Java 8新增的Lambda表達式，我們可以用簡潔高效的代碼來處理List。
1、遍歷
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();
User user1 = new User(1L, "張三", 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());
});
}
運行結果：
2、list轉為Map
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user對象集合
User user1 = new User(1L, "張三", 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為key，轉為Map
Map<Long,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, a -> a,(k1, k2)->k1));
System.out.println(userMap);
}
運行結果：
3、將List分組：List里面的對象元素，以某個屬性來分組
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user對象集合
User user1 = new User(1L, "張三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "張三", 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);
//根據name來將userList分組
Map<String, List<User>> groupBy = userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println(groupBy);
}
運行結果：
4、過濾：從集合中過濾出來符合條件的元素
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user對象集合
User user1 = new User(1L, "張三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "張三", 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);
//取出名字為張三的用戶
List<User> filterList = userList.stream().filter(user -> user.getName().equals("張三")).collect(Collectors.toList());
filterList.stream().forEach(user ->{
System.out.println(user.getName());
});
}
運行結果：
5、求和：將集合中的數據按照某個屬性求和
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();//存放user對象集合
User user1 = new User(1L, "張三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "張三", 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);
//取出名字為張三的用戶
int totalAge = userList.stream().mapToInt(User::getAge).sum();
System.out.println("和：" + totalAge);
}
運行結果：
6、從List轉為Map，key與value 一 一對應
public static void main(String[] args) {
List<User> userList = Lists.newArrayList();
User user1 = new User(1L, "張三", 24);
User user2 = new User(2L, "李四", 27);
User user3 = new User(3L, "王五", 21);
User user4 = new User(4L, "張三", 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));
}
運行結果：
