Requirements

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。

如下实现List转换为Map

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());

Console
class java.util.LinkedHashMap
class java.util.HashMap
{v1=g1, v3=g3}
Exception in thread “main” java.lang.IllegalStateException: Duplicate key g1
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
…

问题分析

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。

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)); };
}

补充

关于合并函数

List<GroupBrandCateBO> list = new ArrayList<>(
       Arrays.asList(
               new GroupBrandCateBO("v1", "g1", "b1"),
               new GroupBrandCateBO("v1", "g2", "b2"),
               new GroupBrandCateBO("v1", "g2", "b2"),
               new GroupBrandCateBO("v3", "g3", "b3")
       )
);
Map<String, String> map00 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> currVal));
Map<String, String> map01 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal + currVal));
System.out.println(map00.toString());
System.out.println(map01.toString());

Console
{v1=g2, v3=g3}
{v1=g1g2g2, v3=g3}

传入Lambda表达式将转化为BinaryOperator<U> mergeFunction对象，合并处理value，非Key！！！
比如：

(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