Class

• Definition: A class is a blueprint or template that defines the structure and behavior (data members and methods) of objects.

• Example: class Car { int speed; void drive(); }

Object

• Definition: An object is an instance of a class. It has its own identity, state (values of attributes), and behavior (methods).

• Example: Car myCar = new Car();

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Constructor & Destructor

• Constructor: A special method used to initialize objects. Called automatically when an object is created.

• Destructor: A special method used to clean up resources when an object is destroyed (C++), or garbage collected (Java/C#).

Association, Aggregation, Composition

• Explanation:

  • Association: Relationship between two classes.

  • Aggregation: "Has-a" relationship, weaker. (Car has an Engine, Engine can exist independently.)

  • Composition: Strong "has-a", lifecycle tied. (Human has a Heart, cannot exist separately.)

Interfaces & Abstract Classes

• Abstract Class: Can have both abstract methods (no body) and concrete methods. Cannot be instantiated.

• Interface: Only method signatures (in Java < 8). Defines a contract. A class must implement it.

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Four pillars of OOPS:-

1. Encapsulation

• Definition: Encapsulation is the concept of binding data (variables) and behavior (methods) together inside a class, and restricting direct access to some components using access modifiers (private, public, protected).

• Public: Accessible everywhere.

• Private: Accessible only within the same class.

• Protected: Accessible within the same class, subclasses, and same package (Java).

• Default (Java): Accessible within the same package.

2. Abstraction

• Definition: Abstraction is the process of hiding implementation details and exposing only the essential features. Achieved using abstract classes and interfaces.

• Benefit: Reduces complexity and increases flexibility.

3. Inheritance

• Definition: Inheritance is the mechanism where one class (child/subclass) can inherit properties and methods from another class (parent/superclass).

• Benefit: Promotes code reuse and hierarchy.

• Types: Single, Multiple (not in Java, but in C++), Multilevel, Hierarchical, Hybrid.

4. Polymorphism

• Definition: Polymorphism means "many forms". It allows the same method or operator to behave differently based on context.

• Types:

  • Compile-time (Overloading) → Same method name, different parameters.

  • Runtime (Overriding) → Subclass provides its own implementation of a met

Copyclass Test {
    void show(int a) {}
    void show(double b) {} // Overloading

    void display() { System.out.println("Parent"); }
}
class SubTest extends Test {
    @Override
    void display() { System.out.println("Child"); } // Overriding
}

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Overloading vs Overriding

• Overloading: Same method name but different parameter list (compile-time).

• Overriding: Same method name and parameters in subclass, with different implementation (runtime).

Virtual Functions (C++)

• A function in the base class that can be overridden in the derived class, enabling runtime polymorphism.

Dynamic Binding vs Static Binding

• Static Binding (Early): Method call is resolved at compile time (overloading).

• Dynamic Binding (Late): Method call is resolved at runtime (overriding with virtual methods).