Java Optional优雅处理空指针异常的实现步骤

// 1. 创建一个空的 Optional
Optional<String> emptyOpt = Optional.empty();

// 2. 创建包含非null值的Optional (如果传入null会抛出NullPointerException)
Optional<String> nonNullOpt = Optional.of("Hello");

// 3. 创建可能为null的Optional
Optional<String> nullableOpt = Optional.ofNullable(null);

// 检查值是否存在
boolean isPresent = optional.isPresent();
// Java 11+ 新增的相反方法
boolean isEmpty = optional.isEmpty();
// 如果值存在则执行操作
optional.ifPresent(value -> System.out.println("Value: " + value));
// Java 9+ 的 ifPresentOrElse
optional.ifPresentOrElse(
    value -> System.out.println("Value: " + value),
    () -> System.out.println("Value is absent")
);

// 不安全的方式 - 可能抛出NoSuchElementException
String value = optional.get();

// 安全方式1 - 提供默认值
String value = optional.orElse("default");

// 安全方式2 - 通过Supplier提供默认值
String value = optional.orElseGet(() -> "computed default");

// 安全方式3 - 值不存在时抛出指定异常
String value = optional.orElseThrow(() -> new IllegalArgumentException("Value missing"));

// map - 转换值并自动包装为Optional
Optional<String> upperOpt = optional.map(String::toUpperCase);

// flatMap - 用于嵌套Optional的情况
Optional<String> flatMapped = optional.flatMap(v -> Optional.of(v.toUpperCase()));

// filter - 基于条件过滤
Optional<String> filtered = optional.filter(v -> v.length() > 3);

String city = Optional.ofNullable(person)
    .map(Person::getAddress)
    .map(Address::getCity)
    .orElse("Unknown City");

if (user != null) {
    Address address = user.getAddress();
    if (address != null) {
        Country country = address.getCountry();
        if (country != null) {
            String isocode = country.getIsocode();
            if (isocode != null) {
                isocode = isocode.toUpperCase();
            }
        }
    }
}

String isocode = Optional.ofNullable(user)
    .map(User::getAddress)
    .map(Address::getCountry)
    .map(Country::getIsocode)
    .map(String::toUpperCase)
    .orElse("DEFAULT");

List<Order> orders = ...;
Optional<Order> mostExpensive = orders.stream()
    .max(Comparator.comparing(Order::getPrice));
mostExpensive.ifPresent(order -> System.out.println("最贵订单: " + order));

public final class Optional<T> {
    private static final Optional<?> EMPTY = new Optional<>();
    private final T value;
    
    private Optional() {
        this.value = null;
    }
    
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }
    
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }
    
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }
    
    // 其他方法...
}