添加 'W4'

W4

@@ -0,0 +1,57 @@
+// 抽象图形类
+abstract class Shape {
+    public abstract double getArea();
+}
+
+// 圆形
+class Circle extends Shape {
+    private double r;
+
+    public Circle(double r) {
+        this.r = r;
+    }
+
+    @Override
+    public double getArea() {
+        return Math.PI * r * r;
+    }
+}
+
+// 矩形
+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;
+    }
+}
+
+// 三角形
+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;
+    }
+}
+
+// 工具类
+class ShapeUtil {
+    public static void printArea(Shape shape) {
+        System.out.println("面积：" + shape.getArea());
+    }
+}