什么是空对象模式

用一个空对象取代 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。

适用场景

提供默认行为，提供默认值。

abstract class AbstractCustomer {
    protected String name;
    public abstract boolean isNull();
    public abstract String getName();
}
class RealCustomer extends AbstractCustomer {
    public RealCustomer(String name) {
        this.name = name;
    }
    @Override
    public String getName() {
        return name;
    }
    @Override
    public boolean isNull() {
        return false;
    }
}
class NullCustomer extends AbstractCustomer {
    @Override
    public String getName() {
        return "Not Available in Customer Database";
    }
    @Override
    public boolean isNull() {
        return true;
    }
}
class CustomerFactory {
    public static final String[] NAMES = {"Rob", "Joe", "Julie"};
    public static AbstractCustomer getCustomer(String name) {
        for (String s : NAMES) {
            if (s.equalsIgnoreCase(name)) {
                return new RealCustomer(name);
            }
        }
        return new NullCustomer();
    }
}
/**
 * 空对象模式  测试类
 */
public class NullObjectPattern {
    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());
        //输出：
        //Customers
        //Rob
        //Not Available in Customer Database
        //Julie
        //Not Available in Customer Database
    }
}

Java8提供的Optional

public class OptionalDemo {
    public static void main(String[] args) {
        //JAVA的败笔之一 就是提供了null这个默认值 使得NPE到处出现，空指针异常编译时无法发现，运行时系统崩溃
//        String s = null;
//        System.out.println(s.length()); //NullPointerException
        //改进如下
//        if (null != s) {
//            System.out.println(s.length());
//        }
        //JDK8 提供了Optional
        //默认EMPTY值
        Optional<Object> empty = Optional.empty();
        System.out.println(empty);//Optional.empty
        System.out.println("-----------------------------");
        String str1=null;
//        Optional<String> s1 = Optional.of(str1);//NullPointerException
        Optional<String> s1 = Optional.ofNullable(str1);//NullPointerException
        System.out.println(s1);//Optional.empty
//        System.out.println(s1.get());//NullPointerException
        System.out.println(s1.isPresent());//false
        System.out.println(s1.orElse("默认值"));//默认值
        System.out.println("-----------------------------");
        String str2="test";
        Optional<String> s2 = Optional.of(str2);
        System.out.println(s2);//Optional[test]
        Optional<String> s3 = Optional.ofNullable(str2);
        System.out.println(s3);//Optional[test]
        System.out.println(s3.get());//test
        System.out.println(s3.isPresent());//true
        System.out.println(s3.orElse("默认值"));//test
    }
}