1. What is Java?

Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It runs on millions of devices and is "Write Once, Run Anywhere" (WORA).

2. What are the main features of Java?

Object-Oriented
Platform Independent
Robust and Secure
Multithreaded
High Performance (via JIT)
Garbage Collected

3. What is the JVM, JRE, and JDK?

JVM (Java Virtual Machine): The abstract machine that provides the runtime environment in which Java bytecode can be executed.
JRE (Java Runtime Environment): A set of software tools used for developing Java applications; includes JVM, class libraries, and other files.
JDK (Java Development Kit): The full software development environment; includes JRE plus development tools like the compiler (javac).

4. What is the JIT compiler?

The Just-In-Time compiler is a part of the JVM that improves performance by compiling bytecode into native machine code at runtime.

5. What is the difference between `checked` and `unchecked` exceptions?

Checked Exceptions: Checked at compile-time (e.g., IOException). You must either handle them or declare them in the throws clause.
Unchecked Exceptions: Occur at runtime (e.g., NullPointerException, ArrayIndexOutOfBoundsException). They extend RuntimeException.

6. What is the difference between `StringBuffer` and `StringBuilder`?

StringBuffer: Synchronized (thread-safe) but slower.
StringBuilder: Not synchronized (not thread-safe) but faster; preferred for single-threaded operations.

7. What is an "Interface"?

A blueprint of a class that contains only abstract methods and static constants. It is used to achieve complete abstraction and multiple inheritance in Java.

8. What is an "Abstract Class"?

A class that cannot be instantiated and may or may not contain abstract methods. It is used to provide a common base for related classes.

9. What is the difference between `Interface` and `Abstract Class`?

Interface: Supports multiple inheritance; methods are public abstract by default.
Abstract Class: Supports single inheritance; can have constructors, private members, and non-abstract methods.

10. What are the "Access Modifiers" in Java?

public: Accessible everywhere.
protected: Accessible within the same package and by subclasses.
default: Accessible only within the same package.
private: Accessible only within the same class.

11. What is "Polymorphism"?

The ability of an object to take on many forms. In Java, this is achieved through method overloading (compile-time) and method overriding (runtime).

12. What is the difference between "Method Overloading" and "Method Overriding"?

Overloading: Multiple methods with the same name but different parameters in the same class.
Overriding: A method in a subclass with the same name, parameters, and return type as in its superclass.

13. What is "Encapsulation"?

The process of wrapping code and data together into a single unit (class). This is achieved by making class variables private and providing public getter and setter methods.

14. What is "Inheritance"?

A mechanism where one class acquires the properties (fields) and behaviors (methods) of another class. Use the extends keyword.

15. What is the `static` keyword?

Used for memory management. It makes a field or method belong to the class rather than to instances of the class.

16. What is the `final` keyword?

Used to restrict the user:

final variable: Value cannot be changed.
final method: Cannot be overridden.
final class: Cannot be inherited.

17. What is the `finally` block?

A block of code that is always executed after a try-catch block, regardless of whether an exception is thrown or caught. It's used for cleanup (like closing resources).

18. What is the `finalize()` method?

A method called by the Garbage Collector before an object is destroyed. (It's deprecated in modern Java).

19. What is "Garbage Collection"?

The process by which Java automatically reclaims the memory used by objects that are no longer reachable by the application.

20. What is the `super` keyword?

A reference variable used to refer to immediate parent class objects (e.g., to call parent constructors or methods).

21. What is the `this` keyword?

A reference variable that refers to the current object.

22. What is "Diamond Problem" in inheritance?

A situation where a class inherits from two classes that both inherit from a common superclass, potentially leading to ambiguity. Java avoids this by not supporting multiple inheritance through classes.

23. What are "Wrapper Classes"?

Classes that wrap primitive data types as objects (e.g., Integer, Double, Character). They are useful for using primitives in collections.

24. What is "Autoboxing" and "Unboxing"?

Autoboxing: Automatically converting a primitive into its corresponding wrapper class (e.g., int to Integer).
Unboxing: Automatically converting a wrapper class object back into its primitive type.

25. What is the "Java Collection Framework"?

A set of classes and interfaces that provide a unified architecture for storing and manipulating groups of objects (e.g., List, Set, Map).

26. What is the difference between `ArrayList` and `LinkedList`?

ArrayList: Uses a dynamic array; better for random access; slower for insertions/deletions in the middle.
LinkedList: Uses a doubly linked list; better for insertions/deletions; slower for random access.

27. What is the difference between `HashMap` and `Hashtable`?

HashMap: Not synchronized; allows one null key and multiple null values; non-legacy.
Hashtable: Synchronized (thread-safe); does not allow nulls; legacy class.

28. What is the difference between `HashSet` and `TreeSet`?

HashSet: Does not maintain any order; fastest operations.
TreeSet: Maintains elements in natural or custom sorted order; slower.

29. What is an "Iterator"?

An interface used to traverse through a collection of objects one by one. It provides methods like hasNext() and next().

30. What is "Generics" in Java?

A feature that allows you to specify the type of objects that a collection can hold, providing compile-time type safety.

31. What are "Lambda Expressions" (Java 8)?

A way to represent anonymous functions in a concise and readable way. They help in achieving functional programming in Java.

32. What is the "Stream API" (Java 8)?

A powerful tool for processing collections of data in a functional and declarative way (e.g., filtering, mapping, reducing).

33. What is the "Optional" class (Java 8)?

A container object used to represent a value that may or may not be present, helping to avoid NullPointerException.

34. What is the "Default Method" (Java 8) in an Interface?

A method with a body inside an interface, allowed to provide backward compatibility without breaking existing implementers.

35. What is a "Thread"?

The smallest unit of execution within a program. Java supports multithreading by extending the Thread class or implementing the Runnable interface.

36. What is "Deadlock"?

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

37. What is "Synchronization"?

The process of controlling the access of multiple threads to a shared resource, preventing data inconsistency.

38. What is the `volatile` keyword?

Indicates that a variable's value will be modified by different threads, ensuring that the updated value is always read from the main memory.

39. What is "Reflection"?

The ability of a program to examine and modify its own structure (classes, methods, fields) at runtime.

40. What is "Serialization" and "Deserialization"?

Serialization: Converting an object into a byte stream (to save or transmit).
Deserialization: Reconstructing the object from the byte stream.

41. What is the `transient` keyword?

Used during serialization to indicate that a field should not be serialized.

42. What is the difference between `Comparator` and `Comparable`?

Comparable: Used for a class's "natural" sorting order (one implementation inside the class).
Comparator: Used for custom sorting orders (multiple implementations outside the class).

43. What is "Type Casting"?

Converting one data type into another (Implicit: Widening, Explicit: Narrowing).

44. What is a "Constructor"?

A special method with the same name as the class, used to initialize an object. It has no return type.

45. What is "Cloneable" interface?

A marker interface used to indicate that an object can be cloned using the clone() method.

46. What is the difference between `==` and `.equals()`?

==: Compares reference (memory address) for objects.
.equals(): Compares the content/value of objects (if overridden).

47. What is the "Singleton" pattern?

A design pattern that ensures a class has only one instance and provides a global point of access to it.

48. What is the "Factory" pattern?

A design pattern used to create objects without specifying the exact class of object that will be created.

49. What is "Java Reflection API"?

A set of classes used to inspect and manipulate classes, methods, and fields at runtime.

50. What is the "Java Module System" (Java 9)?

A way to package and organize Java code into modules, providing better encapsulation and reduced footprint.

Java Interview Questions

Table of Contents

What is Java?

What are the main features of Java?

What is the JVM, JRE, and JDK?

What is the JIT compiler?

What is the difference between <code>checked</code> and <code>unchecked</code> exceptions?

What is the difference between <code>StringBuffer</code> and <code>StringBuilder</code>?

What is an "Interface"?

What is an "Abstract Class"?

What is the difference between <code>Interface</code> and <code>Abstract Class</code>?

What are the "Access Modifiers" in Java?

What is "Polymorphism"?

What is the difference between "Method Overloading" and "Method Overriding"?

What is "Encapsulation"?

What is "Inheritance"?

What is the <code>static</code> keyword?

What is the <code>final</code> keyword?

What is the <code>finally</code> block?

What is the <code>finalize()</code> method?

What is "Garbage Collection"?

What is the <code>super</code> keyword?

What is the <code>this</code> keyword?

What is "Diamond Problem" in inheritance?

What are "Wrapper Classes"?

What is "Autoboxing" and "Unboxing"?

What is the "Java Collection Framework"?

What is the difference between <code>ArrayList</code> and <code>LinkedList</code>?

What is the difference between <code>HashMap</code> and <code>Hashtable</code>?

What is the difference between <code>HashSet</code> and <code>TreeSet</code>?

What is an "Iterator"?

What is "Generics" in Java?

What are "Lambda Expressions" (Java 8)?

What is the "Stream API" (Java 8)?

What is the "Optional" class (Java 8)?

What is the "Default Method" (Java 8) in an Interface?

What is a "Thread"?

What is "Deadlock"?

What is "Synchronization"?

What is the <code>volatile</code> keyword?

What is "Reflection"?

What is "Serialization" and "Deserialization"?

What is the <code>transient</code> keyword?

What is the difference between <code>Comparator</code> and <code>Comparable</code>?

What is "Type Casting"?

What is a "Constructor"?

What is "Cloneable" interface?

What is the difference between <code>==</code> and <code>.equals()</code>?

What is the "Singleton" pattern?

What is the "Factory" pattern?

What is "Java Reflection API"?

What is the "Java Module System" (Java 9)?