更新

Animal.java

@@ -0,0 +1,3 @@
+public abstract class Animal {
+    public abstract void makeSound();
+}

Cat.java

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+public class Cat extends Animal {
+    @Override
+    public void makeSound() {
+        System.out.println("Meow!");
+    }
+}

Dog.java

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+public class Dog extends Animal implements Swimmable {
+    @Override
+    public void makeSound() {
+        System.out.println("Woof!");
+    }
+    
+    @Override
+    public void swim() {
+        System.out.println("Dog is swimming.");
+    }
+}

Main.java

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+public class Main {
+    public static void main(String[] args) {
+        // 测试多态
+        Animal animal1 = new Dog();
+        Animal animal2 = new Cat();
+        
+        System.out.println("Testing polymorphism:");
+        animal1.makeSound(); // 应该输出 Woof!
+        animal2.makeSound(); // 应该输出 Meow!
+        
+        // 测试 Dog 的 swim 方法
+        System.out.println("\nTesting Dog's swim method:");
+        if (animal1 instanceof Swimmable) {
+            ((Swimmable) animal1).swim();
+        }
+        
+        // 测试 Cat 是否实现了 Swimmable
+        System.out.println("\nTesting if Cat is Swimmable:");
+        if (animal2 instanceof Swimmable) {
+            ((Swimmable) animal2).swim();
+        } else {
+            System.out.println("Cat is not Swimmable.");
+        }
+    }
+}

Swimmable.java

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+public interface Swimmable {
+    void swim();
+}