Java里如何使用Optional.map对对象进行转换_Optional映射操作说明

Optional<String> nameOpt = Optional.of("alice");
Optional<String> upperOpt = nameOpt.map(String::toUpperCase);
System.out.println(upperOpt.orElse("EMPTY")); // 输出: ALICE

Optional<String> emptyOpt = Optional.empty();
Optional<String> result = emptyOpt.map(String::toUpperCase);
System.out.println(result.isPresent()); // 输出: false

class User {
    private String email;
    private int age;

    public User(String email, int age) {
        this.email = email;
        this.age = age;
    }

    public String getEmail() { return email; }
    public int getAge() { return age; }
}

Optional<User> userOpt = Optional.of(new User("bob@example.com", 25));
Optional<String> emailOpt = userOpt.map(User::getEmail);
System.out.println(emailOpt.orElse("No email")); // 输出: bob@example.com

Optional<User> userOpt = Optional.of(new User("carol@example.com", 30));
Optional<Integer> lengthOpt = userOpt
    .map(User::getEmail)
    .map(String::length);

System.out.println(lengthOpt.orElse(0)); // 输出: 19

Optional<String> result = userOpt
    .flatMap(u -> Optional.ofNullable(u.getEmail()))
    .map(String::toLowerCase);