1. Can an interface have a constructor? Why or why not?

• Answer: No, an interface cannot have a constructor because it is not designed to be instantiated. Interfaces are meant to define a contract that implementing classes adhere to. Constructors are associated with creating object instances, which is not applicable for interfaces.

2. What is the significance of the default keyword in Java 8 interfaces?

• Answer: The default keyword allows interfaces to have method implementations. This was introduced in Java 8 to provide backward compatibility for interfaces when new methods are added. Classes implementing the interface can either use the default implementation or override it.

3. Can a class implement multiple interfaces in Java? If yes, explain how.

• Answer: Yes, a class can implement multiple interfaces in Java. It does so by using the implements keyword followed by a comma-separated list of interface names. Example:

class MyClass implements Interface1, Interface2 {

    // Implement methods of both interfaces

}

4. Can an interface extend another interface? If so, what is the use case?

• Answer: Yes, an interface can extend another interface using the extends keyword. This allows the child interface to inherit the methods of the parent interface, enabling modular design and reusability. Example:

interface ParentInterface {

    void method1();

}

interface ChildInterface extends ParentInterface {

    void method2();

}

5. What happens if a class implements two interfaces with conflicting default methods?

• Answer: If a class implements two interfaces with conflicting default methods, it must explicitly override the conflicting method to resolve the ambiguity. Example:

class MyClass implements Interface1, Interface2 {

    @Override

    public void defaultMethod() {

        // Custom implementation to resolve conflict

    }

}

6. Can you explain the difference between public, private, protected, and package-private access modifiers?

• Answer:
  • public: Accessible from anywhere.
  • private: Accessible only within the same class.
  • protected: Accessible within the same package and by subclasses in other packages.
  • (Default) Package-private: Accessible only within the same package.

7. What is the purpose of the protected access modifier? Can you access a protected method from another package?

• Answer: protected allows visibility to subclasses and within the same package. A protected method can be accessed from another package only through inheritance. Example:

class Parent {

    protected void display() {

        System.out.println(“Protected method”);

    }

}

class Child extends Parent {

    public void access() {

        display(); // Accessible via inheritance

    }

}

8. What is the default access level for a class, method, or variable if no access modifier is specified?

• Answer: The default access level is package-private, meaning it is accessible only within the same package.

9. Can you access a private variable from a subclass?

• Answer: No, private variables are not accessible from a subclass. However, they can be accessed indirectly through public or protected getter and setter methods.

10. What is a functional interface? Can you give an example?

• Answer: A functional interface is an interface with exactly one abstract method. It can be used as a target for lambda expressions. Example:

@FunctionalInterface

interface MyFunctionalInterface {

    void perform();

}

11. Explain the concept of Lambda Expressions in Java 8. How do they improve code readability?

• Answer: Lambda expressions provide a concise way to represent anonymous functions. They improve code readability by reducing boilerplate code for anonymous classes, especially in functional programming scenarios. Example:

(int a, int b) -> a + b;

12. What is the Stream API in Java 8, and how is it different from a collection?

• Answer: The Stream API is a sequence of elements supporting functional-style operations like filtering, mapping, and reducing. Unlike collections, streams do not store elements; they process data lazily and can be parallelized for better performance.

Key Difference:

• Collections: Store and manage data in memory.
• Streams: Focus on computation and transformation of data in a pipeline.

13. What is the difference between map() and flatMap() in Java 8 Streams?

• map(): Transforms each element in the stream and wraps the result in a stream of individual objects.
  • Example: Convert a list of strings to their lengths.
• flatMap(): Transforms each element into a stream and flattens the resulting streams into a single stream.
  • Example: Flatten a list of lists into a single list.
• Key Difference: flatMap() removes nested structures, whereas map() retains them.

14. How do you use the Optional class in Java 8 to avoid NullPointerException?

• Answer: The Optional class wraps a value that may or may not be null. It provides methods like isPresent(), ifPresent(), and orElse() to handle values safely.
  • Example:

Optional<String> optional = Optional.ofNullable(value);

optional.ifPresent(System.out::println); // Prints value if not null

String result = optional.orElse(“Default Value”); // Provides default if null

15. What are the major improvements introduced in java.time package in Java 8?

• Answer: The java.time package provides a modern API for date and time operations. Key improvements include:
  • Immutable and thread-safe classes (LocalDate, LocalTime, ZonedDateTime).
  • Better handling of time zones.
  • Parsing and formatting dates using DateTimeFormatter.
  • More intuitive APIs for operations like adding/subtracting dates.

16. Can you create an instance of a class with only static methods? How would you call the methods?

• Answer: While you can technically create an instance of such a class, it is unnecessary. Static methods are called using the class name directly.
  • Example:

MyClass.staticMethod();

17. What is the difference between a static method and a non-static method?

• Answer:
  • Static Method:
    • Belongs to the class, not an instance.
    • Can be called using the class name.
    • Cannot access non-static members directly.
  • Non-Static Method:
    • Belongs to an instance.
    • Can access both static and non-static members.

18. Can you access a non-static variable from a static method? Why or why not?

• Answer: No, you cannot directly access a non-static variable from a static method because static methods are associated with the class, not an instance. Non-static variables require an object instance to access.

19. What is a static block in Java, and when is it executed?

• Answer: A static block is a block of code that is executed when the class is loaded into memory. It is typically used to initialize static variables.
  • Example:

static {

    System.out.println(“Static block executed”);

}

20. How would you call a static method in another class?

• Answer: You call a static method in another class using the class name followed by the method name.
  • Example:

AnotherClass.staticMethod();

21. Can a static variable be overridden by a subclass? Why or why not?

• Answer: No, static variables cannot be overridden because they are associated with the class rather than any instance. However, they can be hidden by declaring a static variable with the same name in the subclass.
• Example of hiding:

class Parent {

    static int value = 10;

}

class Child extends Parent {

    static int value = 20;

}

System.out.println(Parent.value); // 10

System.out.println(Child.value);  // 20