Java 8流-收集可能为空的值

Question

    private static <T> Map<String, ?> getDifference(final T a, final T b, final Map<String, Function<T, Object>> fields) {
    return fields.entrySet().stream()
            .map(e -> {
                final String name = e.getKey();
                final Function<T, Object> getter = e.getValue();
                final Object pairKey = getter.apply(a);
                final Object pairValue = getter.apply(b);
                if (Objects.equals(pairKey, pairValue)) {
                    return null;
                } else {
                    return Pair.of(name, pairValue);
                }
            })
            .filter(Objects::nonNull)
            .collect(Collectors.toMap(Pair::getKey, Pair::getValue));
    }

.collect(Collectors.toMap(Pair::getKey,Optional.ofNullable(Pair::getValue).orElse(""));

.collect(Collectors.toMap(pair -> pair.getKey(), Optional.ofNullable(pair -> pair.getValue()).orElse(""));

Answer 1

您的语法不正确。tomap()的第二个参数必须是lambda，因此

.collect(Collectors.toMap(
             pair -> pair.getKey(),
             pair -> Optional.ofNullable(pair.getValue()).orElse("")
));

或

您可以修改map()节，如下所示

return Pair.of(name, Optional.ofNullable(pairValue).orElse(""));