java-集合

单列：collection 重复：list 不可重复：set
双列：map 配置：properties
转化：Collection values = map.values();

userList.stream().collect(Collectors.toMap(User::getId, User::getName));

Map> map1 = prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
Set>> entries1 = map1.entrySet();
for (Map.Entry> entry : entries1) {
    System.out.println(entry);
}