Sovannaro

1. What is Java?

Answer: Java is a high-level, object-oriented programming language developed by Sun Microsystems. It is platform-independent due to its “write once, run anywhere” capability, achieved through the Java Virtual Machine (JVM).

2. Explain the concept of OOP in Java.

Answer: Object-Oriented Programming (OOP) in Java is based on four main principles: Encapsulation, Inheritance, Polymorphism, and Abstraction. These principles help in creating modular and reusable code.

3. What are the main features of Java?

Answer: Key features of Java include:

• Platform independence
• Object-oriented
• Robustness
• Security
• High performance
• Multithreading
• Dynamic and extensible

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

Answer:

• JDK (Java Development Kit): A software development kit for developing Java applications, includes JRE and development tools.
• JRE (Java Runtime Environment): Provides libraries and JVM to run Java applications.
• JVM (Java Virtual Machine): An engine that executes Java bytecode, allowing Java programs to run on any device.

5. What is a constructor in Java?

Answer: A constructor is a special method that is called when an object is instantiated. It has the same name as the class and does not have a return type.

6. What is the difference between == and equals() in Java?

Answer: == checks for reference equality (whether two references point to the same object), while equals() checks for value equality (whether two objects are logically equivalent).

7. What are the access modifiers in Java?

Answer: The access modifiers in Java are:

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

8. What is inheritance in Java?

Answer: Inheritance is a mechanism where one class (subclass) can inherit fields and methods from another class (superclass), promoting code reusability.

9. What is polymorphism?

Answer: Polymorphism is the ability of a single function or method to operate in different ways based on the object it is acting upon. It can be achieved through method overloading and method overriding.

10. What is encapsulation?

Answer: Encapsulation is the bundling of data (attributes) and methods (functions) that operate on the data into a single unit, usually a class, and restricting access to some of the object’s components.

11. What is an abstract class?

Answer: An abstract class is a class that cannot be instantiated and may contain abstract methods (without implementation) as well as concrete methods (with implementation).

12. What is an interface in Java?

Answer: An interface is a reference type in Java that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces cannot contain instance fields or constructors.

13. What is the purpose of the final keyword?

Answer: The final keyword can be used to declare constants, prevent method overriding, and prevent inheritance of classes.

14. What is the Java Collections Framework?

Answer: The Java Collections Framework is a unified architecture for representing and manipulating collections, providing interfaces like List, Set, Map, and classes like ArrayList, HashSet, and HashMap.

15. What is the difference between ArrayList and LinkedList?

Answer: ArrayList is based on a dynamic array, providing fast random access but slower insertions/deletions. LinkedList is based on a doubly linked list, providing faster insertions/deletions but slower access time.

16. What is exception handling in Java?

Answer: Exception handling is a mechanism to handle runtime errors, allowing the program to continue its normal flow. It uses try, catch, finally, throw, and throws keywords.

17. What is the difference between checked and unchecked exceptions?

Answer: Checked exceptions are checked at compile-time, requiring explicit handling (e.g., IOException). Unchecked exceptions are checked at runtime and do not require handling (e.g., NullPointerException).

18. What is the try-with-resources statement?

Answer: The try-with-resources statement is a feature introduced in Java 7 that automatically closes resources (like files or sockets) when they are no longer needed, ensuring that resources are released properly without requiring explicit finally blocks.

19. What are lambda expressions in Java?

Answer: Lambda expressions, introduced in Java 8, provide a clear and concise way to represent a single method interface using an expression. They enable functional programming features in Java.

20. What is the Stream API in Java?

Answer: The Stream API, introduced in Java 8, allows for functional-style operations on streams of elements, enabling operations like filtering, mapping, and reducing collections in a more readable and efficient manner.

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

Answer:

• String is immutable, meaning once created, its value cannot be changed.
• StringBuilder is mutable and not synchronized, making it faster for single-threaded operations.
• StringBuffer is also mutable but synchronized, making it thread-safe at the cost of performance.

22. What is the purpose of the volatile keyword?

Answer: The volatile keyword in Java is used to indicate that a variable’s value will be modified by different threads. It ensures visibility of changes to variables across threads.

23. What is multithreading in Java?

Answer: Multithreading is a programming concept that allows concurrent execution of two or more threads, enabling efficient use of CPU resources and improving application performance.

24. What is the difference between synchronized and non-synchronized methods?

Answer: A synchronized method locks the object for the duration of the method call, preventing other threads from accessing it. A non-synchronized method does not have this restriction, allowing multiple threads to access it simultaneously.

25. What is the Java Memory Model?

Answer: The Java Memory Model defines how threads interact through memory and what behaviors are allowed in concurrent programming. It specifies rules for visibility, ordering, and atomicity of variables.

26. What is garbage collection in Java?

Answer: Garbage collection is the process of automatically identifying and reclaiming memory that is no longer in use, helping to prevent memory leaks and optimize memory usage.

27. What are the different types of garbage collectors in Java?

Answer: Java provides several garbage collectors, including:

• Serial Garbage Collector
• Parallel Garbage Collector
• Concurrent Mark-Sweep (CMS) Collector
• G1 Garbage Collector
• Z Garbage Collector (ZGC)

28. What is the main method in Java?

Answer: The main method is the entry point of any Java application. It is defined as public static void main(String[] args) and is where the program execution begins.

29. What is the difference between throw and throws?

Answer: throw is used to explicitly throw an exception from a method or block of code, while throws is used in a method signature to declare that a method can throw exceptions, allowing the caller to handle them.

30. What is method overloading?

Answer: Method overloading is a feature that allows a class to have more than one method with the same name but different parameters (type, number, or order), enabling different behaviors based on the input.

31. What is method overriding?

Answer: Method overriding occurs when a subclass provides a specific implementation of a method that is already defined in its superclass, allowing for dynamic method dispatch.

32. What is the instanceof operator?

Answer: The instanceof operator is used to test whether an object is an instance of a specific class or interface, returning a boolean value.

33. What is a static block in Java?

Answer: A static block is a block of code that is executed when the class is loaded into memory. It is used for static initializations of a class.

34. What is the difference between == and equals() for objects?

Answer: == checks if two references point to the same object in memory, while equals() checks if two objects are logically equivalent based on their content.

35. What is a Java package?

Answer: A package is a namespace that organizes a set of related classes and interfaces, providing a way to group them and avoid naming conflicts.

36. What is the purpose of the super keyword?

Answer: The super keyword is used to refer to the immediate parent class object, allowing access to parent class methods and constructors.

37. What is the difference between abstract class and interface?

Answer: An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods (until Java 8, after which it can have default and static methods). A class can implement multiple interfaces but can inherit from only one abstract class.

38. What is the this keyword in Java?

Answer: The this keyword is a reference variable that refers to the current object within an instance method or constructor, helping to distinguish between instance variables and parameters.

39. What is the purpose of the static keyword?

Answer: The static keyword is used to indicate that a particular member (variable or method) belongs to the class rather than instances of the class. Static members can be accessed without creating an instance of the class.

40. What is a nested class in Java?

Answer: A nested class is a class defined within another class. It can be static or non-static and is used to logically group classes that are only used in one place, increasing encapsulation.

41. What is the difference between HashMap and Hashtable?

Answer: HashMap is not synchronized and allows null keys and values, while Hashtable is synchronized and does not allow null keys or values. This makes HashMap faster but less thread-safe than Hashtable.

42. What is the volatile keyword used for?

Answer: The volatile keyword is used to indicate that a variable’s value will be modified by different threads, ensuring that changes made by one thread are visible to others.

43. What is the purpose of the transient keyword?

Answer: The transient keyword is used in serialization to indicate that a field should not be serialized. When an object is serialized, transient fields are ignored.

44. What is the difference between String and StringBuilder?

Answer: String is immutable, meaning its value cannot be changed once created, while StringBuilder is mutable, allowing for modification of its content without creating new objects.

45. What is the default keyword in interfaces?

Answer: The default keyword allows the definition of default methods in interfaces, providing a method implementation that can be inherited by implementing classes.

46. What is the purpose of the assert keyword?

Answer: The assert keyword is used for debugging purposes to make an assertion about a condition that must be true at a specific point in the program. If the condition is false, an AssertionError is thrown.

47. What is the difference between Runnable and Callable?

Answer: Runnable is a functional interface that does not return a result and cannot throw checked exceptions, while Callable can return a result and can throw checked exceptions.

48. What is the Fork/Join framework?

Answer: The Fork/Join framework is a framework introduced in Java 7 that helps in parallel processing by breaking down tasks into smaller subtasks, which can be executed concurrently and then combined to produce a final result.

49. What is the CompletableFuture class?

Answer: The CompletableFuture class is part of the Java Concurrency API and represents a future result of an asynchronous computation. It allows for non-blocking operations and provides methods for combining multiple futures.

50. What are the new features introduced in Java 17?

Answer: Java 17 introduced several new features, including:

• Sealed classes
• Pattern matching for instanceof
• New macOS rendering pipeline
• Enhanced switch expressions
• New APIs and improvements in performance and security.

This list provides a solid foundation for preparing for Java programming interviews in 2025. Each question and answer can be expanded upon based on the interview context and the depth of knowledge required.