在空对象模式（Null Object Pattern）中，一个空对象取代 NULL 对象实例的检查。Null 对象不是检查空值，而是反应一个不做任何动作的关系。这样的 Null 对象也可以在数据不可用的时候提供默认的行为。

在空对象模式中，我们创建一个指定各种要执行的操作的抽象类和扩展该类的实体类，还创建一个未对该类做任何实现的空对象类，该空对象类将无缝地使用在需要检查空值的地方

public abstract class AbstractCustomer {protected String name;public abstract boolean isNil();public abstract String getName();
}

public class RealCustomer extends AbstractCustomer{public RealCustomer(String name) {this.name = name;}@Overridepublic boolean isNil() {return false;}@Overridepublic String getName() {return name;}
}

public class NullCustomer extends AbstractCustomer{@Overridepublic boolean isNil() {return true;}@Overridepublic String getName() {return "Not Available in Customer Database";}
}

public class CustomerFactory {public static final String[] names = {"Rob", "Joe", "Julie"};public static AbstractCustomer getCustomer(String name){for (int i = 0; i < names.length; i++) {if (names[i].equalsIgnoreCase(name)){return new RealCustomer(name);}}return new NullCustomer();}
}

@Test
public void test20(){AbstractCustomer customer1 = CustomerFactory.getCustomer("Rob");AbstractCustomer customer2 = CustomerFactory.getCustomer("Bob");AbstractCustomer customer3 = CustomerFactory.getCustomer("Julie");AbstractCustomer customer4 = CustomerFactory.getCustomer("Laura");System.out.println("Customers");System.out.println(customer1.getName());System.out.println(customer2.getName());System.out.println(customer3.getName());System.out.println(customer4.getName());
}

Customers
Rob
Not Available in Customer Database
Julie
Not Available in Customer Database