Check this article our for reading from a file.
Then, you can use Java's Scanner class to read individual items and put them into a list.
For example:
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.ArrayList;
public class ReadFromFile {
    public static void main(String[] args) {
        File file = new File("file-name.txt");
        try {
            Scanner scanner = new Scanner(file);
            List<Integer> myIntList = new ArrayList<>(); 
            while (scanner.hasNext()) {
                int i = scanner.nextInt();
                myIntList.add(i);
            }
            scanner.close();
            // now you have an ArrayList with the numbers in it that you can use
        } 
        catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}