java - 如果我的内部对象为空，Optional.ofNullable 似乎不起作用？

Question

我有一个嵌套类结构，用于反序列化我的数据:

Class A has a single property that is an object of Class B (say b)
Class B has a single property that is an object of Class C (say c)
Class C has a single property that is an object of Class D (say d)
Class D has a single property that is a a string (say e)

我的数据看起来像

Map<String, List<Map<String, Map<String, Map<String, Map<String, String>>>>>> input =
                ImmutableMap.of("Key",
                        ImmutableList.of(ImmutableMap.of("a",
                                ImmutableMap.of("b",
                                        ImmutableMap.of("c",
                                                ImmutableMap.of("d", "e"))))));

我想解析这个多级映射并将结果放入映射中

Map<String, String> result = new HashMap<>();

最后，我希望 result 映射在末尾包含以下内容:["key", "e"]

如果 map 包含所有中间键a、b、c和d，我的这段代码就可以工作

mapping.entrySet()
            .stream()
            .map(l -> l.getValue().stream()
                    .map(Optional::ofNullable)
                    .map(opta -> opta.map(A::getB))
                    .map(optb -> optb.map(B::getC))
                    .map(optc -> optc.map(C::getD))
                    .map(optd -> optd.map(D::getE))
                    .map(optv -> optv.orElse("default"))
                    .map(m -> result.put(l.getKey(), m))
                    .count())
            .count();

但是例如说如果输入是

Map<String, List<Map<String, Map<String, Map<String, Map<String, String>>>>>> input =
                    ImmutableMap.of("Key",
                        ImmutableList.of(ImmutableMap.of("a",
                                ImmutableMap.of("b",null))));

然后我的代码失败了:

java.lang.NullPointerException: null value in entry: b=null

为什么我的Optional.isNullable不工作？

Answer 1

您正在使用ImmutableMap ，和ImmutableMap不喜欢null键或值:https://guava.dev/releases/23.0/api/docs/com/google/common/collect/ImmutableMap.html#of-K-V-

public static <K,V> ImmutableMap<K,V> of(K k1, V v1)

Returns an immutable map containing a single entry. This map behaves and performs comparably to Collections.singletonMap(K, V) but will not accept a null key or value. It is preferable mainly for consistency and maintainability of your code.

该消息是通过此方法生成的:https://github.com/google/guava/blob/82b3e9806dc3422e51ecb9400d8f50404b083dde/guava/src/com/google/common/collect/CollectPreconditions.java#L28

  static void checkEntryNotNull(Object key, Object value) {
    if (key == null) {
      throw new NullPointerException("null key in entry: null=" + value);
    } else if (value == null) {
      throw new NullPointerException("null value in entry: " + key + "=null");
    }
  }

另一方面，我认为您应该使用专门的而不是 map ...我很确定这种类型会让您的团队或其他读者感到头疼:

Map<String, List<Map<String, Map<String, Map<String, Map<String, String>>>>>>