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java8 stream 的toMap方法，如果key有重复的就会跑出上面的异常，比如：
List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(1);System.out.println(list.stream().collect(Collectors.toMap(e->e, e->"hello")));Exception in thread "main" java.lang.IllegalStateException: Duplicate key 1
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1254)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
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 com.liyao.AAA.main(AAA.java:32)
解决办法是：使用重载方法，多传入一个merge function来处理冲突
List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(1);
// System.out.println(list.stream().collect(Collectors.toMap(e->e, e->"hello")));System.out.println(list.stream().collect(Collectors.toMap(e->e, e->"1", (k,v)->k + "world"))); /*** Returns a {@code Collector} that accumulates elements into a* {@code Map} whose keys and values are the result of applying the provided* mapping functions to the input elements.** <p>If the mapped* keys contains duplicates (according to {@link Object#equals(Object)}),* the value mapping function is applied to each equal element, and the* results are merged using the provided merging function.** @apiNote* There are multiple ways to deal with collisions between multiple elements* mapping to the same key. The other forms of {@code toMap} simply use* a merge function that throws unconditionally, but you can easily write* more flexible merge policies. For example, if you have a stream* of {@code Person}, and you want to produce a "phone book" mapping name to* address, but it is possible that two persons have the same name, you can* do as follows to gracefully deals with these collisions, and produce a* {@code Map} mapping names to a concatenated list of addresses:* <pre>{@code* Map<String, String> phoneBook* people.stream().collect(toMap(Person::getName,* Person::getAddress,* (s, a) -> s + ", " + a));* }</pre>** @implNote* The returned {@code Collector} is not concurrent. For parallel stream* pipelines, the {@code combiner} function operates by merging the keys* from one map into another, which can be an expensive operation. If it is* not required that results are merged into the {@code Map} in encounter* order, using {@link #toConcurrentMap(Function, Function, BinaryOperator)}* may offer better parallel performance.** @param <T> the type of the input elements* @param <K> the output type of the key mapping function* @param <U> the output type of the value mapping function* @param keyMapper a mapping function to produce keys* @param valueMapper a mapping function to produce values* @param mergeFunction a merge function, used to resolve collisions between* values associated with the same key, as supplied* to {@link Map#merge(Object, Object, BiFunction)}* @return a {@code Collector} which collects elements into a {@code Map}* whose keys are the result of applying a key mapping function to the input* elements, and whose values are the result of applying a value mapping* function to all input elements equal to the key and combining them* using the merge function** @see #toMap(Function, Function)* @see #toMap(Function, Function, BinaryOperator, Supplier)* @see #toConcurrentMap(Function, Function, BinaryOperator)*/public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction) {return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);}
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