Java Stream 流常用操作大全

---

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());

---

2. map 元素映射

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

---

3. filter 元素过滤

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

---

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(), "admin"))
		.findFirst().orElse(null);

---

5. groupingBy 分组

（1）单个字段分组

Map<String, List<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, List<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()));

---