行为型模式-空对象模式

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;
    }

    @Override
    public boolean isNil() {
        return false;
    }

    @Override
    public String getName() {
        return name;
    }
}

public class NullCustomer extends AbstractCustomer{
    @Override
    public boolean isNil() {
        return true;
    }

    @Override
    public 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