Optional的使用

Optional 被设计出来，主要是为了解决空指针异常。

以一个 Student 类为例：

@Getter
class Student {
    private String name;
    private int age;
    private int score;

    public Student(String name, int age, int score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    public Student() {
    }
}

---

代替 if 语句判空

public void optionalBasicUsage(Student student) {
    // if 语句判空
    if (student != null) {
        System.out.println("一般方法：" + student.getName());
    }

    // 使用 Optional 无需判空
    Optional<Student> optionalStudent = Optional.ofNullable(student);
    // 如果Name存在，执行一段逻辑
    optionalStudent.map(Student::getName).ifPresent(this::callOther);
}

private Object callOther(String name) {
    LOGGER.info("call other api {}...", name);
    return null;
}

---

判空时，通过 orElseGet 方法进行懒加载

public void optionalAdvanceUsage(Student student) {
    Optional<Student> optionalStudent = Optional.ofNullable(student);
    // 如果存在，执行一段逻辑
    optionalStudent.ifPresent(this::callOther);

    // orElseGet实行懒加载，orElse不会懒加载，因此orElseGet性能更好
    // Student student1 = optionalStudent.orElse(createStudent());
    Student student1 = optionalStudent.orElseGet(this::createStudent);
}

private Student createStudent() {
    LOGGER.info("called from createStudent");
    return new Student();
}

private Object callOther(Student student) {
    LOGGER.info("call other api {}...", student);
    return null;
}

---

使用 filter 进行过滤

public void optionalAdvanceUsage(Student student) {
    Optional<Student> optionalStudent = Optional.ofNullable(student);
    // 如果存在，执行一段逻辑
    optionalStudent.ifPresent(this::callOther);
	
     // 过滤
    optionalStudent.filter(stu -> stu.getAge() > 10)
            .ifPresent(stu -> LOGGER.info("age > 10 {}", stu.getName()));
}

---

map 和 flatMap 方法

public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent()) {
        return empty();
    } else {
        // Optional.ofNullable 会自动在外层用Optional进行封装
        return Optional.ofNullable(mapper.apply(value));
    }
}

public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
    Objects.requireNonNull(mapper);
    if (!isPresent()) {
        return empty();
    } else {
        @SuppressWarnings("unchecked")
        // mapper的apply方法要求返回值类型是Optional<? extends U>，所以无需再用Optional封装
        Optional<U> r = (Optional<U>) mapper.apply(value);
        return Objects.requireNonNull(r);
    }
}