// Sealed classpublic sealed class Shape    permits Circle, Rectangle, Triangle {        public abstract double area();}// Final subclass (cannot be extended)public final class Circle extends Shape {    private final double radius;        public Circle(double radius) {        this.radius = radius;    }        @Override    public double area() {        return Math.PI * radius * radius;    }}// Final subclasspublic final class Rectangle extends Shape {    private final double width, height;        public Rectangle(double width, double height) {        this.width = width;        this.height = height;    }        @Override    public double area() {        return width * height;    }}// Non-sealed subclass (can be extended)public non-sealed class Triangle extends Shape {    protected double base, height;        public Triangle(double base, double height) {        this.base = base;        this.height = height;    }        @Override    public double area() {        return 0.5 * base * height;    }}// Can extend non-sealed classpublic class EquilateralTriangle extends Triangle {    private double side;        public EquilateralTriangle(double side) {        super(side, side * Math.sqrt(3) / 2);        this.side = side;    }}// Sealed interfacepublic sealed interface Expression    permits Constant, Variable, Addition, Multiplication {        double evaluate();}public final class Constant implements Expression {    private final double value;        public Constant(double value) {        this.value = value;    }        @Override    public double evaluate() {        return value;    }}public final class Variable implements Expression {    private final String name;    private final double value;        public Variable(String name, double value) {        this.name = name;        this.value = value;    }        @Override    public double evaluate() {        return value;    }}public record Addition(Expression left, Expression right) implements Expression {    @Override    public double evaluate() {        return left.evaluate() + right.evaluate();    }}public record Multiplication(Expression left, Expression right) implements Expression {    @Override    public double evaluate() {        return left.evaluate() * right.evaluate();    }}

// Create instancesShape circle = new Circle(5.0);Shape rectangle = new Rectangle(4.0, 6.0);Shape triangle = new Triangle(3.0, 4.0);// Pattern matching with instanceof (Java 16+)public static double calculateArea(Shape shape) {    if (shape instanceof Circle c) {        return c.area();    } else if (shape instanceof Rectangle r) {        return r.area();    } else if (shape instanceof Triangle t) {        return t.area();    }    // Compiler knows all cases are covered    return 0;}// Switch expressions with pattern matching (Java 17+)public static String describe(Shape shape) {    return switch (shape) {        case Circle c -> "Circle with radius " + c.radius;        case Rectangle r -> "Rectangle with dimensions " + r.width + "x" + r.height;        case Triangle t -> "Triangle with base " + t.base + " and height " + t.height;        // Compiler ensures exhaustiveness    };}// Expression evaluationExpression expr1 = new Constant(5.0);Expression expr2 = new Variable("x", 3.0);Expression expr3 = new Addition(expr1, expr2);Expression expr4 = new Multiplication(expr3, new Constant(2.0));double result = expr4.evaluate(); // (5 + 3) * 2 = 16// Pattern matching on expressionspublic static String format(Expression expr) {    return switch (expr) {        case Constant c -> String.valueOf(c.value());        case Variable v -> v.name();        case Addition a -> "(" + format(a.left()) + " + " + format(a.right()) + ")";        case Multiplication m -> "(" + format(m.left()) + " * " + format(m.right()) + ")";    };}