What are the four pillars of OOP, and how does Java support each?

• Encapsulation: bundling state and behavior together and restricting direct access to internals via access modifiers (private fields, public getters/setters or methods that validate input).

class Account {
    private double balance;
    public void deposit(double amt) {
        if (amt <= 0) throw new IllegalArgumentException();
        balance += amt;
    }
}

• Abstraction: exposing what an object does while hiding how, via abstract classes and interfaces that define a contract without full implementation.
• Inheritance: a subclass (extends) reuses and specializes a superclass's fields/methods, modeling an "is-a" relationship. Java supports single inheritance of classes but multiple inheritance of interfaces.
• Polymorphism: the same method call behaves differently depending on the actual runtime type (overriding, resolved dynamically via virtual dispatch) or on argument types at compile time (overloading).

Shape s = new Circle(5);
s.area(); // calls Circle.area() at runtime, even though s is typed as Shape

Java's design (single class inheritance + interfaces, mandatory access modifiers, and virtual-by-default instance methods) makes all four pillars first-class, idiomatic parts of the language rather than conventions layered on top.