ArrayList stores only the last element

Question

What I tried to achieve is to separate the text input into groups when there's an empty line. The way I wanted to achieve this is by adding the lines to ArrayList< String > temp until there's an empty line. After that I added temp to ArrayList<ArrayList< String >> group. But It seems like It only adds the last temp values to group. I can't see why. Here's the code:

public static void main(String... args) throws IOException {
    List<String> data = new ArrayList<>();
    File f = new File("input6.txt");

    try (Scanner sc = new Scanner(f)) {
        while (sc.hasNext()) {
            data.add(sc.nextLine());
        }
    } catch(FileNotFoundException e) {
        e.printStackTrace();
    }
    
    List<List<String>> group = new ArrayList<>();
    List<String> temp = new ArrayList<>();
    
    for (String line : data) {
        temp.add(line);
        if (line.trim().isEmpty()) {

            group.add(temp);
            temp.clear();
        }
    }
}

Answer 1

The problem is that you are always clearing the list ( ie, temp.clear(); ) that you have added to the list group . Instead of reusing the same reference, just reassign the temp to a new list. Do the following instead:

    ArrayList<ArrayList<String>> group = new ArrayList<ArrayList<String>>();
    ArrayList<String> temp  = new ArrayList<String>();
    for(String line : data) {
        temp.add(line);
        if(line.trim().isEmpty()) {
            group.add(temp);
            temp  = new ArrayList<String>();
        }
    } 

Answer 2

Your problem is you clear the temp list, because when you add it the result, it's been added by a reference . You have to add a copy to the result.

public static void main(String... args) throws FileNotFoundException {
    File file = new File("input6.txt");
    List<List<String>> groups = readFile(file);
}

public static List<List<String>> readFile(File file) throws FileNotFoundException {
    List<List<String>> groups = new ArrayList<>();
    List<String> group = new ArrayList<>();

    try (Scanner scan = new Scanner(file)) {
        while (scan.hasNext()) {
            String line = scan.nextLine();

            if (line.trim().isEmpty()) {
                if (group.isEmpty())
                    continue;

                groups.add(group);
                group = new ArrayList<>();
            } else
                group.add(line);
        }

        if (!group.isEmpty())
            groups.add(group);
    }

    return groups;
}

Answer 3

When you are adding the temp ArrayList to group it is not creating a new list but using the reference of the temp list. Hence when you clear the temp list the group values also get cleared so you will have to store a copy of temp into the group arraylist as below.

        public static void main(String... args) throws IOException {
            List<String> data = new ArrayList<>();

            try {
                File f = new File("input6.txt");
                Scanner sc = new Scanner(f);

                while (sc.hasNext()) {
                    data.add(sc.nextLine());
                }
            } catch(FileNotFoundException e) {
                e.printStackTrace();
            }

            List<List<String>> group = new ArrayList<>();
            List<String> temp = new ArrayList<>();

            for (String line : data) {
                temp.add(line);
                if (line.trim().isEmpty()) {

                    group.add(new ArrayList<>(temp));
                    temp.clear();
                }
            }
        }