練習使用Optional來避免NullPointerException。
User
public class User {
private Integer id;
private String name;
private Integer age;
public User(Integer id, String name, Integer age) {
super();
this.id = id;
this.name = name;
this.age = age;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
}
OptionalUser與User內容幾乎一樣，只是getter會以Optional的方式回傳。注意此為錯誤用法。Optional主要用在一般方法的回傳值，不應用在類別成員、方法或建構式傳入參數、POJO getter等
OptionalUser
public class OptionalUser {
private Integer id;
private String name;
private Integer age;
public OptionalUser(Integer id, String name, Integer age) {
super();
this.id = id;
this.name = name;
this.age = age;
}
public Optional<Integer> getId() {
return Optional.ofNullable(id);
}
public void setId(Integer id) {
this.id = id;
}
public Optional<String> getName() { // return Optional. wrong use
return Optional.ofNullable(name);
}
public void setName(String name) {
this.name = name;
}
public Optional<Integer> getAge() { // return Optional. wrong use
return Optional.ofNullable(age);
}
public void setAge(Integer age) {
this.age = age;
}
}
UserService用來取得User與OptionalUser的資料。
UserService
public class UserService {
private List<User> userList = null;
private List<OptionalUser> optionalUserList = null;
public UserService() {
userList = Arrays.asList(
new User(1, "John", null),
new User(2, null, 17)
);
optionalUserList = Arrays.asList(
new OptionalUser(1, "John", null),
new OptionalUser(2, null, 17)
);
}
public User getUserById(Integer id) {
for (User user : userList) {
if(user.getId() == id) {
return user;
}
}
return null;
}
public Optional<OptionalUser> getOptionalUserById(Integer id) {
for (OptionalUser optionalUser : optionalUserList) {
if(optionalUser.getId().orElse(-1) == id) {
return Optional.ofNullable(optionalUser);
}
}
return Optional.empty();
}
public boolean isAdult(OptionalUser ou) {
return ou.getAge().filter(a -> a >= 18).map((a) -> {
System.out.println(ou.getName().get() + " is adult");
return true;
}).orElseGet(() -> {
System.out.println(ou.getName().get() + " is not adult");
return false;
});
}
}
Main
public class Main {
private static final int DEFAULT_AGE = 1;
public static void main(String[] args) throws Exception {
UserService service = new UserService();
// 使用if來檢查null
User user = service.getUserById(2);
if(user != null) {
Integer age = user.getAge();
if(age == null) {
age = DEFAULT_AGE;
}
if(user.getName() == null) {
user.setName("Anonymouse");
}
if(age >= 18) {
System.out.println(user.getName() + " is adult");
} else {
System.out.println(user.getName() + " is not adult");
}
} else {
throw new Exception("No user found");
}
// 使用Optional
OptionalUser ou = service.getOptionalUserById(2).orElseThrow(() -> new Exception("No user found"));
String name = ou.getName().orElseGet(()->{
ou.setName("Anonymouse");
return ou.getName().get();
});
System.out.println(service.isAdult(ou));
}
}
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