What are the four pillars of object-oriented programming?

Object-oriented programming rests on four pillars:

• Encapsulation: keep state private and expose behaviour through methods, so invariants cannot be broken from outside.
• Abstraction: model only the essential behaviour, hiding implementation detail behind an interface or abstract type.
• Inheritance: derive a specialised type from a base type to reuse and extend behaviour.
• Polymorphism: call the same method on different types and let the runtime pick the right implementation.

interface Shape { double area(); }
class Circle implements Shape {
    private final double r;
    Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
}

Here Circle encapsulates r, implements the abstraction Shape and participates in polymorphism. In practice inheritance is the least important and most abused pillar; composition and interfaces usually age better. The SOLID principles refine these ideas to keep designs maintainable as systems grow.