1. What is Java?

Answer: Java is a high-level, class-based, object-oriented programming language designed to have as few implementation dependencies as possible.

2. What are the features of Java?

Answer: Some key features of Java include:

• Object-Oriented
• Platform-Independent
• Simple and Secure
• Robust
• Multithreading

3. What is JVM, JRE, and JDK?

Answer:

• JVM (Java Virtual Machine): Executes Java bytecode.
• JRE (Java Runtime Environment): Provides libraries for running Java applications.
• JDK (Java Development Kit): Contains tools for Java development, including JRE and compiler.

4. What is a class in Java?

Answer: A class is a blueprint for creating objects in Java. Example:

class Dog {
    String name;
    void bark() {
        System.out.println("Woof!");
    }
}

5. What is an object in Java?

Answer: An object is an instance of a class. Example:

Dog dog = new Dog();

6. What is Inheritance in Java?

Answer: Inheritance allows a class to inherit properties and methods from another class. Example:

class Animal {}
class Dog extends Animal {}

7. What is Polymorphism in Java?

Answer: Polymorphism allows objects to be treated as instances of their parent class. Example:

Animal a = new Dog();

8. What is Encapsulation in Java?

Answer: Encapsulation is the bundling of data and methods into a single unit, the class, and restricting access to some of the object's components.

9. What is Abstraction in Java?

Answer: Abstraction is hiding complex implementation details and showing only the necessary features of an object. Example:

abstract class Animal {
    abstract void sound();
}

10. What are interfaces in Java?

Answer: Interfaces are abstract types used to specify methods that a class must implement. Example:

interface Animal {
    void sound();
}

11. What is the difference between Abstract Class and Interface?

Answer:

• An abstract class can have method implementations, whereas an interface cannot.
• A class can implement multiple interfaces but can only extend one abstract class.

12. What is the final keyword in Java?

Answer: The final keyword can be applied to variables, methods, and classes, indicating that they cannot be modified, overridden, or inherited.

13. What is a static method in Java?

Answer: A static method belongs to the class rather than the instance of the class. Example:

class Example {
    static void display() {
        System.out.println("Static method");
    }
}

14. What is the this keyword in Java?

Answer: The this keyword refers to the current object in a method or constructor.

15. What is the super keyword in Java?

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

16. What is method overloading in Java?

Answer: Method overloading is having multiple methods with the same name but different parameter lists. Example:

class MathOperations {
    int add(int a, int b) { return a + b; }
    double add(double a, double b) { return a + b; }
}

17. What is method overriding in Java?

Answer: Method overriding occurs when a subclass provides a specific implementation of a method declared in its superclass. Example:

class Animal {
    void sound() { System.out.println("Animal sound"); }
}
class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

18. What is a constructor in Java?

Answer: A constructor is a block of code used to initialize objects. It has the same name as the class and no return type.

19. What is the default constructor?

Answer: A default constructor is provided by Java if no constructors are explicitly defined by the programmer.

20. What are the access modifiers in Java?

Answer:

• Private: Only accessible within the same class.
• Default: Accessible within the same package.
• Protected: Accessible within the same package and subclasses.
• Public: Accessible from anywhere.

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

Answer: == compares reference equality, while equals() compares content.

22. What is a package in Java?

Answer: A package is a namespace that organizes related classes and interfaces. Example:

package com.example;

23. What is exception handling in Java?

Answer: Exception handling is the process of managing runtime errors using try-catch blocks. Example:

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

24. What is a checked exception in Java?

Answer: Checked exceptions are exceptions that are checked at compile-time. Example: IOException.

25. What is an unchecked exception?

Answer: Unchecked exceptions are exceptions that are checked at runtime. Example: NullPointerException.

26. What is the difference between throw and throws?

Answer:

• throw is used to explicitly throw an exception.
• throws is used to declare exceptions in a method signature.

27. What is the difference between an error and an exception?

Answer: Errors are serious problems that an application should not try to handle, while exceptions are conditions that a program can catch.

28. What are Java collections?

Answer: Java collections provide a way to store and manage groups of objects. Example: ArrayList, HashMap, etc.

29. What is the difference between ArrayList and LinkedList?

Answer: ArrayList is backed by an array, while LinkedList is implemented as a doubly-linked list.

30. What is a HashMap in Java?

Answer: HashMap is a collection that stores key-value pairs where each key is unique. Example:

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);

31. What is a Set in Java?

Answer: A Set is a collection that does not allow duplicate elements.

32. What is the difference between HashSet and TreeSet?

Answer: HashSet is unordered, while TreeSet maintains a sorted order.

33. What is multithreading in Java?

Answer: Multithreading is the concurrent execution of two or more threads. Example:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running");
    }
}

34. What is synchronization in Java?

Answer: Synchronization ensures that only one thread accesses a resource at a time.

35. What is the volatile keyword in Java?

Answer: The volatile keyword ensures that a variable is read from the main memory and not from a thread's local cache.

36. What is garbage collection in Java?

Answer: Garbage collection is the process of automatically reclaiming memory by removing unused objects.

37. What is the purpose of finalize() method in Java?

Answer: The finalize() method is invoked before an object is garbage collected.

38. What is a lambda expression in Java?

Answer: Lambda expressions provide a concise way to express behavior. Example:

(int x, int y) -> x + y

39. What is Stream API in Java?

Answer: The Stream API provides functional programming features to process collections of objects.

40. What is the Optional class in Java?

Answer: Optional is a container object used to avoid null pointer exceptions. Example:

Optional<String> value = Optional.of("Hello");

41. What is reflection in Java?

Answer: Reflection is the ability to inspect and manipulate classes and objects at runtime.

42. What is the Singleton pattern in Java?

Answer: The Singleton pattern restricts the instantiation of a class to one object. Example:

class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

43. What is the Factory pattern in Java?

Answer: The Factory pattern provides a way to create objects without specifying the exact class of the object. Example:

class AnimalFactory {
    public Animal createAnimal(String type) {
        if (type.equals("Dog")) {
            return new Dog();
        }
        return null;
    }
}

44. What is Dependency Injection in Java?

Answer: Dependency Injection is a design pattern that allows objects to be injected into a class, rather than being created by the class.

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

Answer:

• String is immutable.
• StringBuilder is mutable and non-synchronized.
• StringBuffer is mutable and synchronized.

46. What is a Functional Interface in Java?

Answer: A functional interface is an interface with a single abstract method. Example:

@FunctionalInterface
interface MyFunction {
    void apply();
}

47. What is the transient keyword in Java?

Answer: The transient keyword is used to indicate that a variable should not be serialized.

48. What is the synchronized keyword in Java?

Answer: The synchronized keyword ensures that a method or block is accessed by only one thread at a time.

49. What is the difference between wait() and sleep() in Java?

Answer:

• wait() releases the lock on the object.
• sleep() pauses the thread but does not release the lock.

50. What is a Stream in Java?

Answer: Streams allow functional-style operations on collections. Example:

List<String> list = Arrays.asList("A", "B", "C");
list.stream().filter(s -> s.equals("A")).forEach(System.out::println);

This set of questions covers both beginner and advanced concepts in Java to prepare for an interview.