你可以用三元的，

List<String> finalStringList = myString == null ?
        Collections.emptyList()
        : Optional.ofNullable(myStringList)
        .map(strings -> strings.stream()
                .filter(obj -> StringUtils.isEmpty(myString)
                        || myString.equals(obj))
                .collect(Collectors.toList()))
        .orElseGet(ArrayList::new);

或者一个基本的if，默认为空列表。

List<String> finalStringList = Collections.emptyList();
if (myString != null) {
    finalStringList = Optional.ofNullable(myStringList)
            .map(strings -> strings.stream()
                    .filter(obj -> StringUtils.isEmpty(myString)
                            || myString.equals(obj))
                    .collect(Collectors.toList()))
            .orElseGet(ArrayList::new);
}

这可能适合您（使用Java 9+）：

List<String> result2 = Optional.ofNullable(myString)
  .filter(Objects::nonNull)
  .map(mystr -> Optional.ofNullable(myStringList)
                        .filter(Objects::nonNull)
                        .or(() -> Optional.of(new ArrayList<>()))
                        .stream()
                        .flatMap(List::stream)
                        .filter(mystr::equals)
                        .collect(Collectors.toList())) //ArrayList not guaranteed
    .orElse(myStringList);

如果您使用的是Java 8，下面的代码应该可以工作，但看起来有点难读：

List<String> result = Optional.ofNullable(myString)
  .filter(Objects::nonNull)
  .map(mystr -> Optional.ofNullable(myStringList)
                .filter(Objects::nonNull)
                .map(list -> list.stream()
                            .filter(mystr::equals)
                            .collect(Collectors.toList()))
                .orElseGet(ArrayList::new))
  .orElse(myStringList);

其他解决方案

List<String> myStringList = Arrays.asList("abc", "aef"); // may return null
String myString = "abc"; // may be null

Map<Predicate, Supplier<List<String>>> predicateMap = Map.of(
      (v) -> myStringList == null, () -> new ArrayList<>(), // do a null check on the list. If null, return a new empty list.
      (v) -> myString == null, () -> myStringList, // do a null check on myString. If null, return the list as is WITHOUT LOOPING.
      (v) -> true, () -> myStringList.stream() // if myString is not null, loop through the list and return the filtered list.
                .filter(myString::equals)
                .collect(Collectors.toList())
      );
List<String> outputList = predicateMap.entrySet().stream()
      .filter(p -> p.getKey().test(null))
      .findFirst().get().getValue().get();

System.out.println(outputList);

就我个人而言，我认为if-else的代码将更具可读性/可维护性/效率：

if (myStringList == null || myStringList.size() == 0) {
    return Collections.emptyList();
} else if (StringUtils.isEmpty(myString)) {
    return new ArrayList<>(myStringList); // or return myStringList;
} else {
    return myStringList.stream().filter(e -> myString.equals(e))
                                .collect(Collectors.toList());
}

好吧，如果你真的想避免写几行代码，试试我的库：abacus-common

return N.isNullOrEmpty(myString) ? N.newArrayList(myStringList) 
                                 : N.filter(myStringList, e -> myString.equals(e));