Java 中遇到null 和为空的情况，使用Optional来解决。

 1 package crazy;
 2 
 3 import java.util.Optional;
 4 
 5 class Company {
 6     private String name;
 7     private Optional<Office> office;
 8 
 9     public Company(String name, Optional<Office> office) {
10         this.name = name;
11         this.office = office;
12     }
13 
14     public String getName() {
15         return name;
16     }
17 
18     public Optional<Office> getOffice() {
19         return office;
20     }
21 }
22 
23 class Office {
24     private String id;
25     private Optional<Address> address;
26 
27     public Office(String id, Optional<Address> address) {
28         this.id = id;
29         this.address = address;
30     }
31 
32     public String getId() {
33         return id;
34     }
35 
36     public Optional<Address> getAddress() {
37         return address;
38     }
39 }
40 
41 class Address {
42     private Optional<String> street;
43     private Optional<String> city;
44 
45     public Address(Optional<String> street, Optional<String> city) {
46         this.street = street;
47         this.city = city;
48     }
49 
50     public Optional<String> getStreet() {
51         return street;
52     }
53 
54     public Optional<String> getCity() {
55         return city;
56     }
57 }
58 
59 public class OptionalDemo1 {
60 
61     public static void main(String[] args) {
62         Optional<Address> address1 = Optional.of(new Address(Optional.ofNullable(null), Optional.of("New York")));
63         Optional<Office> office1 = Optional.of(new Office("OF1", address1));
64         Optional<Company> company1 = Optional.of(new Company("Door Never Closed", office1));
65 
66         // What is the street address of company1?
67         // In which city company1 is located?
68         Optional<Office> maybeOffice = company1.flatMap(Company::getOffice);
69         Optional<Address> maybeAddress = office1.flatMap(Office::getAddress);
70         Optional<String> maybeStreet = address1.flatMap(Address::getStreet);
71 
72         maybeStreet.ifPresent(System.out::println);
73         if (maybeStreet.isPresent()) {
74             System.out.println(maybeStreet.get());
75         } else {
76             System.out.println("Street not found.");
77         }
78         
79         // shorter way
80         String city = company1.flatMap(Company::getOffice)
81                 .flatMap(Office::getAddress)
82                 .flatMap(Address::getStreet)
83                 .orElse("City is not found.");
84         
85         System.out.println("City: " + city);
86         
87          // only print if city is not null
88         company1.flatMap(Company::getOffice)
89                 .flatMap(Office::getAddress)
90                 .flatMap(Address::getCity)
91                 .ifPresent(System.out::println);
92 
93     }
94 }