✅ OOP Interview Questions with Answers Part-3 💡💻

21. What is a final class or method?
• A final class can't be extended.
• A final method can't be overridden.
Useful for security, immutability (e.g., String class in Java is final).

22. What is object cloning?
• Creating an exact copy of an object.
• In Java: use .clone() method from Cloneable interface.
• Shallow vs Deep cloning:
– Shallow copies references.
– Deep copies full object graph.

23. What is a singleton class?
• A class that allows only one instance.
• Ensures shared resource (like a config manager or DB connection).
• Common in design patterns.

public class Singleton {
  private static Singleton instance = new Singleton();
  private Singleton() {}
  public static Singleton getInstance() {
    return instance;
  }
}

24. What are access specifiers?
Control visibility of class members:
• public – accessible everywhere
• private – only inside the class
• protected – inside class subclasses
• (default) – same package

25. What is cohesion in OOP?
• Degree to which class elements belong together.
• High cohesion = focused responsibility → better design.

26. What is coupling?
• Dependency between classes.
• Low coupling = better modularity, easier maintenance.

27. Difference between tight and loose coupling?
• Tight coupling: classes are strongly dependent → harder to modify/test.
• Loose coupling: minimal dependency → promotes reusability, flexibility.

28. What is composition vs aggregation?
• Composition: "part-of" strong relationship → child can't exist without parent.
Example: Engine in a Car
• Aggregation: weak association → child can exist independently.
Example: Student in a University

29. Difference between association, aggregation, and composition?
• Association: General relationship
• Aggregation: Whole-part, but loose
• Composition: Whole-part, tightly bound

30. What is the open/closed principle?
• From SOLID:
“Software entities should be open for extension, but closed for modification.”
• Means add new code via inheritance, not by changing existing logic.

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