Java8中List转Map报错“java.lang.IllegalStateException: Duplicate key”

排查思路

1. 从报错的关键字中可以大致判断是是key冲突,Duplicate key在数据库的主键冲突错误中经常遇到，个人的思维惯性就联想到了数据库,从这个方向去排查,无果。
2. 抓耳挠腮之下，分析如下错误信息

java.lang.IllegalStateException: Duplicate key image(id=1,name='晓明')
  at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
  at java.util.HashMap.merge(HashMap.java:1253)
  at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)

这里是当时的问题代码，本意是想把查出来的文件list再以文件fileId作为Map的key转换成map,结果库里有两个一样的文件ID就导致了错误的发生，因为map的key是唯一的。

Map<String, ImageFile> oldImageFileMap = 
       list(new LambdaQueryWrapper<ImageFile>()
       .eq(ImageFile::getOrderNo, orderNo)
       .eq(ImageFile::getDeleteFlag, 0))
       .stream().collect(Collectors.toMap(ImageFile::getOriginalFileId, Function.identity()));

以下是源码片段

public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper) {
        return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}

public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapSupplier) {
        BiConsumer<M, T> accumulator
                = (map, element) -> map.merge(keyMapper.apply(element),
                                              valueMapper.apply(element), mergeFunction);
        return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
    }

private static <T> BinaryOperator<T> throwingMerger {
    return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };
}

解决方案：
用集合转map的另外的重载方法二

修改后的代码

        Map<String, ImageFile> oldImageFileMap = list(new LambdaQueryWrapper<ImageFile>()
                .eq(ImageFile::getOrderNo, orderNo)
                .eq(ImageFile::getDeleteFlag, 0))
                .stream().collect(Collectors.toMap(ImageFile::getOriginalFileId, Function.identity(),(v1,v2)->v2));