java - 收集器.toMap IllegalStateException : Duplicate key when no duplication is possible

Question

我一直遇到这个错误，但我无法理解它，因为它提示一个值恰好出现一次。

Exception in thread "main" java.lang.IllegalStateException: Duplicate key wp-admin/admin-ajax.php#13236
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1245)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
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 <my code>

这是原始代码:

reader.lines().filter(line -> line.startsWith(TAG_MAPPED_NODE)).map(line -> {
    final String[] splitted = line.split(" ");
    if (splitted.length != 3)
        throw new IllegalStateException("Unexpected line: " + line);
    return splitted;(splitted[1],splitted[2]);
}).collect(Collectors.toMap(t -> t[1], t -> t[2]));

但是，它提示的值只存在一次。验证码:

List<String> usefulLines =
    reader.lines().filter(line -> line.startsWith(TAG_MAPPED_NODE)).collect(Collectors.toList());
List<String> trouble =
    usefulLines.stream().filter(line -> line.contains("wp-admin/admin-ajax.php#13236")).collect(Collectors.toList());
System.out.println("Trouble size: " + trouble.size());
return usefulLines.stream().map(line -> {
    final String[] splitted = line.split(" ");
    if (splitted.length != 3)
        throw new IllegalStateException("Unexpected line: " + line);
    return splitted;
}).collect(Collectors.toMap(t -> t[1], t -> t[2]));

输出是:Trouble size: 1

因此，只有一个条目具有此值(而且，我必须补充一点，我将其存储在 Map 的值端，而不是键)

系统配置:

• Java版本:openjdk版本“1.8.0_72”
• IDE:IntelliJ IDEA 2016.1
• 操作系统:Fedora 22 64 位

那么我到底是怎么得到这个异常(exception)的呢？对我来说这看起来像是一个 JDK 错误 - 有人看到我做错了什么吗？

Answer 1

我的解决方案是用 Guava 的 ImmutableMap.toImmutableMap() 替换有问题的 Collectors.toMap() .耶!