java8 对象转 Map 时 key Duplicate key

报错信息：

Duplicate key SpotDiffBudgetDTO

java.lang.IllegalStateException: Duplicate key...

问题代码：

Map<String, SpotDiffBudgetDTO> spotMap = list.stream().collect(Collectors
    .toMap(SpotDiffBudgetDTO::getSpotId, 
    spotDiffBudgetDTO -> spotDiffBudgetDTO));

原因解析：

生成Map的key时重复，导致错误，key冲突（因为Map的key必须保持唯一性！）

解决方案：

转换时使用另外一种写法，当遇到key冲突时选取第一个获第二个做key

代码：

case1:
Map<String, SpotDiffBudgetDTO> spotMap = list.stream().collect(Collectors
    .toMap(SpotDiffBudgetDTO::getSpotId,
        item->item,
        (k1,k2) ->k1));

case2:
Map<String, SpotDiffBudgetDTO> spotMap = list.stream().collect(Collectors
    .toMap(SpotDiffBudgetDTO::getSpotId,
        spotDiffBudgetDTO -> spotDiffBudgetDTO,
        (getSpotId,spotDiffBudgetDTO) ->getSpotId));

源码：

 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);
    }

参考：blog.csdn.net/TP89757/art…