Java Object-Oriented Design (OOD)

 

1. What is OOPS?

Answer:

OOPS stands for Object-Oriented Programming System.

It is a programming paradigm where the software design is based on objects, which represent real-world entities.
Each object is an instance of a class and contains state (data) and behavior (methods).

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2. What are the basic concepts of OOPS?

Answer:
The four fundamental principles of OOPS are:

1. Abstraction

2. Encapsulation

3. Inheritance

4. Polymorphism

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3. What is a Class?

Answer:
A class is a blueprint or template used to create objects.
It defines:

• Properties (variables / fields)

• Behaviors (methods)

Example:

class Car {
    String color;
    void drive() {}
}

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4. What is an Object?

Answer:
An object is a runtime instance of a class.
It has:

• State → variables

• Behavior → methods

• Identity → unique memory reference

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5. What is Encapsulation?

Answer:
Encapsulation is the process of binding data and methods together and restricting direct access to object data.

It is achieved using access modifiers:

• public

• protected

• default

• private

Example:

private int balance;
public int getBalance() { return balance; }

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6. What is Polymorphism?

Answer:
Polymorphism means “many forms”.
It allows the same method to behave differently based on the object type.

Types:

• Compile-time (Method Overloading)

• Runtime (Method Overriding)

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7. What is Inheritance?

Answer:
Inheritance allows one class to reuse properties and methods of another class.

Types:

• Single Inheritance

• Multilevel Inheritance

• Hierarchical Inheritance

Java does not support multiple inheritance with classes but supports it using interfaces.

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8. What are Manipulators?

Answer:
Manipulators are special functions used with stream operators (<<, >>) in C++ to format input/output.

Examples:

• endl

• setw

⚠️ Not applicable in Java.

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9. What is a Constructor?

Answer:
A constructor is a special method used to initialize an object.

Rules:

• Same name as the class

• No return type

• Automatically invoked during object creation

class Person {
    Person() {}
}

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10. What is a Destructor?

Answer:
A destructor releases resources when an object is destroyed.

⚠️ Java does not support destructors.
Garbage Collection handles memory cleanup.

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11. What is an Inline Function?

Answer:
An inline function is expanded at compile time to reduce function call overhead.

⚠️ Java does not support inline functions explicitly (JIT compiler optimizes internally).

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12. What is Function Overloading?

Answer:
Function overloading allows multiple methods with the same name but different:

• Parameters

• Data types

• Number of arguments

Example:

int add(int a, int b)
double add(double a, double b)

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13. What is a Virtual Function?

Answer:
A virtual function supports runtime polymorphism.

⚠️ In Java:

• All non-static methods are virtual by default

• Implemented using method overriding

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14. What is a Friend Function?

Answer:
Friend functions allow access to private members of a class.

⚠️ Java does NOT support friend functions (encapsulation is strict).

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15. What is Operator Overloading?

Answer:
Operator overloading allows operators to behave differently for different operands.

⚠️ Java does not support operator overloading
(Except + for String concatenation).

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16. What is an Abstract Class?

Answer:
An abstract class:

• Cannot be instantiated

• Can contain abstract and non-abstract methods

abstract class Shape {
    abstract void draw();
}

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17. What is a Ternary Operator?

Answer:
The ternary operator is a conditional operator:

result = (condition) ? value1 : value2;

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18. What is the use of finalize() method?

Answer:
finalize() was used for cleanup before garbage collection.

⚠️ Deprecated in Java 9+
Not recommended for resource management.

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19. What are the types of Arguments?

Answer:

1. Call by Value – Java supports this

2. Call by Reference – Not supported directly in Java

Java always passes object references by value.

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20. What is the super keyword?

Answer:
super is used to:

• Access parent class methods

• Access parent constructor

• Access hidden variables

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21. What is Method Overriding?

Answer:
Method overriding allows a child class to provide its own implementation of a parent class method.

Rules:

• Same method name

• Same parameters

• Same return type

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22. What is an Interface?

Answer:
An interface contains:

• Abstract methods

• Default methods (Java 8+)

• Static methods

Used to achieve multiple inheritance in Java.

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23. What is Exception Handling?

Answer:
Exception handling manages runtime errors using:

• try

• catch

• finally

• throw

• throws

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24. What are Tokens?

Answer:
Tokens are the smallest units of a program.

Examples:

• Keywords

• Identifiers

• Constants

• Operators

• Separators

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25. Difference between Overloading and Overriding?

Feature	Overloading	Overriding
Binding	Compile-time	Runtime
Parameters	Different	Same
Return Type	Can vary	Must match
Class	Same class	Parent-Child