将列表转换为映射和筛选空键

Question

使用Java8 streams，我希望将列表转换为映射，如Java8 list into map 解决方案中所述。但是，我希望过滤以删除具有某些键的条目（例如，如果键为空），而不需要将值转换为键两次。

Map<String, Choice> result =
    choices.stream().filter((choice) -> choice.getName() != null).collect(Collectors.toMap(Choice::getName,
                                 Function.<Choice>identity());

Answer 1

以下是您想要的自定义收集器：

public class FilteredKeyCollector<T, K, V> implements Collector<T, Map<K, V>, Map<K, V>> {

    private final Function<? super T,? extends K> keyMapper;
    private final Function<? super T,? extends V> valueMapper;
    private final Predicate<K> keyFilter;
    private final EnumSet<Collector.Characteristics> characteristics;

    private FilteredKeyCollector(Function<? super T,? extends K> keyMapper, Function<? super T,? extends V> valueMapper, Predicate<K> keyFilter) {

        this.keyMapper = keyMapper;
        this.valueMapper = valueMapper;
        this.keyFilter = keyFilter;
        this.characteristics = EnumSet.of(Collector.Characteristics.IDENTITY_FINISH);
    }

    @Override
    public Supplier<Map<K, V>> supplier() {
        return HashMap<K, V>::new;
    }

    @Override
    public BiConsumer<Map<K, V>, T> accumulator() {
        return (map, t) -> {
            K key = keyMapper.apply(t);
            if (keyFilter.test(key)) {
                map.put(key, valueMapper.apply(t));
            }
        };
    }

    @Override
    public BinaryOperator<Map<K, V>> combiner() {
        return (map1, map2) -> {
            map1.putAll(map2);
            return map1;
        };
    }

    @Override
    public Function<Map<K, V>, Map<K, V>> finisher() {
        return m -> m;
    }

    @Override
    public Set<Collector.Characteristics> characteristics() {
        return characteristics;
    }
}

并使用它：

Map<String, Choice> result = choices.stream()
    .collect(new FilteredKeyCollector<>(
                Choice::getName,    // key mapper
                c -> c,             // value mapper
                k -> k != null));   // key filter