//
// SortList.java
//

import java.io.*;
import java.util.Arrays;
import java.util.Vector;

/** Sorts a text file alphabetically, removing duplicates. */
public class SortList {

  public static void main(String[] args) throws Exception {
    boolean showDups = args[0].equals("-v");
    String file = args[showDups ? 1 : 0];
    Vector v = new Vector();
    BufferedReader fin = new BufferedReader(new FileReader(file));
    while (true) {
      String line = fin.readLine();
      if (line == null) break;
      v.add(line);
    }
    fin.close();
    String[] lines = new String[v.size()];
    v.copyInto(lines);
    Arrays.sort(lines);
    new File(file).renameTo(new File(file + ".old"));
    PrintWriter fout = new PrintWriter(new FileWriter(file));
    for (int i=0; i<lines.length; i++) {
      if (i == 0 || !lines[i].equals(lines[i-1])) fout.println(lines[i]);
      else if (showDups) System.out.println("Duplicate line: " + lines[i]);
    }
    fout.close();
  }

}

//
// SortList.java
//

import java.io.*;
import java.util.Arrays;
import java.util.Vector;

/** Sorts a text file alphabetically, removing duplicates. */
public class SortList {

  public static void main(String[] args) throws Exception {
    String file = args[0];
    Vector v = new Vector();
    BufferedReader fin = new BufferedReader(new FileReader(file));
    while (true) {
      String line = fin.readLine();
      if (line == null) break;
      v.add(line);
    }
    fin.close();
    String[] lines = new String[v.size()];
    v.copyInto(lines);
    Arrays.sort(lines);
    new File(file).renameTo(new File(file + ".old"));
    PrintWriter fout = new PrintWriter(new FileWriter(file));
    for (int i=0; i<lines.length; i++) {
      if (i == 0 || !lines[i].equals(lines[i-1])) fout.println(lines[i]);
    }
    fout.close();
  }

}

--- 
+++ 
@@ -10,7 +10,8 @@
 public class SortList {
 
   public static void main(String[] args) throws Exception {
-    String file = args[0];
+    boolean showDups = args[0].equals("-v");
+    String file = args[showDups ? 1 : 0];
     Vector v = new Vector();
     BufferedReader fin = new BufferedReader(new FileReader(file));
     while (true) {
@@ -26,6 +27,7 @@
     PrintWriter fout = new PrintWriter(new FileWriter(file));
     for (int i=0; i<lines.length; i++) {
       if (i == 0 || !lines[i].equals(lines[i-1])) fout.println(lines[i]);
+      else if (showDups) System.out.println("Duplicate line: " + lines[i]);
     }
     fout.close();
   }
Add command line option to print a warning for each duplicate.

