Collector.toMap，Map中的key不能重复，如果重复的话，会抛出异常

桃扇骨 2023-06-16 02:17 4阅读 0赞

# 1.报错 #

Exception in thread "main" java.lang.IllegalStateException: Duplicate key http://www.google.com
        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)
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)

## 2.解决 ##

通过查看Collectors.toMap的代码及注释我们会发现：

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);
    }

If the mapped keys contains duplicates (according to Object#equals(Object)), 
    an IllegalStateException is thrown when the collection operation is performed. 
    If the mapped keys may have duplicates, use toMap(Function, Function, BinaryOperator) instead.

所以说呢，我们使用的Collectors.toMap的方法是不支持key重复的，并且如果有重复的时候，建议我们使用toMap(Function, Function, BinaryOperator) 方法来替换使用，并且我们还可以定义当key重复的时候，是使用旧的数据还是使用新的数据  
即：

Map<String, String> memberMap = list.stream()
    									.collect(Collectors.toMap(Member::getId, Member::getImgPath,(oldValue,newValue) -> oldValue));