// 抽象类 Shape
public abstract class Shape {
    public abstract double getArea();
}
// Circle 类
public class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}
// Rectangle 类
public class Rectangle extends Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double getArea() {
        return width * height;
    }
}
// Triangle 类
public class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double getArea() {
        return 0.5 * base * height;
    }
}
// ShapeUtil 工具类
public class ShapeUtil {
    public static void printArea(Shape shape) {
        System.out.println("图形的面积是：" + shape.getArea());
    }
}