Sorting numeric strings

Given the following data:

Col1, Col2, Col3
438.23, 991897664, ccc
22.12, 991897631, bbb
100.99, 881897631, aaa
50.12, 991884803, ddd

The class below will sort the data based on the column specified,
except Col1, which contains float values. If you set the SortCol
variable below to 0, sorting does not work. If you set it to 1 or 2,
sorting does work. How can I sort Col1 which is a column of numeric
strings?

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 SortColumn {

    public static void main(String[] args) throws Exception {

        BufferedReader reader = new BufferedReader(new FileReader("file.csv"));
        //BufferedReader reader = new BufferedReader(new
FileReader("jtp-input2-test.csv"));
        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).toString();
           List<String> l = map.get(key);
           if (l == null) {
              l = new LinkedList<String>();
              map.put(key, l);
              System.out.println(key);
           }
        l.add(line);

        }
        reader.close();

        FileWriter writer = new FileWriter("sorted_numbers.txt");
        writer.write("Col1, Col2, Col3\n");
// writer.write("billnumber, Copay, Discount, NonAllow, unknown\n");
        for (List<String> list : map.values()) {
           for (String val : list) {
               writer.write(val);
               writer.write("\n");
           }
        }
        writer.close();
}

    private static String getField(String line) {
       // Column you want to sort on (Zero based)
       int SortCol = 0;
       return line.split(",")[SortCol];
    }
}