Java8 常用Lambda表达式
一、list转map
1.方式一
public static void listToMap(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wzx","12345645",18));
userList.add(new User("was","1215133",18));
//将list转为map,其中username为key,每个user为value,这种写法在key重复时会报错,需要改成方式二
Map<String, User> userMap = userList.stream().collect(Collectors.toMap(User::getUsername, user -> user));
userMap.keySet().forEach(a->System.out.println("---------"+a));
}
2.方式二
public static void listToMap(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wjg","12345645",18));
userList.add(new User("was","1215133",18));
//将list转为map,(k1,k2)->k1表示当key重复时保留k1,就是保留第一条记录,也就是丢弃后面的数据;如果是想覆盖就写成:(k1,k2)->k2, 就是保留后面的记录
Map<String, User> userMap = userList.stream().collect(Collectors.toMap(User::getUsername, user -> user,(k1,k2)->k1));
userMap.values().forEach(a->System.out.println("---------"+a.toString()));
}
二、list转set
public static void listToSet(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wjg","12345645",18));
userList.add(new User("was","1215133",18));
Set<User> collect = userList.stream().collect(Collectors.toSet());
collect.forEach(a->System.out.println("---------"+a.toString()));
}
三、filter()过滤
public static void filter(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wzx","12345645",18));
userList.add(new User("was","1215133",16));
//只留下年纪大于18的用户
List<User> users = userList.stream().filter(user -> user.getAge() > 18).collect(Collectors.toList());
users.forEach(a->System.out.println("---------"+a.toString()));
}
四、groupingBy分组
public static void groupby(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wzx","12345645",18));
userList.add(new User("was","1215133",18));
//通过年纪分组
Map<Integer, List<User>> userMap = userList.stream().collect(Collectors.groupingBy(user -> user.getAge()));
userMap.forEach((k,v)->System.out.println(k+"---------"+v.size()));
五、sorted+Comparator 排序
public static void sort(){
List<User> userList = new ArrayList<User>();
userList.add(new User("wjg","123",24));
userList.add(new User("wzx","12345645",18));
userList.add(new User("was","1215133",18));
//通过年纪排序,默认是升序
List<User> users = userList.stream().sorted(Comparator.comparing(User::getAge)).collect(Collectors.toList());
users.forEach(a->System.out.println("---------"+a.toString()));
//降序使用:reversed()
users = userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());
users.forEach(a->System.out.println("---------"+a.toString()));
}