1. What is the Java programming language? #

Answer:
Java is a high-level, object-oriented, platform-independent programming language. It follows the WORA (Write Once, Run Anywhere) principle, meaning compiled Java code can run on any platform with a compatible JVM.

2. What are the key features of Java? #

Answer:

  • Object-Oriented: Based on concepts like classes and objects.
  • Platform-Independent: Compiled bytecode runs on any platform.
  • Secure: Provides built-in security features (e.g., bytecode verification, access control).
  • Robust: Includes features like exception handling and memory management (via Garbage Collection).
  • Multithreaded: Supports concurrent execution of threads.

3. What is the difference between JDK, JRE, and JVM? #

Answer:

  • JDK (Java Development Kit): A development environment containing tools like the compiler and debugger.
  • JRE (Java Runtime Environment): Provides the libraries and JVM to run Java applications.
  • JVM (Java Virtual Machine): Executes bytecode and provides a runtime environment.

4. What are the access modifiers in Java? #

Answer:

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

5. What is the difference between == and .equals()? #

Answer:

  • == compares object references (memory locations).
  • .equals() compares object contents (logical equality).

6. What is a Java Constructor? #

Answer:
A constructor is a special method used to initialize objects. It has the same name as the class and no return type.
Example:

public class Example {
    Example() {
        System.out.println("Constructor called");
    }
}

7. What is the difference between final, finally, and finalize()? #

Answer:

  • final: A keyword to declare constants or prevent inheritance/method overriding.
  • finally: A block in exception handling that executes regardless of whether an exception is thrown.
  • finalize(): A method called by the Garbage Collector before an object is destroyed.

8. What is the difference between Abstract Class and Interface? #

Answer:

  • Abstract Class: Can have abstract and concrete methods, allows inheritance, and supports fields with access modifiers.
  • Interface: Contains only abstract methods (before Java 8) and constants, supports multiple inheritance, and cannot have instance fields.

9. What are static methods and variables? #

Answer:

  • Static methods: Belong to the class and not instances. They can be called without creating an object.
  • Static variables: Shared across all instances of a class.
    Example:
class Example {
    static int count = 0;
    static void showCount() {
        System.out.println("Count: " + count);
    }
}

10. What is the difference between Array and ArrayList? #

Answer:

  • Array: Fixed size, can store primitive types and objects.
  • ArrayList: Dynamic size, part of the Java Collections framework, stores objects only.

11. What is method overloading and overriding? #

Answer:

  • Overloading: Defining methods with the same name but different parameters.
  • Overriding: Providing a new implementation of a method in a subclass.

12. What is a try-catch block? #

Answer:
A try-catch block handles exceptions in Java.
Example:

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero.");
}

13. What are checked and unchecked exceptions? #

Answer:

  • Checked Exceptions: Must be handled or declared in the method signature (e.g., IOException).
  • Unchecked Exceptions: Occur at runtime and do not need to be explicitly handled (e.g., NullPointerException).

14. What is the difference between String, StringBuilder, and StringBuffer? #

Answer:

  • String: Immutable and stored in the string pool.
  • StringBuilder: Mutable, non-thread-safe.
  • StringBuffer: Mutable, thread-safe due to synchronized methods.

15. What is Garbage Collection in Java? #

Answer:
Garbage Collection automatically deallocates unused memory. The JVM identifies and removes objects no longer referenced. This improves memory efficiency.

16. What is the synchronized keyword? #

Answer:
The synchronized keyword prevents multiple threads from accessing a method or block simultaneously, ensuring thread safety.
Example:

synchronized void increment() {
    count++;
}

17. What are Java Streams? #

Answer:
Streams are used for functional-style operations on collections. They support methods like filter(), map(), and reduce().
Example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
numbers.stream().filter(n -> n % 2 == 0).forEach(System.out::println);

18. What is the purpose of the volatile keyword? #

Answer:
The volatile keyword ensures that a variable's value is always read from and written to main memory, not a thread's cache, ensuring visibility in a multithreaded environment.

19. What is the Java Collections Framework? #

Answer:
The Java Collections Framework is a set of classes and interfaces for storing and manipulating data structures like lists, sets, maps, and queues.

20. What is the difference between HashMap and ConcurrentHashMap? #

Answer:

  • HashMap: Not thread-safe, may cause issues in concurrent environments.
  • ConcurrentHashMap: Thread-safe, allows concurrent access with segment-level locking.