java/w4/Main.java

// Shape.java
public abstract class Shape {
    public abstract double getArea();
}

// Circle.java
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.java
public class Rectangle extends Shape {
    private double length;
    private double width;

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

    @Override
    public double getArea() {
        return length * width;
    }
}

// Triangle.java
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.java
public class ShapeUtil {
    public static void printArea(Shape shape) {
        System.out.printf("图形面积：%.2f\n", shape.getArea());
    }
}

// Demo.java（测试类，展示多态）
public class Demo {
    public static void main(String[] args) {
        Shape circle = new Circle(5.0);
        Shape rectangle = new Rectangle(4.0, 6.0);
        Shape triangle = new Triangle(3.0, 4.0);

        ShapeUtil.printArea(circle);      // 图形面积：78.54
        ShapeUtil.printArea(rectangle);   // 图形面积：24.00
        ShapeUtil.printArea(triangle);    // 图形面积：6.00
    }
}