How to convert a 2d array into a 2d ArrayList in java?

Question

I have a Java program that reads elements from a file, stores them in a 2d array and then it manipulates them according by commiting several operations.

I have already implemented the program by using a 2d array and now I want to find a way to turn this array into a 2d ArrayList, so I can manipulate these elements individually, like i did with the 2d array.

The program reads from a file that looks like this:

Jason,56
Martha,89
James,23
...

Here is my code attempting to convert my 2d array into a 2d ArrayList:

Keep in mind that I want all the names to be stored in the 1st column of the array/ArrayList and the age in the second column:

public class Testr {

    public static void main(String[] args) throws FileNotFoundException, IOException {
            Scanner sc = new Scanner(new BufferedReader(new FileReader("C:\\Users\\test.csv")));

            int num_rows = countLines("C:\\Users\\test.csv");
            System.out.println("Num of rows : " + num_rows);

            int num_cols = countColumns("C:\\Users\\test.csv");
            System.out.println("Num of cols : " + num_cols);

            String[][] Entries_arr = new String[num_rows][num_cols];
while(sc.hasNextLine())
            {
                for(int i = 0; i < Entries_arr.length; i++)
                {
                    String[] line;
                    line = sc.nextLine().trim().split(";");
                    for(int j = 0; j < line.length; j++)
                    {
                        Entries_arr[i][j] = line[j];

                    }
                }
            }
            List<List<String>> Entries = new ArrayList<List<String>>();
            for(int i = 0; i < Entries_arr.length; i++)
            {
                List<String> recs = new ArrayList<String>();
                for(int j = 0; j < Entries_arr[i].length; j++)
                {
                    recs.add(String.valueOf(Entries_arr[i][j]));
                }
                Entries.add(recs);
            }
            System.out.println(Entries);

    }
    //------------------------------------------------------------------------------------------------------------------------   
    public static int countLines(String filename) throws IOException {
    InputStream is = new BufferedInputStream(new FileInputStream(filename));
    try {
        byte[] c = new byte[1024];
        int count = 0;
        int readChars = 0;
        boolean empty = true;
        while ((readChars = is.read(c)) != -1) {
            empty = false;
            for (int i = 0; i < readChars; ++i) {
                if (c[i] == '\n') {
                    ++count;
                }
            }
        }
        return (count == 0 && !empty) ? 1 : count;
    } finally {
        is.close();
    }
}
    //------------------------------------------------------------------------------------------------------
        public static int countColumns(String filename) {
        File file = new File(filename);
        Scanner scanner;
        try {
            scanner = new Scanner(file);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
            return -1;
        }

        int number = 0;
        if (scanner.hasNextLine()) {
            number = scanner.nextLine().split(";").length;
        }
        scanner.close();
        return number;
        }
}

Any help is appreciated.

So what is the problem? I don't see you writing into Entries_arr other than reading from it. — Alanpatchi
– Alanpatchi, Commented Jun 14, 2020 at 23:23
You shouldn't be using a 2D array or a 2D List since with either attempt at a solution, you'd be using parallel arrays or parallel lists, both an invitation to disaster. Better to create a class, say called User that holds both the name String and the age int in private fields, and then create a single List<user> that is instantiated as an ArrayList<User> and then fill this single-dimensional list with User objects. — DontKnowMuchBut Getting Better
– DontKnowMuchBut Getting Better, Commented Jun 15, 2020 at 0:02
You also seem to be grossly over-complicating things with your code. You should simply create your list: List<User> users = new ArrayList<>(); and then iterate through the file creating User objects from each line and filling your List, and a User created from each line. No need to create arrays, no need to convert, just do the whole thing in one simple fell swoop. — DontKnowMuchBut Getting Better
– DontKnowMuchBut Getting Better, Commented Jun 15, 2020 at 0:13
@DontKnowMuchButGettingBetter Thank you for answering, could you please explain a little bit more about that? All I wanted to do is to manipulate the 2d arrayList as I did with the 2d array. Because this way I am getting each entry like this: [Jason,56],... — user47
– user47, Commented Jun 15, 2020 at 7:07

Ivo Mori · Accepted Answer · 2020-09-05 05:32:25Z

Going back to what was pointed out in the comments

Don't use a 2D array (or a 2D list) because you'd be using something similar to parallel arrays or parallel lists as your data structure. See Jon Skeet's Anti-Pattern: Parallel Collections blog for details.
Also by trying to solve your problem with a 2D array/list your code gets much more complicated than actually necessary (and for no good reason).

So how could an approach look like as pointed out in the comments?

Following code

reads a test file line by line,
processes each line into a User instance,
and collects them all into an ArrayList.

import java.net.URL;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FileToList {

    public static void main(String[] args) {
        try (Stream<String> lines = Files.lines(findCsvFile())) {
            List<User> users = lines
                    .map(FileToList::process)
                    .flatMap(Optional::stream)
                    .collect(Collectors.toCollection(ArrayList::new));

            System.out.println(users);
        } catch (Exception e) {
            System.out.println("Error when accessing the file\n");
            e.printStackTrace();
        }
    }

    static Path findCsvFile() throws Exception {
        URL resource = FileToList.class.getResource(FileToList.class.getSimpleName() + ".class");
        return Paths.get(resource.toURI()).resolveSibling("test.csv");
    }

    static Optional<User> process(String line) {
        Optional<User> user = Optional.empty();
        String[] values = line.split(",");
        try {
            String name = values[0];
            int age = Integer.parseInt(values[1]);
            User actualUser = new User(name, age);
            user = Optional.of(actualUser);
        } catch (Exception e) {
            System.out.printf("Cannot process line: \"%s\" | Error: %s\n", line, e);
        }
        return user;
    }

    static class User {
        private final String userFirstName;
        private final int userAge;

        User(String name, int age) {
            this.userAge = age;
            this.userFirstName = name;
        }

        public String getUserFirstName() {
            return userFirstName;
        }

        public int getUserAge() {
            return userAge;
        }

        @Override
        public String toString() {
            return String.format("%s(%d)", userFirstName, userAge);
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            User user = (User) o;
            return userAge == user.userAge &&
                    Objects.equals(userFirstName, user.userFirstName);
        }

        @Override
        public int hashCode() {
            return Objects.hash(userFirstName, userAge);
        }
    }
}

Running the above code against your example CVS file then outputs

[Jason(56), Martha(89), James(23)]

Note that Optional is used in the process method only to cover for the situation when a line from the CSV file cannot be processed into a new User object. The .flatMap(Optional::stream) then removes any potential optional (non-existing) user instances from the stream before collecting the actual users into an ArrayList.

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