另外关于基础语法和遇到的错误,可以参看我的其他文章
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基础语法参考
Java8新特性:HashMap优化、lambda、Stream等新特性详解 -
遇到问题及解决参考
这个文章访问量是真多,应该这个错比较常见,大家通过搜索引擎搜到了它
Java8-Stream: no instance(s) of type variable(s) R exist so that void conforms to R
一、场景概述
1.1、Stream的作用
1.2、使用场景
主要有如下几种场景:
二、Tips
2.1、group by
userList.stream()
.collect(Collectors.groupingBy(User::getSex));
2.2、order by
userList.stream()
.sorted(Comparator.comparing(User::getAge).reversed())
.limit(3)
.collect(Collectors.toList());
2.3、where
2.3.1、最值筛选
// 最小
Optional<User> min = userList.stream()
.min(Comparator.comparing(User::getAge));
// 最大
Optional<User> max = userList.stream()
.max(Comparator.comparing(User::getAge));
// 获得对象
User user = min.get();
2.3.2、条件筛选
userList.stream()
.filter(e -> e.getAge() < 30)
.collect(Collectors.toList());
userList.stream()
.filter(u -> u.getAge() > 20 && u.getSex() == 1)
.collect(Collectors.toList());
userList.stream().
filter(u -> u.getName().equals("小明"))
.findFirst().orElse(ll);
2.4、distinct
userList.stream()
.map(User::getName)
.distinct()
.collect(Collectors.toList());
memberListAll.stream()
.collect(Collectors.collectingAndThen(
Collectors.toCollection(
() -> new TreeSet<>(Comparator.comparing(WorkWxUserInfoVO :: getUserid))), ArrayList::new)
);
2.5、apply
userList.forEach(e -> {
e.setName("hello");
});
userList.stream()
.map(user -> {user.setName(user.getName().replaceAll("\u0000", "")); return user;})
.collect(Collectors.toList());
List<User> userList = userIds.stream()
.map(id -> {
User user = userService.getUserById(id);
return user;
})
.collect(Collectors.toList());
2.6、提取属性
userList.stream()
.map(User::getName)
.distinct()
.collect(Collectors.toList());
userList.stream()
.collect(Collectors.toMap(User::getMenuId, User::getMenuName)));
userList
//去重
.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(
() -> new TreeSet<>(Comparator.comparing(User :: getMenuId))), ArrayList::new))
//转map
.stream().collect(Collectors.toMap(User::getMenuId, User::getMenuName)));
以上仅记录最常见的几个场景,更多场景请参看:Java8新特性:HashMap优化、lambda、Stream等新特性详解