Java Fundamentals

1. What is Java?

   • Java is a high-level, object-oriented, platform-independent programming language.
   • It was developed by Sun Microsystems (now Oracle).

2. What are the key features of Java?

   • Platform Independence: "Write Once, Run Anywhere" - Java bytecode can run on any platform with a Java Virtual Machine (JVM).
   • Object-Oriented: Encapsulates data and methods within objects.
   • Robust: Strong memory management and exception handling.
   • Secure: Features like bytecode verification enhance security.
   • Multithreaded: Supports concurrent execution of multiple threads.

3. Explain the difference between JDK, JRE, and JVM.

   • JDK (Java Development Kit): Contains the necessary tools for Java development, including the compiler, debugger, and JRE.
   • JRE (Java Runtime Environment): Includes the JVM and Java libraries required to run Java programs.
   • JVM (Java Virtual Machine): An abstract machine that executes Java bytecode.

4. What are the different types of access modifiers in Java?

   • public: Accessible from anywhere.
   • private: Accessible only within the same class.
   • protected: Accessible within the same class and its subclasses.  

   • default (no modifier): Accessible within the same package.

5. What is object-oriented programming (OOP)?

   • A programming paradigm based on the concept of "objects," which encapsulate data (attributes) and methods (behavior).
   • Key principles: Encapsulation, Inheritance, Polymorphism, Abstraction.

6. Explain the concept of inheritance in Java.

   • A mechanism where a class (subclass) inherits properties and methods from another class (superclass).
   • Promotes code reusability and establishes a hierarchical relationship between classes.

7. Explain the concept of polymorphism in Java.

   • The ability of an object to take on many forms.
   • Achieved through method overloading (same method name, different parameters) and method overriding (same method name and signature in subclass).

8. What is an interface in Java?

   • A blueprint of a class that contains only abstract methods (methods without implementation) and constant variables.
   • Classes can implement interfaces to achieve multiple inheritance.

9. What is an abstract class in Java?

   • A class that cannot be instantiated directly.
   • Can contain both abstract and concrete methods.
   • Used to provide a common template for subclasses.

10. What are the different types of exceptions in Java?

    • Checked Exceptions: Must be handled explicitly using try-catch blocks.
    • Unchecked Exceptions: (Runtime Exceptions) Do not need to be explicitly handled.

Java Collections

11. What are the different types of collections in Java?

    • List: Ordered collection of elements (e.g., ArrayList, LinkedList).
    • Set: Unordered collection of unique elements (e.g., HashSet, TreeSet).
    • Map: Stores data in key-value pairs (e.g., HashMap, TreeMap).

12. Explain the difference between ArrayList and LinkedList.

    • ArrayList: Uses an array to store elements, provides fast random access but slow insertion/deletion.
    • LinkedList: Uses a doubly linked list, provides fast insertion/deletion but slow random access.

13. What is the difference between HashMap and TreeMap?

    • HashMap: Uses a hash table to store elements, provides fast insertion, deletion, and retrieval.
    • TreeMap: Stores elements in sorted order, provides slower insertion and deletion but efficient retrieval of elements in sorted order.

14. How do you iterate over a collection in Java?

    • For-each loop: Enhanced for loop for iterating over collections.
    • Iterator interface: Provides methods for traversing and removing elements from a collection.

Java Multithreading

15. What is multithreading in Java?

    • The ability of a program to execute multiple threads concurrently.
    • Improves performance and responsiveness by allowing multiple tasks to run simultaneously.

16. What is a thread in Java?

    • A lightweight unit of execution within a program.

17. How do you create a thread in Java?

    • Extending the Thread class: Create a new class that extends the Thread class and override the run() method.
    • Implementing the Runnable interface: Create a new class that implements the Runnable interface and implement the run() method.

18. What is thread synchronization?

    • A mechanism to control access to shared resources among multiple threads to prevent data corruption.
    • Achieved using synchronized blocks or methods.

19. What is deadlock?

    • A situation where two or more threads are blocked forever, waiting for each other to release resources.

20. What is a thread pool?

    • A collection of reusable threads that can be used to execute tasks efficiently.

Java Input/Output

21. What are the different types of streams in Java?

    • Byte streams: Read and write bytes of data (e.g., FileInputStream, FileOutputStream).
    • Character streams: Read and write characters of data (e.g., FileReader, FileWriter).

22. What is the difference between FileInputStream and FileReader?

    • FileInputStream: Reads bytes from a file.
    • FileReader: Reads characters from a file using a default character encoding.

Java Serialization

23. What is serialization in Java?

    • The process of converting an object into a stream of bytes so that it can be stored in a file or transmitted over a network.  

24. How do you serialize an object in Java?

    • Implement the Serializable interface in the class.
    • Use ObjectOutputStream to write the object to a stream.

Java Generics

25. What are generics in Java?

    • A mechanism for creating type-safe classes and methods.
    • Allows you to write code that can work with different types of objects.

26. What are the benefits of using generics?

    • Improved type safety: Reduces the risk of runtime errors.
    • Increased code reusability: Write code that can work with different types.
    • Improved code readability: Makes code more concise and easier to understand.

Java Reflection

27. What is reflection in Java?
    • The ability of a Java program to examine or modify itself at runtime.
    • Allows you to inspect classes, methods, fields, and other parts of the Java code at runtime.

Java Memory Management

28. What is the Java Virtual Machine (JVM) memory model?

    • Defines how memory is allocated and managed by the JVM.
    • Includes components like the heap, stack, method area, etc.

29. What is the garbage collector in Java?

    • An automatic memory management system that reclaims memory occupied by objects that are no longer in use.

Java 8 Features

30. What are lambda expressions in Java 8?

    • Anonymous functions that can be treated as values.
    • Provide a concise way to represent short blocks of code.

31. What are streams in Java 8?

    • A sequence of elements that supports various operations like filtering, mapping, and reducing.

32. What are default and static methods in Java 8 interfaces?

    • Default methods: Allow you to add new methods to existing interfaces without breaking existing implementations.
    • Static methods: Can be called directly on the interface itself without creating an instance.

Java 9 Features

33. What is the Java Module System (Project Jigsaw)?
    • A new modularity system introduced in Java 9.
    • Allows you to define and encapsulate modules to improve application modularity and maintainability.

Java 10 Features

34. What is local variable type inference (var)?
    • Allows you to declare local variables without explicitly specifying their type.

Java 11 Features

35. What is the HTTP Client in Java 11?
    • A new, improved HTTP client API for making HTTP requests.

Java 12 Features

36. What is the switch expression in Java 12?
    • An improved version of the switch statement with a more concise and expressive syntax.

Java 13 Features

37. What is text blocks in Java 13?
    • A new way to define multi-line strings in Java.

Java 14 Features

38. What are records in Java 14?
    • A concise way to define immutable data classes.

Java 15 Features

39. What are sealed classes in Java 15?
    • Restrict the subclasses of a class to be defined within the same file.

Java Fundamentals

1. What is Java?

   • Java is a high-level, object-oriented, platform-independent programming language.
   • It was developed by Sun Microsystems (now Oracle).

2. What are the key features of Java?

   • Platform Independence: "Write Once, Run Anywhere" - Java bytecode can run on any platform with a Java Virtual Machine (JVM).
   • Object-Oriented: Encapsulates data and methods within objects.
   • Robust: Strong memory management and exception handling.
   • Secure: Features like bytecode verification enhance security.
   • Multithreaded: Supports concurrent execution of multiple threads.

3. Explain the difference between JDK, JRE, and JVM.

   • JDK (Java Development Kit): Contains the necessary tools for Java development, including the compiler, debugger, and JRE.
   • JRE (Java Runtime Environment): Includes the JVM and Java libraries required to run Java programs.
   • JVM (Java Virtual Machine): An abstract machine that executes Java bytecode.

4. What are the different types of access modifiers in Java?

   • public: Accessible from anywhere.
   • private: Accessible only within the same class.
   • protected: Accessible within the same class and its subclasses.
   • default (no modifier): Accessible within the same package.

5. What is object-oriented programming (OOP)?

   • A programming paradigm based on the concept of "objects," which encapsulate data (attributes) and methods (behavior).
   • Key principles: Encapsulation, Inheritance, Polymorphism, Abstraction.

6. Explain the concept of inheritance in Java.

   • A mechanism where a class (subclass) inherits properties and methods from another class (superclass).
   • Promotes code reusability and establishes a hierarchical relationship between classes.

7. Explain the concept of polymorphism in Java.

   • The ability of an object to take on many forms.
   • Achieved through method overloading (same method name, different parameters) and method overriding (same method name and signature in subclass).

8. What is an interface in Java?

   • A blueprint of a class that contains only abstract methods (methods without implementation) and constant variables.
   • Classes can implement interfaces to achieve multiple inheritance.

9. What is an abstract class in Java?

   • A class that cannot be instantiated directly.
   • Can contain both abstract and concrete methods.
   • Used to provide a common template for subclasses.

10. What are the different types of exceptions in Java?

    • Checked Exceptions: Must be handled explicitly using try-catch blocks.
    • Unchecked Exceptions: (Runtime Exceptions) Do not need to be explicitly handled.

Java Collections

11. What are the different types of collections in Java?

    • List: Ordered collection of elements (e.g., ArrayList, LinkedList).
    • Set: Unordered collection of unique elements (e.g., HashSet, TreeSet).
    • Map: Stores data in key-value pairs (e.g., HashMap, TreeMap).

12. Explain the difference between ArrayList and LinkedList.

    • ArrayList: Uses an array to store elements, provides fast random access but slow insertion/deletion.
    • LinkedList: Uses a doubly linked list, provides fast insertion/deletion but slow random access.

13. What is the difference between HashMap and TreeMap?

    • HashMap: Uses a hash table to store elements, provides fast insertion, deletion, and retrieval.
    • TreeMap: Stores elements in sorted order, provides slower insertion and deletion but efficient retrieval of elements in sorted order.

14. How do you iterate over a collection in Java?

    • For-each loop: Enhanced for loop for iterating over collections.
    • Iterator interface: Provides methods for traversing and removing elements from a collection.

Java Multithreading

15. What is multithreading in Java?

    • The ability of a program to execute multiple threads concurrently.
    • Improves performance and responsiveness by allowing multiple tasks to run simultaneously.

16. What is a thread in Java?

    • A lightweight unit of execution within a program.

17. How do you create a thread in Java?

    • Extending the Thread class: Create a new class that extends the Thread class and override the run() method.
    • Implementing the Runnable interface: Create a new class that implements the Runnable interface and implement the run() method.

18. What is thread synchronization?

    • A mechanism to control access to shared resources among multiple threads to prevent data corruption.
    • Achieved using synchronized blocks or methods.

19. What is deadlock?

    • A situation where two or more threads are blocked forever, waiting for each other to release resources.

20. What is a thread pool?

    • A collection of reusable threads that can be used to execute tasks efficiently.

Java Input/Output

21. What are the different types of streams in Java?

    • Byte streams: Read and write bytes of data (e.g., FileInputStream, FileOutputStream).
    • Character streams: Read and write characters of data (e.g., FileReader, FileWriter).

22. What is the difference between FileInputStream and FileReader?

    • FileInputStream: Reads bytes from a file.
    • FileReader: Reads characters from a file using a default character encoding.

Java Serialization

23. What is serialization in Java?

    • The process of converting an object into a stream of bytes so that it can be stored in a file or transmitted over a network.

24. How do you serialize an object in Java?

    • Implement the Serializable interface in the class.
    • Use ObjectOutputStream to write the object to a stream.

Java Generics

25. What are generics in Java?

    • A mechanism for creating type-safe classes and methods.
    • Allows you to write code that can work with different types of objects.

26. What are the benefits of using generics?

    • Improved type safety: Reduces the risk of runtime errors.
    • Increased code reusability: Write code that can work with different types.
    • Improved code readability: Makes code more concise and easier to understand.

Java Reflection

27. What is reflection in Java?
    • The ability of a Java program to examine or modify itself at runtime.
    • Allows you to inspect classes, methods, fields, and other parts of the Java code at runtime.

Java Memory Management

28. What is the Java Virtual Machine (JVM) memory model?

    • Defines how memory is allocated and managed by the JVM.
    • Includes components like the heap, stack, method area, etc.

29. What is the garbage collector in Java?

    • An automatic memory management system that reclaims memory occupied by objects that are no longer in use.

Java 8 Features

30. What are lambda expressions in Java 8?

    • Anonymous functions that can be treated as values.
    • Provide a concise way to represent short blocks of code.

31. What are streams in Java 8?

    • A sequence of elements that supports various operations like filtering, mapping, and reducing.

32. What are default and static methods in Java 8 interfaces?

    • Default methods: Allow you to add new methods to existing interfaces without breaking existing implementations.
    • Static methods: Can be called directly on the interface itself without creating an instance.

Java 9 Features

33. What is the Java Module System (Project Jigsaw)?
    • A new modularity system introduced in Java 9.
    • Allows you to define and encapsulate modules to improve application modularity and maintainability.

Java 10 Features

34. What is local variable type inference (var)?
    • Allows you to declare local variables without explicitly specifying their type.

Java 11 Features

35. What is the HTTP Client in Java 11?
    • A new, improved HTTP client API for making HTTP requests.

Java 12 Features

36. What is the switch expression in Java 12?
    • An improved version of the switch statement with a more concise and expressive syntax.

Java 13 Features

37. What is text blocks in Java 13?
    • A new way to define multi-line strings in Java.

Java 14 Features

38. What are records in Java 14?
    • A concise way to define immutable data classes.

Java 15 Features

39. What are sealed classes in Java 15?
    • Restrict the subclasses of a class to be defined within the same file.