添加 'W4'

W4

@@ -0,0 +1,53 @@
+// 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());
+    }
+}