详解Java中Optional类的使用方法

@Data
@AllArgsConstructor
@NoArgsConstructor
class Student {
    private String name;
    private Integer age;
}

public void test1() {
    // 声明一个空Optional
    Optional<Object> empty = Optional.empty();
    
    // 依据一个非空值创建Optional
    Student student = new Student();
    Optional<Student> os1 = Optional.of(student);
    
    // 可接受null的Optional
    Student student1 = null;
    Optional<Student> os2 = Optional.ofNullable(student1);
}    

public void test1() {
    Student student = new Student();
    Optional<Student> os1 = Optional.ofNullable(student);
    boolean present = os1.isPresent();
    System.out.println(present);

    // 利用Optional的ifPresent方法做出如下：当student不为空的时候将name赋值为张三
    Optional.ofNullable(student).ifPresent(p -> p.setName("张三"));
}

public void test1() throws Exception {
    Student student = null;
    Optional<Student> os1 = Optional.ofNullable(student);
    // 使用get一定要注意，假如student对象为空，get是会报错的
    // java.util.NoSuchElementException: No value present
    Student student1 = os1.get();

    // 当student为空的时候,返回我们新建的这个对象,有点像三目运算的感觉
    Student student2 = os1.orElse(new Student("张三", 18));

    // orElseGet就是当student为空的时候，返回通过Supplier供应商函数创建的对象
    Student student3 = os1.orElseGet(() -> new Student("张三", 18));

    // orElseThrow就是当student为空的时候，可以抛出我们指定的异常
    os1.orElseThrow(() -> new Exception());
}

public void test1() {
    Student student = new Student("李四", 3);
    Optional<Student> os1 = Optional.ofNullable(student);
    os1.filter(p -> p.getName().equals("张三")).ifPresent(x -> System.out.println("OK"));
}

public void test1() {
     Student student = new Student("李四", 3);
     Optional<Student> os1 = Optional.ofNullable(student);
     // 如果student对象不为空，就加一岁
     Optional<Student> emp = os1.map(e ->
     {
         e.setAge(e.getAge() + 1);
         return e;
     });
 }

public static Optional<Integer> stringToInt(String s) {
     try {
         return Optional.of(Integer.parseInt(s));
     } catch (NumberFORMatException e) {
         e.printStackTrace();
         return Optional.empty();
     }
 }

Optional.ofNullable(props.getProperty(name))
        .flatMap(OptionalUtils::stringToInt)
        .filter(i -> i>0)
        .orElse(0);

PatientInfo patientInfo = patientInfoDao.getPatientInfoById(consultOrder.getPatientId());
if (patientInfo != null) {
    consultInfoResp.setPatientHead(patientInfo.getHead());
}

// 使用Optional 和函数式编程，一行搞定，而且像说话一样
Optional.ofNullable(patientInfo).ifPresent(p -> consultInfoResp.setPatientHead(p.getHead()));

public void test1() throws Exception {
    Student student = new Student(null, 3);
    if (student == null || isEmpty(student.getName())) {
        throw new Exception();
    }
    String name = student.getName();
    // 业务省略...

    // 使用Optional改造
    Optional.ofNullable(student).filter(s -> !isEmpty(s.getName())).orElseThrow(() -> new Exception());
}

public static boolean isEmpty(CharSequence str) {
    return str == null || str.length() == 0;
}

public static String getChampionName(Competition comp) throws IllegalArgumentException {
    if (comp != null) {
        CompResult result = comp.getResult();
        if (result != null) {
            User champion = result.getChampion();
            if (champion != null) {
                return champion.getName();
            }
        }
    }
    throw new IllegalArgumentException("The value of param comp isn't available.");
}

comp.getResult().getChampion().getName()

public static String getChampionName(Competition comp) throws IllegalArgumentException {
    return Optional.ofNullable(comp)
            .map(Competition::getResult)  // 相当于c -> c.getResult()，下同
            .map(CompResult::getChampion)
            .map(User::getName)
            .orElseThrow(()->new IllegalArgumentException("The value of param comp isn't available."));
}

int timeout = Optional.ofNullable(RedisProperties.getTimeout())
                      .map(x -> Long.valueOf(x.toMillis()).intValue())
                      .orElse(10000);