空对象模式（Null Object Pattern）

public abstract class Duck {
    public String name;
    public abstract void getName();
}

public class ConcreteDuck extends Duck {
    public ConcreteDuck(String name) {
        this.name = name;
    }
    public void getName() {
        System.out.println("ConcreteDuck");
    }
}

public class NullDuck extends Duck {
    public void getName() {
    }
}

public class Client {
    public static final String[] names = {"A","B","C"};
    public static Duck getDuck(String name) {
       for(int i = 0; i < names.length;i++) {
            if(name[i].equalsIgnoreCase(name)) {
                 return new ConcreteDuck(name);
            }
       }
       return new NullDuck();
    }
    public static void main(String[] args) {
         Duck d1 = new Client.getDuck("A");
         Duck d2 = new Client.getDuck("D");
         System.out.println(d1.getName());
         System.out.println(d2.getName());//没有name=D的Duck，空对象的getName方法不打印字符串
    }
}