱𝗶𝗻𝗴
- DSA Related Questions
- Sorting and searching using Java API.
- Stream API coding Questions

Tech Jobs and Internships
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𝗧𝗼𝗽𝗶𝗰 𝟳: 𝗗𝗲𝘃𝗼𝗽𝘀 𝗾𝘂𝗲𝘀𝘁𝗶𝗼𝗻𝘀 𝗼𝗻 𝗱𝗲𝗽𝗹𝗼𝘆𝗺𝗲𝗻𝘁 𝗧𝗼𝗼𝗹𝘀
- These types of topics are mostly asked by managers or leads who are heavily working on it, That's why they may grill you on DevOps/deployment-related tools, You should have an understanding of common tools like Jenkins, Kubernetes, Kafka, Cloud, and all.

𝗧𝗼𝗽𝗶𝗰𝘀 𝟴: 𝗕𝗲𝘀𝘁 𝗽𝗿𝗮𝗰𝘁𝗶𝗰𝗲
- The interviewer always wanted to ask about some design patterns, it may be Normal design patterns like singleton, factory, or observer patterns to know that you can use these in coding.

PDFs and Notes 📝


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𝐉𝐚𝐯𝐚 𝐈𝐧𝐭𝐞𝐫𝐯𝐢𝐞𝐰 𝐐𝐮𝐞𝐬𝐭𝐢𝐨𝐧𝐬 𝐟𝐨𝐫 𝐅𝐫𝐞𝐬𝐡𝐞𝐫𝐬 𝐭𝐨 𝐂𝐫𝐚𝐜𝐤 𝐭𝐡𝐞 𝐍𝐞𝐱𝐭 𝐉𝐨𝐛 𝐈𝐧𝐭𝐞𝐫𝐯𝐢𝐞𝐰

1. What is Java?
2. What are the features of Java?
3. What are the OOP concepts?
4. What is data encapsulation, and why is it useful?
5. What is polymorphism?
6. What are the types of polymorphism, and how do they differ?
7. What does an interface in Java refer to?
8. What is a class in Java?
9. What is a Java object?
10. What is a singleton class, and how can it be used?
11. What is a constructor in Java?
12. What does the term constructor overloading mean?
13. How are non-primitive variables used in Java?
14. In Java, what is a static variable?
15. What are Java data types, and how are they grouped?
16. How do you define primitive data types and describe each by size and denoscription?
17. What do the terms autoboxing and unboxing mean in Java?
18. What are wrapper classes in Java?
19. What is inheritance in Java?
20. What types of inheritance does Java support?
21. What is Java exception handling?
22. What are the differences between unchecked exceptions, checked exceptions, and errors?
23. What are loops in Java?
24. What are the types of loops in Java, and how are they used?
25. What is an infinite loop?
26. What is the difference between the continue and break statement?
27. What is the entry point in Java, and how is it written?
28. In Java, what’s the purpose of static methods and static variables?
29. How do you use, call, and access a static method in Java?
30. How do you use, call, and access a non-static method in Java?

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In 2024, if you want to become a Java Developer, then.

follow this roadmap.

➤ 𝗝𝗮𝘃𝗮 + 𝗔𝗱𝘃𝗮𝗻𝗰𝗲 𝗝𝗮𝘃𝗮

• Variables, data types, and operators
• Control structures (if-else, loops)
• Functions/methods
• Basic data structures (arrays, lists, stacks, queues)
• Setup Java development environment (JDK, IDEs like IntelliJ IDEA or Eclipse)
• Basic syntax and structure of Java programs
• Object-Oriented Programming (OOP) concepts: classes, objects, inheritance, polymorphism, encapsulation, abstraction
• Exception handling
• Input/Output (I/O) operations
• Java Collections Framework (List, Set, Map)
• Multithreading and Concurrency
• Generics and Enums
• Lambda expressions and Stream API
• Java 8+ features (functional programming concepts, new date and time API)
• Annotations and Reflection

➤ 𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲

• Relational Databases: MySQL, PostgreSQL
• SQL: CRUD operations, Joins,
• Indexes, Transactions
• JDBC (Java Database Connectivity)
• NoSQL Databases: MongoDB, Cassandra
• Understanding of when and why to use NoSQL databases

➤ 𝗦𝗽𝗿𝗶𝗻𝗴 𝗙𝗿𝗮𝗺𝗲𝘄𝗼𝗿𝗸

1. Core Spring Concepts:
• Spring Boot for rapid application development
• Dependency Injection (DI) and Inversion of Control (IoC)

2. Spring Modules:
• Spring Data JPA (for database interactions)
• Spring MVC (for creating web applications)
• Spring Security (for authentication and authorization)
• Spring Cloud (for developing cloud- native applications)

3. RESTful Web Services:
• Building REST APIs with Spring Boot
• Understanding of REST principles and HTTP methods (GET, POST, PUT, DELETE)
• JSON (de) serialization with Jackson

➤ 𝗔𝗣𝗜'𝘀 𝗮𝗻𝗱 𝗠𝗶𝗰𝗿𝗼𝘀𝗲𝗿𝘃𝗶𝗰𝗲𝘀

1. RESTful APIs:
• Best practices for designing and documenting APIs (Swagger/ OpenAPI)
• Microservices Architecture:
• Fundamentals of microservices
• Communication between microservices (REST, gRPC, messaging)
• Service discovery and configuration management
• Using Spring Cloud components (Eureka, Config Server, etc.)

➤ 𝗖𝗼𝗱𝗲 𝗦𝘁𝗿𝘂𝗰𝘁𝘂𝗿𝗲 𝗯𝗲𝘀𝘁 𝗽𝗿𝗮𝗰𝘁𝗶𝗰𝗲𝘀

- Keep it simple: Break down complex logic into smaller methods.
- Single Responsibility Principle (SRP): Each class should handle one responsibility.
- Don't Repeat Yourself (DRY): Avoid duplication by extracting reusable code.
- Proper formatting: Use consistent indentation and formatting for readability.

➤ Best Java Resources:

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Here are the top 5 Java tricks that can enhance your coding efficiency and performance:

### 1. **Use StringBuilder for String Manipulation**
Whyhy**: Strings in Java are immutable, meaning every time you modify a String, a new object is created, which impacts memory and performance.
Trickck**: Use StringBuilder (or StringBuffer if thread safety is required) for concatenating multiple strings.

   StringBuilder sb = new StringBuilder();
   sb.append("Hello");
   sb.append(" ");
   sb.append("World");
   System.out.println(sb.toString()); // Outputs: Hello World
   

### 2. **Use try-with-resources for Auto-Closing ResourcesWhy **Why**: Java’s try-with-resources statement ensures that resources like streams, connections, etc., are closed automatically, which avoids resource leakTrick*Trick**: Instead of manually closing resources, use this feature to clean up resources efficiently.

   try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
       String line;
       while ((line = br.readLine()) != null) {
           System.out.println(line);
       }
   } catch (IOException e) {
       e.printStackTrace();
   }
   

### 3. **Efficient HashMap InitializaWhy - **Why**: If you know the approximate number of entries that a HashMap will hold, it's efficient to initialize it with the right capacity to avoid rehaTrick - **Trick**: Set the initial capacity to the nearest power of two (plus a load factor margin) when you create the HashMap.

   int expectedSize = 100;
   HashMap<String, Integer> map = new HashMap<>(expectedSize * 4 / 3); // Adjusting for the load factor of 0.75
   

### 4. **Leverage Optional to Avoid NullPointerException**
- **Why**: Java 8 introduced Optional to handle potential null values in a cleaner way, reducing the chances of encountering NullPointerException.
- **Trick**: Use Optional when returning a value that may be null, allowing you to chain methods with greater safety.

   Optional<String> name = Optional.ofNullable(getUserName());
   name.ifPresent(n -> System.out.println(n));
   

### 5.WhyCWhyta Processing**
- **Why**: Java Streams (introduced in Java 8) simplify data processing pipelinTrickrickore readable.
- **Trick**: Use Stream operations like filter(), map(), and collect() to process collections more concisely.

   List<String> names = Arrays.asList("John", "Jane", "Jack", "Doe");
   List<String> filteredNames = names.stream()
       .filter(name -> name.startsWith("J"))
       .map(String::toUpperCase)
       .collect(Collectors.toList());
   System.out.println(filteredNames); // Outputs: [JOHN, JANE, JACK]
   

These tricks can help you write more efficient, cleaner, and maintainable Java code!

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Java in one notes.pdf
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Zero To Hero Java Programming In 6months
1. Basic Understanding (1-2 months):

> Syntax and Fundamentals: Learning about variables, data types, operators, control structures (if-else, loops), and basic input/output.
>Object-Oriented Programming (OOP) Concepts: Classes, objects, inheritance, polymorphism, encapsulation, and abstraction.
> > Practice: Writing small programs to reinforce these concepts.

2. Intermediate Level (2-4 months):

>Collections Framework: Lists, sets, maps, and queues.
>Exception Handling: Understanding try-catch blocks and custom exceptions.
>Basic I/O: Reading and writing files.
>>Practice: Working on small projects or coding exercises.

3. Advanced Level (4-6 months):

>Multithreading and Concurrency: Understanding threads, synchronization, and parallel processing.
>Java Streams and Lambdas: Functional programming in Java.
>Advanced OOP Concepts: Design patterns, interfaces, and abstract classes.
>>Practice: Developing more complex applications or contributing to open-source projects.

➤ Best Java Resources:

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7 Baby Steps to Learn Java

1. Grasp the Basics: Start with the fundamentals of Java, such as understanding data types, variables, operators, control flow (if-else, loops), and basic syntax. Learn how Java differs from other programming languages, particularly in its object-oriented nature.

2. Write Simple Programs: Begin by writing simple Java programs to solidify your understanding of the basics. Try creating programs that handle basic tasks like calculating the Fibonacci sequence, checking if a number is even or odd, or converting units (e.g., Celsius to Fahrenheit).

3. Explore Object-Oriented Concepts: Java is an object-oriented programming (OOP) language, so it’s crucial to get comfortable with OOP concepts like classes, objects, inheritance, polymorphism, and encapsulation. Practice by creating small programs that implement these concepts, such as a basic inventory system.

4. Build Small Projects: Start working on small projects to apply what you’ve learned. Create a simple calculator, a to-do list app, or even a basic text-based game. These projects will help you understand how to structure your code and utilize Java’s standard libraries.

5. Study Other Java Code: Examine code written by others to see how they structure their programs and solve problems. GitHub is a great resource for this. By studying existing projects, you’ll learn best practices and discover new ways to approach coding challenges.

6. Engage with Java Documentation: Java’s official documentation is a treasure trove of information. Explore it to learn about the various classes and methods available in the Java Development Kit (JDK). This will deepen your understanding and help you write more efficient code.

7. Join Java Communities: Participate in online Java communities like StackOverflow, Java forums, and Reddit’s Java subreddit. Engaging with these communities will give you access to a wealth of knowledge and support from experienced developers.

Work on coding problems, participate in coding challenges, and keep experimenting with new projects. The more you code, the more proficient you’ll become.

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Preparing for a Java developer interview can be a bit overwhelming,
but breaking it down by difficulty and experience level can make it more manageable.

Whether you're a fresher or an experienced developer, here's a guide to help you focus your preparation and walk into your interview with confidence.

𝗙𝗼𝗿 𝗔𝗹𝗹 𝗟𝗲𝘃𝗲𝗹𝘀 (𝗜𝗻𝗰𝗹𝘂𝗱𝗶𝗻𝗴 𝗙𝗿𝗲𝘀𝗵𝗲𝗿𝘀)

➤ Topic 1: Project Flow and Architecture (Medium)
- These questions are designed to gauge your understanding of project development, teamwork, and problem-solving. Be ready to discuss a project you've worked on, including the tech stack used, the challenges you faced, and how you overcame them.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗖𝗼𝗿𝗲 𝗝𝗮𝘃𝗮 𝗦𝗸𝗶𝗹𝗹𝘀 (𝟭-𝟯 𝗬𝗲𝗮𝗿𝘀 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲)

➤ Topic 2: Core Java (Medium to Hard)
- Fundamental Java concepts. You'll likely face questions on strings, object-oriented programming (OOP), collections, exception handling, and multithreading.

𝗙𝗼𝗿 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲𝗱 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 (𝟯+ 𝗬𝗲𝗮𝗿𝘀 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲)

➤ Topic 3: Java 8/11/17 Features (Hard)
- This is where the interview gets more challenging. You'll asked advanced features introduced in recent Java versions, such as lambda expressions, functional interfaces, the Stream API, and modules.

➤ Topic 4: Spring Framework, Spring Boot, Microservices, and REST API (Hard)
- Expect questions on popular frameworks and backend development architectures. Be prepared to explain concepts like dependency injection, Spring MVC, and microservices.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲

➤ Topic 5: Hibernate/Spring Data JPA/Database (Hard)
- This section focuses on data persistence with JPA and working with relational (SQL) or NoSQL databases. Be ready to discuss JPA repositories, entity relationships, and complex querying techniques.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗔𝗱𝗱𝗶𝘁𝗶𝗼𝗻𝗮𝗹 𝗦𝗸𝗶𝗹𝗹𝘀

➤ Topic 6: Coding (Medium to Hard)
- You'll likely encounter coding challenges related to data structures and algorithms (DSA), as well as using the Java Stream API.

➤ Topic 7: DevOps Questions on Deployment Tools (Advanced)
- These questions are often posed by managers or leads, especially if you're applying for a role that involves DevOps. Be prepared to discuss deployment tools like Jenkins, Kubernetes, and cloud platforms.

➤ Topic 8: Best Practices (Medium)
- Interviewers may ask about design patterns like Singletons, Factories, or Observers to see how well you write clean, reusable code.

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240 Java Interview Questions😎 #resources

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Preparing for a Java Interview?

Here are some essential Java questions to help you review important concepts:

Core Java Concepts:
1. What is the difference between JDK and JRE?
2. Why is Java platform-independent?
3. What is the difference between an abstract class and an interface?
4. What is the difference between final, finally, and finalize?
5. What is the difference between stack and heap memory?
6. What is method overloading vs. method overriding?
7. What’s the difference between a private and protected modifier?
8. What is constructor overloading in Java?
9. How is the super keyword used in Java?
10. What is the difference between static methods, variables, and classes?
11. What is System.out.println in Java?
12. What part of memory (Stack or Heap) is cleaned during garbage collection?

Object-Oriented Programming:
1. What Object-Oriented features are supported by Java?
2. What are the different access specifiers in Java?
3. What’s the difference between composition and inheritance?
4. What is the purpose of an abstract class?
5. What’s the difference between a constructor and a method?
6. What is the diamond problem in Java, and how is it resolved?
7. How do local and instance variables differ?
8. What is a Marker interface in Java?

Data Structures and Algorithms:
1. Why are strings immutable in Java?
2. What’s the difference between creating a String using new() and as a literal?
3. What is the Collections framework?
4. How do ArrayList and LinkedList differ?
5. What is the difference between a HashMap and a TreeMap?
6. How do HashSet and TreeSet differ?
7. What’s the difference between an Iterator and a ListIterator?
8. What is the purpose of the Comparable interface?
9. What is the java.util.concurrent package for?

Exception Handling:
1. What is an exception in Java?
2. How does exception propagation work?
3. What’s the difference between checked and unchecked exceptions?
4. What is the use of the try-catch block in Java?
5. How do throw and throws differ?
6. What is the use of the finally block?
7. What’s the base class of all exception classes?

Multithreading:
1. What is a thread, and what are its stages in Java?
2. How do threads differ from processes?
3. What are the different types of thread priorities in Java?
4. What is context switching in Java?
5. What’s the difference between user threads and Daemon threads?
6. What is synchronization in Java?
7. What is a deadlock in Java?
8. How are the wait() and notify() methods used?
9. What’s the difference between synchronized and volatile in Java?
10. What’s the purpose of the sleep() method?
11. How do wait() and sleep() differ?
12. What’s the difference between notify() and notifyAll()?

Java Enterprise Edition (Java EE):
1. What is Java EE?
2. How do Servlets and JSP differ?
3. What is the purpose of the Java Persistence API (JPA)?
4. What’s the difference between stateful and stateless session beans?

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Java interview questions along with brief answers

1. What is the difference between == and .equals() in Java?
- == compares object references, while .equals() compares the content of objects.

2. Explain the difference between ArrayList and LinkedList.
- ArrayList uses a dynamic array to store elements, providing fast random access. LinkedList uses a doubly-linked list, offering efficient insertion and deletion.

3. What is the significance of the static keyword in Java?
- The static keyword is used to create variables and methods that belong to the class rather than instances of the class. These can be accessed without creating an object.

4. What is the difference between an abstract class and an interface?
- An abstract class can have both abstract (unimplemented) and concrete (implemented) methods, while an interface can only have abstract methods. A class can implement multiple interfaces, but it can extend only one abstract class.

5. What is the purpose of the finally block in exception handling?
- The finally block is used to ensure that a specific block of code is always executed, regardless of whether an exception is thrown or not.

6. Explain the concept of multithreading in Java.
- Multithreading is the concurrent execution of two or more threads. Threads allow programs to run tasks in parallel, improving performance. In Java, you can create threads by extending the Thread class or implementing the Runnable interface.

7. What is the NullPointerException and how can it be avoided?
- NullPointerException occurs when you try to access a method or field of an object that is null. To avoid it, ensure that objects are properly initialized before accessing their members.

8. How does garbage collection work in Java?
- Garbage collection is the process of automatically reclaiming memory occupied by unreferenced objects. Java uses a garbage collector to identify and delete objects that are no longer reachable.

9. Explain the final keyword in Java.
- The final keyword can be applied to variables, methods, and classes. When applied to a variable, it means the variable's value cannot be changed. When applied to a method, it prevents method overriding. When applied to a class, it prevents the class from being subclassed.

10. What is the Java Virtual Machine (JVM)?
- JVM is a virtual machine that executes Java bytecode. It abstracts the hardware and operating system details, allowing Java programs to run on any device with a compatible JVM.

These questions cover a range of Java concepts commonly discussed in interviews. Be prepared to elaborate on your answers and provide examples if requested during an interview.

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.

https://news.1rj.ru/str/open_source7

Data Structures and Algorithms in Java
Robert Lafore, 2002