Stream ToMap(Collectors.toMap)

Stream ToMap(Collectors.toMap)

List TO Map

List Stream 转换 Map时向collect()方法中传递Collector对象，对象由Collectors.toMap()方法返回。

实例：

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List<GroupBrandCateBO> list = new ArrayList<>(  
      Arrays.asList(  
              new GroupBrandCateBO("v1", "g1", "b1"),  
              new GroupBrandCateBO("v1", "g1", "b1"),  
              new GroupBrandCateBO("v3", "g3", "b3")  
      )  
);  
    Map<String, String> map = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal, LinkedHashMap::new));   
       System.out.println(map.getClass());  
    Map<String, String> map0 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal));  
      System.out.println(map0.getClass());  
      System.out.println(map0.toString());  
    Map<String, String> map1 = list.stream().collect(Collectors.toMap(GroupBrandCateBO::getVersion, GroupBrandCateBO::getGroupCode));  
      System.out.println(map1.toString());

问题

当存在Key相同的情况下会出现如下异常：

Exception in thread “main” java.lang.IllegalStateException: Duplicate key……

原因分析

toMap()函数重载：
未指定合并函数mergeFunction情况下，传入throwingMerger()返回BinaryOperator对象，当出现key重复时，调用合并函数！
未指定Supplier实例情况下，默认生成HashMap实例！

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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>  
    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);  
}  
  
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：
加上：

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(oldVal, currVal) -> currVal) // key相同时当前值替换原始值  
(oldVal, currVal) -> oldVal + currVal //key相同时保留原始值和当前值，合并value！！！！！！

注：合并的是值value，而不是key！！！

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