diff --git a/w4/Circle.java b/w4/Circle.java
new file mode 100644
index 0000000..f1a73d6
--- /dev/null
+++ b/w4/Circle.java
@@ -0,0 +1,23 @@
+/**
+ * 圆：面积 = π * r²
+ */
+public class Circle extends Shape {
+
+    private final double radius;
+
+    public Circle(double radius) {
+        if (radius <= 0) {
+            throw new IllegalArgumentException("半径必须为正数");
+        }
+        this.radius = radius;
+    }
+
+    public double getRadius() {
+        return radius;
+    }
+
+    @Override
+    public double getArea() {
+        return Math.PI * radius * radius;
+    }
+}
diff --git a/w4/Rectangle.java b/w4/Rectangle.java
new file mode 100644
index 0000000..7ca4f00
--- /dev/null
+++ b/w4/Rectangle.java
@@ -0,0 +1,29 @@
+/**
+ * 矩形：面积 = 宽 * 高
+ */
+public class Rectangle extends Shape {
+
+    private final double width;
+    private final double height;
+
+    public Rectangle(double width, double height) {
+        if (width <= 0 || height <= 0) {
+            throw new IllegalArgumentException("宽和高必须为正数");
+        }
+        this.width = width;
+        this.height = height;
+    }
+
+    public double getWidth() {
+        return width;
+    }
+
+    public double getHeight() {
+        return height;
+    }
+
+    @Override
+    public double getArea() {
+        return width * height;
+    }
+}
diff --git a/w4/Shape.java b/w4/Shape.java
new file mode 100644
index 0000000..dd41697
--- /dev/null
+++ b/w4/Shape.java
@@ -0,0 +1,10 @@
+/**
+ * 图形抽象基类：统一多态入口，具体面积由子类实现。
+ */
+public abstract class Shape {
+
+    /**
+     * @return 图形面积（具体单位由子类语义决定，如平方厘米）
+     */
+    public abstract double getArea();
+}
diff --git a/w4/ShapeCalculatorDemo.java b/w4/ShapeCalculatorDemo.java
new file mode 100644
index 0000000..4995e77
--- /dev/null
+++ b/w4/ShapeCalculatorDemo.java
@@ -0,0 +1,27 @@
+/**
+ * 演示：多态 —— 同一 {@link ShapeUtil#printArea(Shape)} 处理圆、矩形、三角形。
+ * 运行：javac *.java 后执行 java ShapeCalculatorDemo
+ */
+public class ShapeCalculatorDemo {
+
+    public static void main(String[] args) {
+        Shape circle = new Circle(3.0);
+        Shape rectangle = new Rectangle(4.0, 5.0);
+        Shape triangle = new Triangle(6.0, 4.0);
+
+        System.out.println("—— 圆 ——");
+        ShapeUtil.printArea(circle);
+
+        System.out.println("—— 矩形 ——");
+        ShapeUtil.printArea(rectangle);
+
+        System.out.println("—— 三角形 ——");
+        ShapeUtil.printArea(triangle);
+
+        System.out.println("—— 多态数组统一处理 ——");
+        Shape[] shapes = {circle, rectangle, triangle};
+        for (Shape s : shapes) {
+            ShapeUtil.printArea(s);
+        }
+    }
+}
diff --git a/w4/ShapeUtil.java b/w4/ShapeUtil.java
new file mode 100644
index 0000000..03127cc
--- /dev/null
+++ b/w4/ShapeUtil.java
@@ -0,0 +1,22 @@
+/**
+ * 图形工具类：通过多态统一打印任意 {@link Shape} 的面积。
+ */
+public final class ShapeUtil {
+
+    private ShapeUtil() {
+        // 工具类禁止实例化
+    }
+
+    /**
+     * 打印给定图形的面积（保留两位小数，便于实验输出阅读）。
+     *
+     * @param shape 任意 {@link Shape} 子类实例，可为 null（将给出提示）
+     */
+    public static void printArea(Shape shape) {
+        if (shape == null) {
+            System.out.println("图形引用为空，无法计算面积。");
+            return;
+        }
+        System.out.printf("该图形的面积为：%.2f%n", shape.getArea());
+    }
+}