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 list = new ArrayList<>(
      Arrays.asList(
              new GroupBrandCateBO("v1", "g1", "b1"),
              new GroupBrandCateBO("v1", "g1", "b1"),
              new GroupBrandCateBO("v3", "g3", "b3")
      )
);
Map map = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> oldVal, LinkedHashMap::new)); 
System.out.println(map.getClass());
Map 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 map1 = list.stream().collect(Collectors.toMap(GroupBrandCateBO::getVersion, GroupBrandCateBO::getGroupCode));
System.out.println(map1.toString());
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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 
Collector> toMap(Function keyMapper,
                                Function valueMapper) {
    return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}

public static 
Collector> toMap(Function keyMapper,
                                Function valueMapper,
                                BinaryOperator mergeFunction) {
    return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
}

public static >
Collector toMap(Function keyMapper,
                            Function valueMapper,
                            BinaryOperator mergeFunction,
                            Supplier mapSupplier) {
    BiConsumer accumulator
            = (map, element) -> map.merge(keyMapper.apply(element),
                                          valueMapper.apply(element), mergeFunction);
    return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
}

private static  BinaryOperator throwingMerger() {
    return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };
}
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List 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 map00 = list.stream().collect(Collectors.toMap(item -> item.getVersion(), item -> item.getGroupCode(), (oldVal, currVal) -> currVal));
Map 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());
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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
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