OOP stands for Object-Oriented Programming, a paradigm centered around objects and classes.
2. What is a Class in Java?
a) A blueprint for creating objects
b) A specific instance of an object
c) A method
d) A variable
Answer:
a) A blueprint for creating objects
Explanation:
In Java, a class is a blueprint from which individual objects are created.
3. What is an Object in Java?
a) A reference to a class
b) A type of variable
c) An instance of a class
d) A method
Answer:
c) An instance of a class
Explanation:
An object is an instance of a class, containing both state (attributes) and behavior (methods).
4. What is Inheritance in Java?
a) Sharing variables between classes
b) Copying properties and behaviors from one class to another
c) A method calling another method
d) Dividing a class into subparts
Answer:
b) Copying properties and behaviors from one class to another
Explanation:
Inheritance is a mechanism where one class acquires the properties (fields) and behaviors (methods) of another class.
5. What is Encapsulation in Java?
a) The binding of data and methods that manipulate the data
b) The separation of data from methods
c) The ability to inherit methods
d) Creating multiple copies of the same method
Answer:
a) The binding of data and methods that manipulate the data
Explanation:
Encapsulation in Java is about binding the data (variables) and the methods (functions) that manipulate the data into a single unit, typically a class.
6. What is Polymorphism in Java?
a) The ability of a variable to hold multiple types
b) The ability of a method to perform different tasks based on the object
c) Changing the structure of a class
d) Breaking down methods into smaller parts
Answer:
b) The ability of a method to perform different tasks based on the object
Explanation:
Polymorphism in Java is the ability of an object to take on many forms, allowing methods to perform different operations based on the object type.
7. What is Abstraction in Java?
a) Hiding the implementation details and showing only functionality
b) Removing data from a class
c) Splitting a class into multiple classes
d) Combining similar methods into a single method
Answer:
a) Hiding the implementation details and showing only functionality
Explanation:
Abstraction in Java is about hiding the internal implementation and showing only the necessary functionalities to the user.
8. What is the main feature of OOP?
a) Speed
b) Simplicity
c) Reusability
d) Performance
Answer:
c) Reusability
Explanation:
One of the main features of OOP is reusability, which allows code to be reused through mechanisms like inheritance and polymorphism.
9. Which principle of OOP helps to reduce redundancy?
a) Abstraction
b) Encapsulation
c) Inheritance
d) Polymorphism
Answer:
c) Inheritance
Explanation:
Inheritance helps to reduce redundancy in code by inheriting common properties from a base class.
10. What is a Constructor in Java?
a) A method that constructs variable names
b) A block of code used for initializing an object
c) A special variable inside a class
d) A return type of a method
Answer:
b) A block of code used for initializing an object
Explanation:
A constructor in Java is a special method used to initialize objects.
11. Which of these is a valid class in Java?
a) class@MyClass {}
b) class 123MyClass {}
c) class MyClass {}
d) class *MyClass {}
Answer:
c) class MyClass {}
Explanation:
In Java, a valid class name starts with a letter and uses camel case notation.
12. Can a class in Java be both abstract and final?
a) Yes
b) No
c) Only in special cases
d) Only if it does not have methods
Answer:
b) No
Explanation:
A class cannot be both abstract and final in Java because an abstract class is meant to be inherited, while a final class cannot be inherited.
13. What does overriding a method mean?
a) Changing the method's return type
b) Providing a new implementation for a method in a subclass
c) Renaming a method in a subclass
d) Deleting a method from a superclass
Answer:
b) Providing a new implementation for a method in a subclass
Explanation:
Method overriding means defining a method in a subclass that already exists in the superclass with the same signature (name and parameters).
14. What is an Interface in Java?
a) A concrete class with all methods implemented
b) A collection of abstract methods and constants
c) A special type of class with only static methods
d) A method without a body
Answer:
b) A collection of abstract methods and constants
Explanation:
An interface in Java is a reference type that can contain constants and abstract methods.
15. What does the 'extends' keyword indicate in a Java class declaration?
a) The class is extending its functionality
b) The class is inheriting from a superclass
c) The class is abstract
d) The class will be extended in the future
Answer:
b) The class is inheriting from a superclass
Explanation:
The 'extends' keyword in a class declaration indicates that the class is inheriting from another class, known as the superclass.
16. Which OOP concept is best for separating interface from implementation?
a) Encapsulation
b) Inheritance
c) Abstraction
d) Polymorphism
Answer:
c) Abstraction
Explanation:
Abstraction separates the interface (what the object can do) from the implementation (how the object does it).
17. What is the purpose of the 'this' keyword in Java?
a) To refer to the current class instance
b) To create a new object
c) To call a static method
d) To declare a variable
Answer:
a) To refer to the current class instance
Explanation:
The 'this' keyword in Java is used within an instance method or a constructor to refer to the current object.
18. What is a package in Java?
a) A collection of classes and interfaces
b) A type of data structure
c) A method
d) An operator
Answer:
a) A collection of classes and interfaces
Explanation:
A package in Java is a namespace that groups related classes and interfaces, helping to organize the code.
19. What does it mean for a class to be 'abstract' in Java?
a) The class cannot be instantiated
b) The class does not contain any methods
c) The class is incomplete and must be inherited
d) Both a and c
Answer:
d) Both a and c
Explanation:
An abstract class in Java cannot be instantiated and is meant to be a superclass for other classes.
20. What is a 'final' class in Java?
a) A class that cannot be extended
b) A class that is complete and cannot be changed
c) A class that can be instantiated
d) Both a and b
Answer:
d) Both a and b
Explanation:
A final class cannot be subclassed (extended) and is considered to be complete as it is.
21. Can an interface in Java contain a constructor?
a) Yes
b) No
c) Only if it's an abstract interface
d) Only if it extends a class
Answer:
b) No
Explanation:
Interfaces in Java cannot have constructors because they cannot be instantiated.
22. What are the main pillars of Object-Oriented Programming?
a) Encapsulation, Inheritance, Polymorphism, Abstraction
b) Class, Object, Methods, Variables
c) Syntax, Semantics, Compilation, Execution
d) Functions, Procedures, Modules, Packages
Answer:
a) Encapsulation, Inheritance, Polymorphism, Abstraction
Explanation:
The four main pillars of OOP are Encapsulation, Inheritance, Polymorphism, and Abstraction.
23. Which keyword is used to prevent method overriding?
a) static
b) final
c) abstract
d) private
Answer:
b) final
Explanation:
When a method is marked as 'final', it cannot be overridden by subclasses.
24. What is a 'super' keyword used for in Java?
a) To call a superclass's constructor
b) To refer to the immediate parent class object
c) Both a and b
d) To declare a variable as superior
Answer:
c) Both a and b
Explanation:
The 'super' keyword is used to refer to the immediate parent class object and can be used to call a superclass's constructor or methods.
25. What is the key benefit of encapsulation in Java?
a) Improved performance
b) Better security and data hiding
c) Increased complexity
d) Enhanced polymorphism
Answer:
b) Better security and data hiding
Explanation:
Encapsulation provides better control over the data by hiding it from the outside world and only exposing the necessary parts through methods.
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