1. Which of the following objects can be created in Java?

A. An abstract class object B. An interface object C. An object of a concrete subclass of an abstract class D. None of the above

2. What does inheritance allow when dealing with abstract classes in Java?

A. Abstract classes can inherit from interfaces only B. Concrete subclasses can provide implementations for missing behavior C. Subclasses cannot access any fields of the abstract class D. Subclasses must also be abstract if the parent class is abstract

3. What does it mean when a class is declared abstract but does not contain any abstract method?

A. It's an incomplete class meant to be a superclass B. It must be instantiated directly C. It is a syntax error D. The class cannot contain any methods

4. Which of the following statements is true about constructors in abstract classes?

A. Abstract classes cannot have constructors B. Abstract classes have constructors to be called during subclass instantiation C. Constructors in abstract classes can be called directly D. Constructors in abstract classes must be abstract too

5. What is the output when mailCheck() is called on an Employee reference that points to a Salary object?

A. Employee class mailCheck() is called B. Salary class mailCheck() is called C. Abstract method error occurs D. Null pointer exception

6. Which keyword is used in Java to declare an abstract class?

A. static B. abstract C. final D. interface

7. What is the main purpose of abstraction in Java programming?

A. To expose all implementation details to the user B. To provide both the functionality and internal workings of the class to the user C. To hide implementation details and provide only functionality to the user D. To allow direct instantiation of abstract classes

8. What is the key purpose of abstraction in Object-Oriented Programming?

A. To show all the details of how a class is implemented B. To hide implementation details and show only functionality C. To increase the complexity of the program D. To force a class to be instantiated

9. Can an abstract class contain non-abstract methods?

A. No, all methods must be abstract B. Yes, it can contain both abstract and non-abstract methods C. Only static methods can be non-abstract D. Only private methods can be non-abstract

10. If a class contains one or more abstract methods, what must be true about the class?

A. It must be declared as abstract B. It must be final C. It must be static D. It can be instantiated directly

11. What must a subclass do if it inherits an abstract class with abstract methods?

A. It must implement all the abstract methods or be declared abstract itself B. It can ignore the abstract methods C. It can choose to instantiate the superclass directly D. It must override only some of the abstract methods

12. Why might a method be declared abstract in a parent class?

A. To enforce that subclasses provide their own implementation B. To prevent the method from being used C. To provide a default implementation for all subclasses D. To optimize runtime performance

13. Which of the following is NOT a characteristic of an abstract class?

A. It can have fields and concrete methods B. It can have constructors C. It can be instantiated directly D. It can contain abstract methods

14. In the provided example, which class is abstract?

A. Salary B. Employee C. AbstractDemo D. None of the above

15. In the given Employee example, which constructor statement is true?

A. Abstract classes cannot have constructors B. The Employee abstract class has a constructor to initialize fields C. Abstract classes must have empty constructors D. Constructor is declared abstract in the Employee class

16. What does the term "hiding implementation details" mean in abstraction?

A. Showing only method signatures, not the code within B. Writing code in private methods only C. Removing code for faster execution D. Allowing users to modify internal variables

17. What does the output indicate when calling mailCheck method on Salary reference?

A. It calls the mailCheck method of Employee class B. It calls the mailCheck method of Salary class C. The method is not accessible D. It raises an error since Salary does not implement mailCheck

18. What is the correct way to create an abstract method in Java?

A. void computePay() {} B. abstract void computePay(); C. void computePay(); D. abstract void computePay() {}

19. How is an abstract method declared in Java?

A. public abstract void methodName(); B. public void methodName() {}; C. public void methodName(); D. abstract methodName() {}

20. What would happen if Salary did not override computePay() from Employee?

A. Salary would become abstract class B. Salary would be invalid and cause compile-time error C. computePay() method from Employee would be called at runtime D. computePay() would be ignored

21. How does Java enforce abstraction using abstract classes?

A. By allowing abstract classes to be instantiated B. By requiring subclasses to implement all abstract methods C. By removing all methods from abstract classes D. By hiding all fields of the class

22. Which is NOT true about abstract classes in Java?

A. They cannot be instantiated directly B. They can have instance variables C. They cannot contain constructors D. They can contain implemented methods

23. What does the abstract keyword before a class declaration indicate?

A. The class can be instantiated B. The class contains only static methods C. The class contains at least one abstract method or is designed to be extended D. The class has no constructors

24. How is abstraction achieved in Java?

A. Using constructors and destructors B. Using abstract classes and interfaces C. Using only concrete classes D. Using only public methods

25. What is abstraction in Object-Oriented Programming?

A. The process of writing detailed code for every function B. Hiding implementation details and only exposing functionality C. Writing code without any classes D. Exposing all data of an object to the user

26. Can an abstract class extend another abstract class?

A. Yes, abstract classes can form hierarchies by extending other abstract classes B. No, abstract classes can't extend other classes C. Only concrete classes can extend abstract classes D. Abstract classes can only implement interfaces

27. Which statement about inheritance from an abstract class is correct?

A. The subclass can inherit but cannot override abstract methods B. The subclass can only inherit concrete methods and cannot add new methods C. The subclass must provide implementations for all abstract methods inherited D. The subclass cannot be abstract itself

28. If a class contains an abstract method, what must be true about the class?

A. It must be declared abstract B. It must be declared final C. It must implement all interfaces D. It must have no constructors

29. Why would you declare a class abstract without having abstract methods?

A. To prevent instantiation and design incomplete class B. Because it has no data members C. To allow instantiation only in other packages D. Abstract keyword is mandatory for interface implementation

30. When a subclass inherits an abstract class and does not implement all abstract methods, what must happen?

A. The subclass becomes abstract as well B. The subclass is concrete and can be instantiated C. The subclass will have runtime errors D. The compiler issues a warning but allows instantiation

31. What happens if you try to instantiate an abstract class?

A. The object is created successfully B. Compiler error "class is abstract; cannot be instantiated" C. Runtime exception is thrown D. Warning but code compiles successfully

32. What is required in a subclass of an abstract superclass having abstract methods?

A. To implement all abstract methods or remain abstract itself B. To implement any one of the abstract methods C. To skip implementing abstract methods if not used D. Abstract methods are automatically implemented

33. What is the significance of the semicolon (;) at the end of abstract method declaration?

A. It indicates the method has no implementation B. It ends the class declaration C. It is required only when the method is static D. It is optional syntax

34. From the tutorial example, which method is specifically declared abstract in Employee?

A. mailCheck() B. toString() C. computePay() D. getName()

35. Why does the abstract class Employee have fields and constructors if it cannot be instantiated?

A. To initialize the inherited state for subclasses B. To prevent subclass creation C. To enforce abstract methods D. Constructors are ignored in abstract classes

36. In the example, what does the Salary class method computePay() return?

A. The monthly salary B. The annual salary divided by 12 C. The weekly salary (annual salary divided by 52) D. Always zero

37. Can an abstract class be instantiated directly in Java?

A. Yes, always B. Yes, but only if it has no abstract methods C. No, it cannot be instantiated D. Only through an interface

38. Which keyword is used to specify that a method is abstract?

A. final B. static C. abstract D. void

39. What will happen if you try to instantiate an abstract class directly in code like "Employee e = new Employee(...)”?

A. It will compile and run successfully B. Compilation will fail with “class is abstract, cannot be instantiated” error C. Runtime exception will be thrown D. It will instantiate a default constructor

40. What is the effect of declaring a class as abstract in Java?

A. It can be instantiated like a normal class B. It cannot be instantiated directly C. It cannot have any constructor D. It only contains abstract methods

41. What is the relationship between Employee and Salary classes in the described example?

A. Employee inherits Salary B. Salary is an abstract class extending Employee C. Salary inherits Employee and is a concrete class D. Salary and Employee are unrelated classes

42. What concept does the statement "user will have information on what the object does but not how it does it" refer to?

A. Polymorphism B. Abstraction C. Inheritance D. Encapsulation

43. If a class contains at least one abstract method, what must be true about the class?

A. The class must implement all inherited methods immediately B. The class must be declared as abstract C. The class can be instantiated like any normal class D. The class cannot have any constructors

44. In the example code, how is the constructor of the abstract superclass Employee called?

A. Using super() in subclass constructor B. Implicitly by Java runtime C. Using abstract keyword constructor D. It cannot be called

45. Which of the following is true about abstract methods in Java?

A. They have a body with implementation B. They must be declared private C. They contain only a method signature without a body D. They do not require the parent class to be abstract

46. What is a key difference between an abstract method and a regular method?

A. Abstract methods have a method body; regular methods do not B. Abstract methods do not have a method body and end with a semicolon C. Abstract methods can be static; regular methods cannot D. Regular methods cannot be overridden

47. What is a key characteristic of an abstract class compared to an interface?

A. Abstract classes can have constructors and state (fields) B. Interfaces can have constructors before Java 8 C. Abstract classes cannot have any implemented methods D. Interfaces allow private data members

48. In the example, what method must the Salary class override from Employee?

A. mailCheck() method only B. computePay() method only C. Both mailCheck() and computePay() methods D. No methods need to be overridden

49. Can an abstract class in Java be instantiated directly?

A. Yes, always B. No, never C. Only if it contains no abstract methods D. Only if it contains a constructor

50. What is the main purpose of declaring a method as abstract?

A. To provide a default implementation B. To hide the method from subclasses C. To enforce subclasses to provide method implementation D. To create multiple versions of the method