Redundancy Checking In Java

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

class Cell
{
   Object object;
   Cell next;

   Cell(Object obj, Cell lst) {  object = obj; next = lst;  }
   Object getObject() { return object; }
   void setNext(Cell lst) { next = lst; }
   Cell getNext() { return next; }
}

class Stacker
{
   Cell head;

   Stacker() { head = null; }

   void push(Object obj)
   {
      if (obj == null) return;
      Cell h = new Cell(obj, head);
      head = h;
   }

   Object pop()
   {
      if (head == null) return null;
      Object obj = head.getObject();
      head = head.getNext();
      return obj;
   }

   void display()
   {
      if (head == null) { System.out.println("(empty)");  return; }
      for (Cell t=head ; t != null ; t=t.getNext())
         ((City)(t.getObject())).city();
      System.out.println();
   }

   boolean empty() {  return head == null;  }
}

class City
{
   int cty;
   int prnt;

   City (int c, int p) { cty = c; prnt = p; }
   int parent()  { return prnt; }
   int city()    { return cty; }
}

public class Cycles
{
   /***-------------------------------------------------***/
   /*** Redundancy:                                     ***/
   /***   Input: A network (n by n array of 0-1 values) ***/
   /***          A number of cities (n above)           ***/
   /***   Output: 1 if network is redundant             ***/
   /***           0 otherwise                           ***/
   /*** Checks network links for redundant connections  ***/
   /***-------------------------------------------------***/
   static int redundancy(boolean links[][], int cities)
   {
      Stacker s = new Stacker();
      boolean visited[] = new boolean[cities+1];
      int i;
      City current;

      s.push(new City(0, -1));
      while (!s.empty())
      {
         current = (City)s.pop();
         if (visited[current.city()]) continue;
         visited[current.city()] = true;
         for (i=0 ; i < = cities ; i++)
         {
            if (!links[current.city()][i] || current.city() == i) continue;
            if (!visited[i])
               s.push(new City(i, current.city()));
            else
            if (i != current.parent()) return 1;
         }
      }
      return 0;
   }

   static void display(boolean link[][], int n)
   {
      int i, j;

      for (i=0 ; i < n ; i++)
      {
         for (j=0 ; j < n ; j++)
            System.out.print((link[i][j] ? 1 : 0) + " ");
         System.out.println();
      }
   }

   static int ReadData(boolean links[][], String filename)
   {
      int city1, city2, cities=0;
      try
      {
         DataInputStream is = new DataInputStream(new FileInputStream(filename));
         while (true)
         {
            String s = is.readLine();
            try
            {
               StringTokenizer t = new StringTokenizer(s," ");
               city1 = Integer.parseInt(t.nextToken());
               city2 = Integer.parseInt(t.nextToken());
               links[city1][city2] = true;
               if (city1 > cities) cities = city1;
               if (city2 > cities) cities = city2;
            }
            catch (NullPointerException e){ break; }
         }
      }
      catch (IOException e) {}
      return cities;
   }

   public static void main(String argv[])
   {
      boolean links[][] = new boolean[200][200];
      int city1, city2, cost, cities=0;

      cities = ReadData(links, argv[0]);

      display(links, cities+1);
      System.out.println("<<"+redundancy(links,cities)+">>");
   }
}