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1. 逗号分隔的字符串转 List

（1）转 List<String>

String str = "1,2,3,4,5,6";
List<String> list = Arrays.stream(str.split(",")).map(String::trim).collect(Collectors.toList());

（2）转 List<Long>

String str = "1,2,3,4,5,6";
List<Long> list = Arrays.stream(str.split(",")).map(s -> Long.parseLong(s.trim())).collect(Collectors.toList());

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2. map 元素映射

List<String> usernameList = records.stream().map(SysUser::getUsername()).collect(Collectors.toList());

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3. filter 元素过滤

List<SysUser> sysUserList = records.stream().filter(x -> StrUtil.equals(x.getUsername(), "taibai")).collect(Collectors.toList());

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4. findFirst 查找首个元素

（1）查找 filter 过滤后的首个元素

SysUser sysUser = records.stream().filter(x -> StrUtil.equals(x.getUsername(), "taibai")).findFirst().orElse(null);

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5. groupingBy 分组

（1）单个字段分组

Map<String, SysUser> countMap = records.stream().collect(Collectors.groupingBy(SysUser::getDeptId()));

（2）单个字段分组-统计数量

Map<String, Long> countMap = records.stream().collect(Collectors.groupingBy(SysUser::getDeptId(), Collectors.counting()));

（3）多字段分组

Map<String, SysUser> countMap = records.stream().collect(Collectors.groupingBy(x -> x.getDeptId() + "_" + x.getRoleId()));

（4）多字段分组-统计数量

Map<String, Long> countMap = records.stream().collect(Collectors.groupingBy(x -> x.getProductType() + "_" + x.getCountry(), Collectors.counting()));

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