Sort file contents

import java.io.*;
import java.util.*;

public class Sort {

    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(new FileReader("fileToRead"));
        Map<String, String> map=new TreeMap<String, String>();
        String line="";
        while((line=reader.readLine())!=null){
         map.put(getField(line),line);
        }
        reader.close();
        FileWriter writer = new FileWriter("fileToWrite");
        for(String val : map.values()){
         writer.write(val); 
         writer.write('\n');
        }
        writer.close();
    }

    private static String getField(String line) {
     return line.split(" ")[0];//extract value you want to sort on
    }
}