Collectors.toMap中的Java 8 NullPointerException

问题描述

如果其中一个值为null，则Java 8 Collectors.toMap 会抛出 NullPointerException 。我不明白这种行为，map可以包含null指针作为值而没有任何问题。 Collectors.toMap ？

The Java 8 Collectors.toMap throws a NullPointerException if one of the values is 'null'. I don't understand this behaviour, maps can contain null pointers as value without any problems. Is there a good reason why values cannot be null for Collectors.toMap?

还有一个很好的理由为什么值不能为null解决这个问题的方法有哪些，或者我应该恢复原状for for循环？

Also, is there a nice Java 8 way of fixing this, or should I revert to plain old for loop?

我的问题的一个例子：

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));
    }
}

Stacktrace：

Stacktrace:

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)

Java 11中仍存在此问题。

This problem still exists in Java 11.

推荐答案

使用收集器的静态方法是不可能的。 解释 toMap 基于：

It is not possible with the static methods of Collectors. The javadoc of toMap explains that toMap is based on Map.merge:

以及说：

您可以使用列表方法。

You can avoid the for loop by using the forEach method of your list.

Map<Integer,  Boolean> answerMap = new HashMap<>();
answerList.forEach((answer) -> answerMap.put(answer.getId(), answer.getAnswer()));

但它并不比旧方式简单：

but it is not really simple than the old way:

Map<Integer, Boolean> answerMap = new HashMap<>();
for (Answer answer : answerList) {
    answerMap.put(answer.getId(), answer.getAnswer());
}

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