具有空条目值的 Collectors.toMap 中的 NullPointerException [英] NullPointerException in Collectors.toMap with null entry values
问题描述
Collectors.toMap
会抛出一个 NullPointerException
.我不明白这种行为,地图可以包含空指针作为值而没有任何问题.Collectors.toMap
的值不能为空是否有充分的理由?
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
?
另外,有没有一个很好的 Java 8 方法来解决这个问题,还是我应该恢复到普通的 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));
}
}
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.
This problem still exists in Java 11.
推荐答案
您可以解决这个问题 OpenJDK 中的已知错误:
You can work around this known bug in OpenJDK with this:
Map<Integer, Boolean> collect = list.stream()
.collect(HashMap::new, (m,v)->m.put(v.getId(), v.getAnswer()), HashMap::putAll);
它不是那么漂亮,但它有效.结果:
It is not that much pretty, but it works. Result:
1: true
2: true
3: null
(本教程对我帮助最大.)
(this tutorial helped me the most.)
与 Collectors.toMap
不同,如@mmdemirbas 在评论中指出的那样,如果多次使用相同的键,这将无声地替换值.如果您不想要这个,请查看评论中的链接.
Unlike Collectors.toMap
, this will silently replace values if you have the same key multiple times, as @mmdemirbas pointed out in the comments. If you don't want this, look at the link in the comment.
这篇关于具有空条目值的 Collectors.toMap 中的 NullPointerException的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!