OOP and Java 8 Interview Questions

Part A: Object-Oriented Programming (OOP) Interview Questions

Beginner Level OOP Questions (1-15)

Fundamental OOP Concepts

1. What is Object-Oriented Programming?

   • Programming paradigm based on objects containing data and methods

2. What are the four pillars of OOP?

   • Encapsulation, Inheritance, Polymorphism, Abstraction

3. What is a class and object?

   • Class: blueprint/template, Object: instance of a class

4. What is encapsulation? Provide an example.

   • Wrapping data and methods together, hiding internal implementation

   class Person {
       private String name;
       public String getName() { return name; }
       public void setName(String name) { this.name = name; }
   }

5. What is inheritance? What are its types?

   • Mechanism to acquire properties of another class
   • Types: Single, Multilevel, Hierarchical, Multiple (through interfaces), Hybrid

6. What is polymorphism? Explain its types.

   • One interface, multiple implementations
   • Types: Compile-time (method overloading) and Runtime (method overriding)

7. What is abstraction?

   • Hiding implementation details and showing only essential features

8. What is the difference between abstraction and encapsulation?

   • Abstraction: hiding complexity, Encapsulation: hiding data

9. What is method overloading?

   • Same method name with different parameters in same class

10. What is method overriding?

    • Child class provides specific implementation of parent class method

11. What is the difference between overloading and overriding?

    • Overloading: compile-time, same class; Overriding: runtime, different classes

12. What is a constructor? What are its types?

    • Special method to initialize objects
    • Types: Default, Parameterized, Copy constructor

13. What is constructor overloading?

    • Multiple constructors with different parameters

14. What is the difference between constructor and method?

    • Constructor initializes object, method performs operations

15. What is this keyword?

    • Reference to current object instance

Intermediate Level OOP Questions (16-25)

16. What is super keyword?

    • Reference to parent class members

17. What is the difference between this() and super()?

    • this(): calls current class constructor, super(): calls parent constructor

18. What is an abstract class?

    • Class that cannot be instantiated, can have abstract and concrete methods

19. What is an interface?

    • Contract defining method signatures, pure abstraction (before Java 8)

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

    • Abstract class: partial abstraction, Interface: complete abstraction
    • Abstract class: can have constructors, Interface: cannot

21. Can we have multiple inheritance in Java? How?

    • No through classes, Yes through interfaces

22. What is composition?

    • "Has-a" relationship, object contains other objects

23. What is aggregation?

    • Special form of association, "has-a" relationship with independent lifecycle

24. What is the difference between composition and inheritance?

    • Composition: "has-a", Inheritance: "is-a"

25. What is association, aggregation, and composition?

    • Association: relationship between classes
    • Aggregation: weak "has-a"
    • Composition: strong "has-a"

Advanced Level OOP Questions (26-30)

26. What is dynamic method dispatch?

    • Runtime method resolution based on object type

27. What is the diamond problem?

    • Ambiguity in multiple inheritance, solved by interfaces in Java

28. What is SOLID principles?

    • Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion

29. What are design patterns? Name few important ones.

    • Reusable solutions to common problems
    • Singleton, Factory, Observer, Strategy, Decorator

30. What is tight coupling and loose coupling?

    • Tight: high dependency, Loose: low dependency between classes

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Part B: Java 8 Interview Questions

Beginner Level Java 8 Questions (1-15)

Lambda Expressions

1. What are the new features introduced in Java 8?

   • Lambda expressions, Stream API, Functional interfaces, Optional, Method references, Default methods

2. What is a lambda expression?

   • Anonymous function that can be treated as a value

   (parameters) -> expression
   (parameters) -> { statements; }

3. What is a functional interface?

   • Interface with exactly one abstract method (SAM - Single Abstract Method)

4. What is @FunctionalInterface annotation?

   • Ensures interface has only one abstract method

5. What are the built-in functional interfaces in Java 8?

   • Predicate, Function, Consumer, Supplier, UnaryOperator, BinaryOperator

6. What is Predicate functional interface?

   • Takes one argument, returns boolean

   Predicate<String> isEmpty = s -> s.isEmpty();

7. What is Function functional interface?

   • Takes one argument, returns a result

   Function<String, Integer> length = s -> s.length();

8. What is Consumer functional interface?

   • Takes one argument, returns nothing (void)

   Consumer<String> print = s -> System.out.println(s);

9. What is Supplier functional interface?

   • Takes no argument, returns a result

   Supplier<String> supplier = () -> "Hello World";

10. What are method references?

    • Shorthand notation for lambda expressions
    • Types: Static, Instance, Constructor, Instance of arbitrary object

11. What are the types of method references?

    • Static: Class::staticMethod
    • Instance: object::instanceMethod
    • Constructor: Class::new
    • Instance of arbitrary object: Class::instanceMethod

12. What is the difference between lambda expression and method reference?

    • Method reference is shorthand for lambda when calling existing method

13. Can we use lambda expressions with any interface?

    • No, only with functional interfaces

14. What is the difference between lambda and anonymous inner class?

    • Lambda: functional interfaces only, Anonymous class: any interface/abstract class

15. What are default methods in interfaces?

    • Methods with implementation in interfaces (Java 8+)

Intermediate Level Java 8 Questions (16-30)

Stream API

16. What is Stream API?

    • Functional-style operations on collections of objects

17. What are the characteristics of Stream?

    • No storage, Functional, Lazy evaluation, Possibly unbounded, Consumable

18. What is the difference between Collection and Stream?

    • Collection: data structure, Stream: abstraction for processing data

19. What are intermediate and terminal operations in Stream?

    • Intermediate: return Stream (filter, map, sorted)
    • Terminal: return non-Stream result (collect, forEach, reduce)

20. What is the difference between map() and flatMap()?

    • map(): one-to-one mapping
    • flatMap(): one-to-many mapping, flattens nested structures

21. What is filter() method in Stream?

    • Filters elements based on predicate

    list.stream().filter(x -> x > 5).collect(toList());

22. What is collect() method in Stream?

    • Terminal operation to transform stream elements into different form

23. What are Collectors?

    • Utility class with predefined collectors (toList, toSet, groupingBy, etc.)

24. What is reduce() method?

    • Combines stream elements into single result

    Optional<Integer> sum = numbers.stream().reduce(Integer::sum);

25. What is the difference between findFirst() and findAny()?

    • findFirst(): returns first element, findAny(): returns any element

26. What is parallel stream?

    • Stream that can be processed in parallel across multiple threads

    list.parallelStream().filter(predicate);

27. When should you use parallel streams?

    • Large datasets, CPU-intensive operations, independent operations

28. What is Optional class?

    • Container object to handle null values gracefully

29. What are the methods of Optional class?

    • of(), ofNullable(), empty(), isPresent(), ifPresent(), orElse(), orElseGet()

30. What is the difference between orElse() and orElseGet()?

    • orElse(): always executes, orElseGet(): executes only if empty

Advanced Level Java 8 Questions (31-40)

Advanced Features

31. What are static methods in interfaces?

    • Methods that belong to interface, not implementing class

32. What is the diamond problem with default methods?

    • Multiple interfaces with same default method, resolved by implementing class

33. How do you resolve conflicts with multiple default methods?

    • Override in implementing class or use Interface.super.method()

34. What is CompletableFuture?

    • Enhanced Future for asynchronous programming

35. What are the differences between Future and CompletableFuture?

    • CompletableFuture: composable, chainable, better exception handling

36. What is Date and Time API in Java 8?

    • New immutable date-time classes: LocalDate, LocalTime, LocalDateTime, ZonedDateTime

37. What are the advantages of new Date-Time API?

    • Immutable, Thread-safe, Clear API, Better parsing/formatting

38. What is forEach() method?

    • Internal iteration method for collections and streams

39. What is the difference between external and internal iteration?

    • External: for loop (imperative), Internal: forEach (functional)

40. What are StringJoiner and String.join()?

    • Utilities for joining strings with delimiters

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Common Coding Examples

OOP Example:

// Encapsulation + Inheritance + Polymorphism
abstract class Animal {
    protected String name;

    public Animal(String name) { this.name = name; }

    public abstract void makeSound();

    public void sleep() { System.out.println(name + " is sleeping"); }
}

class Dog extends Animal {
    public Dog(String name) { super(name); }

    @Override
    public void makeSound() { System.out.println(name + " barks"); }
}

Java 8 Example:

// Lambda + Stream + Optional
List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David");

Optional<String> result = names.stream()
    .filter(name -> name.startsWith("C"))
    .map(String::toUpperCase)
    .findFirst();

result.ifPresent(System.out::println); // CHARLIE

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Interview Preparation Tips:

For OOP Questions:

• Practice implementing all four pillars with code examples
• Understand real-world scenarios where each concept applies
• Be ready to draw UML diagrams
• Know design patterns and SOLID principles

For Java 8 Questions:

• Practice writing lambda expressions and method references
• Master Stream API operations with examples
• Understand functional programming concepts
• Practice converting imperative code to functional style

Common Follow-up Areas:

• Performance implications
• Best practices and when to use what
• Real-world application scenarios
• Code optimization techniques