类型不匹配：无法从 Optional<Object> 转换为 BasketDTO

Question

我有以下 java 11 代码，但 Eclipse 显示错误 Type mismatch: cannot convert from Optional<Object> to BasketDTO

    public BasketDTO findBasket(String key) {
        return Optional.ofNullable(key).map(basketRepository::findOneByKey)
                .map(b -> b.map(BasketDTO::new).orElseGet(() -> new BasketDTO()));
    }

    Optional<BasketModel> findOneByKey(String key){
    }

Answer 1

因为basketRepository::findOneByKeyreturns一个Optional，你可以用Optional#flatMap代替Optional#map这样你就不用担心嵌套Optionals:

public BasketDTO findBasket(String key) {
    return Optional.ofNullable(key) // Optional<String>
            .flatMap(basketRepository::findOneByKey) // Optional<BasketModel>
            .map(BasketDTO::new)        // b -> new BasketDTO(b)
            .orElseGet(BasketDTO::new); // () -> new BasketDTO()
}

类型不匹配：无法从 Optional<Object> 转换为 BasketDTO

Type mismatch: cannot convert from Optional<Object> to BasketDTO

java

optional