🔹 1. What is OOPs in Java?
OOPs (Object-Oriented Programming System) is a programming approach based on objects and classes. It helps in organizing code into reusable and modular parts.
🔹 2. What are the 4 pillars of OOPs?
The four main pillars are:
- Encapsulation
- Inheritance
- Polymorphism
- Abstraction
🔹 3. What is Encapsulation?
Encapsulation means wrapping data and methods into a single unit (class) and restricting direct access.
Example:
class Student {
private int age;
public void setAge(int a) {
age = a;
}
public int getAge() {
return age;
}
}👉 It protects data using private variables.
🔹 4. What is Inheritance?
Inheritance allows one class to acquire properties of another class.
Example:
class Animal {
void eat() {
System.out.println("Eating...");
}
}
class Dog extends Animal {
void bark() {
System.out.println("Barking...");
}
}🔹 5. Types of Inheritance in Java?
- Single inheritance
- Multilevel inheritance
- Hierarchical inheritance
- Multiple inheritance (through interfaces only)
🔹 6. What is Polymorphism?
Polymorphism means one action, many forms.
Types:
- Compile-time (Method Overloading)
- Run-time (Method Overriding)
🔹 7. What is Method Overloading?
Same method name but different parameters.
Example:
class Test {
void show(int a) {}
void show(String b) {}
}🔹 8. What is Method Overriding?
Child class provides specific implementation of parent method.
Example:
class Animal {
void sound() {
System.out.println("Animal sound");
}
}
class Dog extends Animal {
void sound() {
System.out.println("Barking");
}
}🔹 9. What is Abstraction?
Hiding internal implementation and showing only functionality.
🔹 10. How is abstraction achieved in Java?
- Abstract class
- Interface
🔹 11. What is an Abstract Class?
A class that cannot be instantiated and may contain abstract methods.
🔹 12. What is an Interface?
An interface contains only abstract methods (before Java 8) and is used for full abstraction.
🔹 13. Difference between Abstract Class and Interface?
| Abstract Class | Interface |
|---|---|
| Can have constructors | No constructors |
| Can have normal methods | Only abstract (before Java 8) |
| Supports partial abstraction | Full abstraction |
🔹 14. Can we achieve multiple inheritance in Java?
No, but it is possible using interfaces.
🔹 15. What is a Class?
A class is a blueprint for objects.
🔹 16. What is an Object?
An object is an instance of a class.
🔹 17. Difference between Class and Object?
| Class | Object |
|---|---|
| Blueprint | Real entity |
| Logical | Physical |
🔹 18. What is Constructor?
A constructor is a special method used to initialize objects.
🔹 19. Types of Constructor?
- Default constructor
- Parameterized constructor
🔹 20. Can constructor be overloaded?
Yes, constructors can be overloaded with different parameters.
🔹 21. Can constructor be inherited?
No, constructors are not inherited in Java.
🔹 22. What is this keyword?
It refers to the current class object.
🔹 23. What is super keyword?
It refers to the parent class object.
🔹 24. What is static keyword?
Static means it belongs to the class, not object.
🔹 25. What is final keyword?
Used to restrict:
- Variable → constant
- Method → cannot override
- Class → cannot inherit
🔹 26. What is runtime polymorphism?
Method overriding that is resolved at runtime.
🔹 27. What is compile-time polymorphism?
Method overloading resolved during compilation.
🔹 28. Why Java does not support multiple inheritance with classes?
To avoid ambiguity and complexity (diamond problem).
🔹 29. What is tight coupling?
When classes are highly dependent on each other.
🔹 30. What is loose coupling?
When classes are independent and easier to maintain.
🔹 31. What is object cloning?
Creating a copy of an existing object.
🔹 32. What is constructor chaining?
Calling one constructor from another using this() or super().
🔹 33. What is composition?
Using one class inside another class (HAS-A relationship).
🔹 34. Difference between HAS-A and IS-A relationship?
- IS-A → Inheritance
- HAS-A → Composition
🔹 35. What is interface multiple inheritance?
A class can implement multiple interfaces.
🔹 36. Can interface have method implementation?
Yes, from Java 8 (default methods).
🔹 37. What is default method in interface?
A method with body inside interface.
🔹 38. What is marker interface?
An empty interface (e.g., Serializable).
🔹 39. What is upcasting?
Parent class reference pointing to child object.
🔹 40. What is downcasting?
Converting parent reference back to child.
🔹 41. What is dynamic method dispatch?
Runtime polymorphism using method overriding.
🔹 42. What is aggregation?
Weak HAS-A relationship (independent objects).
🔹 43. What is encapsulation advantage?
- Data security
- Easy maintenance
- Code flexibility
🔹 44. What is abstraction advantage?
- Hides complexity
- Reduces code duplication
🔹 45. What is inheritance advantage?
- Code reusability
- Easy maintenance
🔹 46. What is polymorphism advantage?
- Flexibility
- Extensibility
🔹 47. What is method hiding?
Static method in child hides parent method.
🔹 48. Can we override static method?
No, static methods cannot be overridden.
🔹 49. What is object lifecycle?
Creation → Usage → Destruction (Garbage collection)
🔹 50. What is garbage collection?
Automatic memory management in Java.