Java使用Optional优雅处理null的具体方法

public String getUserAddress(User user) {
    return user.getAddress().getStreet();
}

public String getUserAddress(User user) {
    if (user != null) {
        Address address = user.getAddress();
        if (address != null) {
            return address.getStreet();
        }
    }
    return null; // 或者返回默认值
}

// 1. 创建一个包含非null值的Optional
Optional hello = Optional.of("Hello"); 

// 2. 创建一个可能为空的Optional
Optional empty = Optional.ofNullable(null);

// 3. 创建一个空Optional
Optional empty2 = Optional.empty();

Optional opt = Optional.of("Java");

if (opt.isPresent()) {  // 检查是否有值
    System.out.println(opt.get()); // 获取值(不安全！)
}

String name = Optional.ofNullable(getName()).orElse("默认名称");

String name = Optional.ofNullable(getName())
                      .orElseGet(() -> generateDefaultName()); // 只有需要时才调用

String name = Optional.ofNullable(getName())
                      .orElseThrow(() -> new IllegalArgumentException("名称不能为空"));

Optional user = Optional.ofNullable(getUser());
Optional name = user.map(User::getName); // 把User映射为name

Optional> nested = Optional.of(Optional.of("hello"));
Optional flat = nested.flatMap(x -> x); // 解包为Optional

Optional longName = name.filter(n -> n.length() > 5); // 只保留长度大于5的名字

List users = ...;
List names = users.stream()
    .map(User::getName)       // 转为Stream
    .map(Optional::ofNullable) // 转为Stream>
    .filter(Optional::isPresent)
    .map(Optional::get)
    .collect(Collectors.toList());

String street = Optional.ofNullable(user)
    .map(User::getAddress)
    .map(Address::getStreet)
    .orElse("未知街道");

Optional.ofNullable(getUser())
    .ifPresent(u -> System.out.println("用户存在: " + u.getName()));

// 错误示范！❌
public void processUser(Optional user) {
    // ...
}

// 没必要！❌
Optional> names = Optional.of(Arrays.asList("A", "B"));

// 错误示范！❌
class User {
    private Optional name; // 不要这样做！
}

public String getEmployeeManagerName(Employee employee) {
    if (employee != null) {
        Department dept = employee.getDepartment();
        if (dept != null) {
            Employee manager = dept.getManager();
            if (manager != null) {
                return manager.getName();
            }
        }
    }
    return "无经理";
}

public String getEmployeeManagerName(Employee employee) {
    return Optional.ofNullable(employee)
        .map(Employee::getDepartment)
        .map(Department::getManager)
        .map(Employee::getName)
        .orElse("无经理");
}

public List getAllManagerNames(List employees) {
    return employees.stream()
        .map(Employee::getDepartment)
        .filter(Objects::nonNull)
        .map(Department::getManager)
        .filter(Objects::nonNull)
        .map(Employee::getName)
        .filter(Objects::nonNull)
        .collect(Collectors.toList());
}

public List getAllManagerNames(List employees) {
    return employees.stream()
        .map(Employee::getDepartment)
        .flatMap(dept -> Optional.ofNullable(dept).stream())
        .map(Department::getManager)
        .flatMap(manager -> Optional.ofNullable(manager).stream())
        .map(Employee::getName)
        .flatMap(name -> Optional.ofNullable(name).stream())
        .collect(Collectors.toList());
}

public interface UserRepository extends JpaRepository {
    Optional findByUsername(String username);
}

@GetMapping("/users/{id}")
public ResponseEntity getUser(@PathVariable Long id) {
    return userRepository.findById(id)
           .map(user -> ResponseEntity.ok(user))
           .orElse(ResponseEntity.notFound().build());
}

String timeout = Optional.ofNullable(config.get("timeout"))
                        .map(String::valueOf)
                        .orElse("30");

Optional.ofNullable(legacyMethodThatReturnsNull())...