public static <T> Optional<T> ofNullable(T value)

String name = getName();
if (name != null) {
    System.out.println(name.length());
} else {
    System.out.println("Name is null");
}

String name = null;
Optional<String> optionalName = Optional.ofNullable(name);
// 檢查是否有值
System.out.println(optionalName.isPresent()); // false
// 提供默認值
System.out.println(optionalName.orElse("Unknown")); // "Unknown"

User user = getUserById(123);
String city = Optional.ofNullable(user)
    .map(User::getAddress) // 如果 user 或 address 為 null，不會拋 NPE
    .map(Address::getCity)
    .orElse("Default City");

String value = Optional.ofNullable(getValueFromAPI())
    .orElseThrow(() -> new RuntimeException("Value cannot be null!"));

Optional.of(null); // 拋出 NullPointerException

import java.util.Optional;
public class OptionalExample {
    public static void main(String[] args) {
        // 示例 1：基本用法
        String name = null;
        Optional<String> optionalName = Optional.ofNullable(name);
        System.out.println(optionalName.orElse("Unknown")); // "Unknown"
        // 示例 2：鏈式操作
        User user = new User("Alice", new Address("New York"));
        String city = Optional.ofNullable(user)
            .map(User::getAddress)
            .map(Address::getCity)
            .orElse("Default City");
        System.out.println(city); // "New York"
    }
}
class User {
    private String name;
    private Address address;
    // 構造方法、getter、setter 省略
}
class Address {
    private String city;
    // 構造方法、getter、setter 省略
}