如何在Java中为.txt文件的每个唯一列值创建ArrayList？

Question

I'm writing a program that reads a .txt file line by line, and I want to create for every 'unique' value of one column, an ArrayList .

What the .txt file looks like:

NAME          AGE          COUNTRY          PHONE NUMBER
Peter         28           USA              00112233
John          25           England          11223344
Justin        22           Australia        22334455
Daisey        24           Canada           33445566
Harry         27           England          44556677
Laura         25           England          55667788
Gary          28           USA              66778899

For example, I want to create for each nationality an ArrayList, with the corresponding names as elements.

So, this:

ArrayList USA:       Peter, Gary
ArrayList England:   John, Harry, Laura
ArrayList Australia: Justin
ArrayList Canada:    Daisey

I don't want to create the ArrayLists myself, before running the program. Because the real column I'm talking about has lots of different values, so lots of ArrayList should be made. Does anyone know how I can let the program create an Arraylist for every unique nationality, and add the corresponding names to this ArrayList ?

Answer 1

Well,it's easier to work with domain objects like:

class User {
    String name;
    String country;
    int age;
    String phone;
    // getters and setters omitted
}

Somewhere in your code there will be method like:

List<User> readfromFile(String fileName) {
    // create User for each read line
}

And then use Stream Api to group by needed column like:

Map<String, List<User>> usersByCountry = users.stream().collect(Collectors.groupingBy(User::getCountry));

If you want just user names grouped by country:

Map<String, List<String>> userNamesByCountry = users.stream().collect(Collectors.groupingBy(User::getCountry, LinkedHashMap::new, Collectors.mapping(User::getName, Collectors.toList())));