java8新特性：利用Lambda處理List集合

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));

　　　　　Map<Long/*Id*/,User> userMap = userList.stream().collect(Collectors.toMap(User::getId, user -> user,(k1,k2)->k1))； //去重
　　　　　System.out.println("toMap：" + JSONArray.toJSONString(userMap));

}

運行結果：

 7、在使用forEach過程中 使用return可以達到continue的效果

 public static void main(String[] args) {
        List<String> testList = Lists.newArrayList("1","2","3","4","5");
        testList.forEach(test->{
            if (test.equals("2")){
                return;
            }
            System.out.println(test);
        });
    }

運行結果：