w6

w6/MainSix.java

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+public class MainSix {
+    public static abstract class Animal {
+        protected String name;
+
+        Animal(String name) {
+            this.name = name;
+        }
+
+        abstract public void makeSound();
+    }
+
+    public interface Swimmable {
+        void swim();
+    }
+
+    public static class Dog extends Animal implements Swimmable {
+        Dog(String name) {
+            super(name);
+        }
+
+        @Override
+        public void makeSound() {
+            System.out.printf("This is dog %s.%n", this.name);
+        }
+
+        @Override
+        public void swim() {
+            System.out.printf("Dog %s swimming.%n", this.name);
+        }
+    }
+
+    public static class Cat extends Animal {
+        Cat(String name) {
+            super(name);
+        }
+
+        @Override
+        public void makeSound() {
+            System.out.printf("This is cat %s.%n", this.name);
+        }
+    }
+
+    public static void trySwim(Animal animal) {
+        System.out.printf("Moving %s to swimming pool.%n", animal.name);
+
+        if (animal instanceof Swimmable) {
+            ((Swimmable) animal).swim();
+        } else {
+            System.out.printf("Oops, looks like %s cannot swim...%n", animal.name);
+        }
+    }
+
+    public static void main(String[] args) {
+        Animal dog1 = new Dog("Adam");
+        Animal dog2 = new Dog("Belly");
+        Animal cat1 = new Cat("Carl");
+
+        dog1.makeSound();
+        dog2.makeSound();
+        cat1.makeSound();
+
+        trySwim(dog1);
+        trySwim(dog2);
+        trySwim(cat1);
+    }
+}

w6/usage.md

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+# 运行记录
+
+```console
+$ javac MainSix.java && java MainSix
+This is dog Adam.
+This is dog Belly.
+This is cat Carl.
+Moving Adam to swimming pool.
+Dog Adam swimming.
+Moving Belly to swimming pool.
+Dog Belly swimming.
+Moving Carl to swimming pool.
+Oops, looks like Carl cannot swim...
+```