在收集器中的NullPointerException。使用空条目值的toMap

收藏家。如果其中一个值为空，toMap将抛出NullPointerException异常。我不理解这种行为，映射可以包含空指针作为值而没有任何问题。是否有很好的理由说明collections . tomap的值不能为空?

此外，是否有一个很好的Java 8的方法来解决这个问题，或者我应该恢复到普通的老循环?

举个例子:

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;


class Answer {
    private int id;

    private Boolean answer;

    Answer() {
    }

    Answer(int id, Boolean answer) {
        this.id = id;
        this.answer = answer;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Boolean getAnswer() {
        return answer;
    }

    public void setAnswer(Boolean answer) {
        this.answer = answer;
    }
}

public class Main {
    public static void main(String[] args) {
        List<Answer> answerList = new ArrayList<>();

        answerList.add(new Answer(1, true));
        answerList.add(new Answer(2, true));
        answerList.add(new Answer(3, null));

        Map<Integer, Boolean> answerMap =
        answerList
                .stream()
                .collect(Collectors.toMap(Answer::getId, Answer::getAnswer));
    }
}

堆栈跟踪：

Exception in thread "main" java.lang.NullPointerException
    at java.util.HashMap.merge(HashMap.java:1216)
    at java.util.stream.Collectors.lambda$toMap$168(Collectors.java:1320)
    at java.util.stream.Collectors$$Lambda$5/1528902577.accept(Unknown Source)
    at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1359)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    at Main.main(Main.java:48)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

这个问题在Java 11中仍然存在。

当前回答

保留所有问题id与小调整

Map<Integer, Boolean> answerMap = 
  answerList.stream()
            .collect(Collectors.toMap(Answer::getId, a -> 
                       Boolean.TRUE.equals(a.getAnswer())));

2017-06-23 08:06:24

其他回答

我稍微修改了Emmanuel Touzery的空安全映射收集器实现。

这个版本:

允许空键允许空值检测重复的键(即使它们为空)并像原始JDK实现一样抛出IllegalStateException 当键已经映射到空值时，也会检测重复的键。换句话说，将具有空值的映射与无映射分开

public static <T, K, U> Collector<T, ?, Map<K, U>> toMapOfNullables(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper) {
    return Collectors.collectingAndThen(
        Collectors.toList(),
        list -> {
            Map<K, U> map = new LinkedHashMap<>();
            list.forEach(item -> {
                K key = keyMapper.apply(item);
                U value = valueMapper.apply(item);
                if (map.containsKey(key)) {
                    throw new IllegalStateException(String.format(
                            "Duplicate key %s (attempted merging values %s and %s)",
                            key, map.get(key), value));
                }
                map.put(key, value);
            });
            return map;
        }
    );
}

单元测试:

@Test
public void toMapOfNullables_WhenHasNullKey() {
    assertEquals(singletonMap(null, "value"),
        Stream.of("ignored").collect(Utils.toMapOfNullables(i -> null, i -> "value"))
    );
}

@Test
public void toMapOfNullables_WhenHasNullValue() {
    assertEquals(singletonMap("key", null),
        Stream.of("ignored").collect(Utils.toMapOfNullables(i -> "key", i -> null))
    );
}

@Test
public void toMapOfNullables_WhenHasDuplicateNullKeys() {
    assertThrows(new IllegalStateException("Duplicate key null"),
        () -> Stream.of(1, 2, 3).collect(Utils.toMapOfNullables(i -> null, i -> i))
    );
}

@Test
public void toMapOfNullables_WhenHasDuplicateKeys_NoneHasNullValue() {
    assertThrows(new IllegalStateException("Duplicate key duplicated-key"),
        () -> Stream.of(1, 2, 3).collect(Utils.toMapOfNullables(i -> "duplicated-key", i -> i))
    );
}

@Test
public void toMapOfNullables_WhenHasDuplicateKeys_OneHasNullValue() {
    assertThrows(new IllegalStateException("Duplicate key duplicated-key"),
        () -> Stream.of(1, null, 3).collect(Utils.toMapOfNullables(i -> "duplicated-key", i -> i))
    );
}

@Test
public void toMapOfNullables_WhenHasDuplicateKeys_AllHasNullValue() {
    assertThrows(new IllegalStateException("Duplicate key duplicated-key"),
        () -> Stream.of(null, null, null).collect(Utils.toMapOfNullables(i -> "duplicated-key", i -> i))
    );
}

2019-08-08 09:13:40

你可以在OpenJDK中使用以下方法来解决这个已知的错误:

Map<Integer, Boolean> collect = list.stream()
        .collect(HashMap::new, (m,v)->m.put(v.getId(), v.getAnswer()), HashMap::putAll);

虽然不是很漂亮，但很好用。结果:

1: true
2: true
3: null

(这篇教程对我帮助最大。)

编辑:

不像收藏家。toMap，如果你有相同的键多次，它会默默地替换值，就像@mmdemirbas在评论中指出的那样。如果你不想要这个，看看评论中的链接。

2014-07-08 14:09:22

如果值是一个字符串，那么这可能会起作用: .stream map.entrySet () () .collect(收藏者。toMap(e -> e.getKey()， e -> Optional.ofNullable(e.getValue()).orElse(""))))

2016-04-27 20:36:44

只需将可空值包装为可选值。非常优雅的解决方法。

    listOfValues.stream()
    .collect(Collectors.toMap(e -> e.getKey(), e -> 
    Optional.ofNullable(e.getValue())))

2023-02-08 09:01:31

保留所有问题id与小调整

Map<Integer, Boolean> answerMap = 
  answerList.stream()
            .collect(Collectors.toMap(Answer::getId, a -> 
                       Boolean.TRUE.equals(a.getAnswer())));

2017-06-23 08:06:24

在收集器中的NullPointerException。使用空条目值的toMap

推荐文章

最新文章

标签