在Null Object模式中,null对象替换NULL对象实例的检查. Null Object反映了无关系,而不是检查是否为空值.如果数据不可用,这样的Null对象也可以用来提供默认行为.
在Null Object模式中,我们创建一个抽象类,指定要完成的各种操作,扩展这个的具体类class和null对象类提供此类的任何实现,并且将在我们需要检查空值的地方无用地使用.
我们将创建一个定义操作的 AbstractCustomer 抽象类.这里是客户的名称和扩展 AbstractCustomer 类的具体类.创建工厂类 CustomerFactory ,以根据传递给它的客户名称返回 RealCustomer 或 NullCustomer 对象.
$ b我们的演示类$ b
NullPatternDemo 将使用 CustomerFactory 来演示Null对象模式的使用.
创建一个抽象类.
AbstractCustomer.java
public abstract class AbstractCustomer { protected String name; public abstract boolean isNil(); public abstract String getName(); }
创建扩展上述类的具体类.
RealCustomer.java
public class RealCustomer extends AbstractCustomer { public RealCustomer(String name) { this.name = name; } @Override public String getName() { return name; } @Override public boolean isNil() { return false; } }
NullCustomer.java
public class NullCustomer extends AbstractCustomer { @Override public String getName() { return "Not Available in Customer Database"; } @Override public boolean isNil() { return true; } }
创建 CustomerFactory Class.
CustomerFactory.java
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(); } }
使用 CustomerFactory 根据传递给它的客户名称获取 RealCustomer 或 NullCustomer 对象.
NullPatternDemo.java
public class NullPatternDemo { public static void main(String[] args) { 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()); } }
验证输出.
Customers Rob Not Available in Customer Database Julie Not Available in Customer Database