Sorting in Java from external text file [closed]

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This data is saved in data.txt I am trying to write a program that can arrange

18b0885  // this is the registration number, bullet points to show indentation 
   SS844 Parallel Algorithms        //  These are course taken by student
   SS555 Calculus for Distributed Computing
   SS501 Quantum Communication

17b0585
   SS828 Problem Based Programming
   SS660 Genetic Computation
   SS567 Hacking Quantum Network

17b2582
   SS567 Hacking Quantum Network
   SS876 Positronics
   SS880 Quark-based Logic

A big list of data like this, And need to write a program to short this data is ascending order by registration number and course will follow the registration number. so the expected out put is like this.

17b2582
   SS567 Hacking Quantum Network
   SS876 Positronics
   SS880 Quark-based Logic

17b0585
   SS828 Problem Based Programming
   SS660 Genetic Computation
   SS567 Hacking Quantum Network

18b0885  
   SS844 Parallel Algorithms       
   SS555 Calculus for Distributed Computing
   SS501 Quantum Communication

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

public class Sort {
    public static void main(String[] args) throws Exception {
     BufferedReader reader = new BufferedReader(new FileReader("data.txt"));
     Map<String, List<String>> map = new TreeMap<String, List<String>>();
     String line = reader.readLine();//read header
     while ((line = reader.readLine()) != null) {
      String key = getField(line);
      List<String> l = map.get(key);
      if (l == null) {
       l = new LinkedList<String>();
       map.put(key, l);
      }
      l.add(line);

     }
     reader.close();
     FileWriter writer = new FileWriter("sorted_numbers3.txt");
     writer.write("");
     for (List<String> list : map.values()) {
      for (String val : list) {
       writer.write(val);
       writer.write("\n");
      }
     }
     writer.close();
    }

    private static String getField(String line) {
     return line.split(",")[0];// 
    }
}

The above program out puts into another text file like this

    SS501 Quantum Communication
    SS555 Calculus for Distributed Computing
    SS567 Hacking Quantum Network
    SS567 Hacking Quantum Network
    SS660 Genetic Computation
    SS828 Problem Based Programming
    SS844 Parallel Algorithms
    SS876 Positronics
    SS880 Quark-based Logic

    17b2582
    17b0585
    18b0885

It shorts all the coureses by ascending order then registration numbers but not what I want. What should I change?

You should really just include the raw text formatted as is in the data file and also how you want the output. You can use spaces (like code formatting) to make sure the line/space formatting is retained. Most people will ignore the syntax highlighting if you say it is input/output text (or you can tell the syntax highlighter that it is text) — Java Devil
– Java Devil, Commented Nov 5, 2013 at 2:44
I don't understand what sorting you expect to happen. Your sample output doesn't seem to be sorted...? — iluxa
– iluxa, Commented Nov 5, 2013 at 3:14

Gene · Accepted Answer · 2013-11-05 05:17:09Z

Well, as others have said your example output is not in any sorted order I can see. If you actually want alphabetical order of serials, then this will do it.

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class Sorter {

    static List<Data> read() throws FileNotFoundException, IOException {
        BufferedReader reader = new BufferedReader(new FileReader("data.txt"));
        List<Data> list = new ArrayList<>();
        String line;
        Data data = null;
        while ((line = reader.readLine()) != null) {
            if (line.matches("\\s*")) {
                continue; // skip blank lines
            }
            // Assume line that begins with space is a course.
            if (Character.isSpaceChar(line.charAt(0))) {
                //  Add new course to data if it's there.
                if (data == null) {
                    System.err.println("Missing serial for course " + line);
                } else {
                    data.courses.add(line);
                }
            } else {
                // Add completed data to list if there is one.
                if (data != null) {
                    list.add(data);
                }
                // Make new data with this serial.
                data = new Data(line);
            }
        }
        // Add the last data to the list if there is one.
        if (data != null) {
            list.add(data);
        }
        return list;
    }

    public static void main(String[] args) {
        try {
            // Read.
            List<Data> list = read();

            // Sort based on serials.
            Collections.sort(list, new Comparator<Data>() {
                @Override
                public int compare(Data a, Data b) {
                    return a.serial.compareTo(b.serial);
                }
            });

            // Print.
            for (Data data : list) {
                data.print();
            }
        } catch (Exception ex) {
            System.err.println("Read failed.");
        }
    }
}

// Local class to hold data: a serial and related courses.
class Data {

    String serial;
    List<String> courses;

    Data(String serial) {
        this.serial = serial;
        courses = new ArrayList<>();
    }

    void print() {
        System.out.println(serial);
        for (String course : courses) {
            System.out.println(course);
        }
    }
}

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