Inheritance

A class has one direct superclass, establishing single inheritance. A subclass can itself be extended, producing multilevel inheritance; several subclasses of one base form a hierarchy. Java avoids multiple class inheritance but allows multiple interfaces.

Specialization

abstract class Shape {
    abstract double area();
}

final class Rectangle extends Shape {
    private final double width, height;
    Rectangle(double width, double height) {
        this.width = width; this.height = height;
    }
    @Override double area() { return width * height; }
}

super accesses superclass construction or behavior. Constructors are not inherited; private members remain private. An override must honor the base contract and may widen visibility, narrow checked exceptions, and return a covariant reference type. Use final to close a class or method when extension would break invariants. Prefer composition when the relationship is “has-a,” behavior changes independently, or a hierarchy exists only to reuse code.