Lambda expression to deal with collection
With the new Lambda expression in Java 8, we can use simple and efficient code to process List.
1. Traverse
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.stream().forEach(user ->{
System.out.println(user.getName());
});
}
operation result:
2. Convert list to Map
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
userList.add(user1);
userList.add(user2);
userList.add(user3);
//ID为key，转为Map
Map<Long,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1, k2)->k1));
System.out.println(userMap);
}
operation result:
3. Group List: Object elements in List are grouped by a certain attribute
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//根据name来将userList分组
Map<String, List<User>> groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println(groupBy);
}
operation result:
4. Filtering: Filter out eligible elements from the collection
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//取出名字为张三的用户
List<User> filterList = userList.stream().filter(user -> user.getName().equals("张三")).collect(Collectors.toList());
filterList.stream().forEach(user ->{
System.out.println(user.getName());
});
}
operation result:
5. Sum: sum the data in the collection according to a certain attribute
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user对象集合
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
//取出名字为张三的用户
int totalAge = userList.stream().mapToInt(User::getAge).sum();
System.out.println("和：" + totalAge);
}
operation result:
6. Convert from List to Map, with one-to-one correspondence between key and value
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();
User user1 = new User(1L, "张三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "张三", 22);
User user5 = new User(5L, "李四", 20);
User user6 = new User(6L, "王五", 28);
userList.add(user1);
userList.add(user2);
userList.add(user3);
userList.add(user4);
userList.add(user5);
userList.add(user6);
Map<Long/*Id*/,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, user -> user));
System.out.println("toMap：" + JSONArray.toJSONString(userMap));
}
operation result: