import java.util.ArrayList;
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;

/* WordsInReverse.java
** Java application having as its purpose to illustrate the use of the
** ArrayList class.
** 
** It reads lines of text from the input source (either a file or the
** keyboard, depending upon whether or not, respectively, a file name
** is provided via a command line argument) and prints the tokens in 
** each line in reverse order.  For example if a line of input were
** 
**    The cat in the hat ate a rat.
**
** what would be printed is
**
**    rat. a ate hat the in cat The
**
** End of input is indicated by a line having no characters.
*/

public class WordsInReverse {

   public static void main(String[] args) throws FileNotFoundException {
      Scanner input;
      boolean echo;
      if (args.length != 0) {   // command line arg is input file name
         input = new Scanner(new File(args[0]));
         echo = true;
      }
      else { 
         input = new Scanner(System.in);
         echo = false;
      }

      System.out.print("> ");
      String inputLine = input.nextLine();
      while (inputLine.length() != 0) {

         if (echo) { System.out.println(inputLine); }

         // Create a Scanner to parse the line of tokens just read in
         Scanner lineScanner = new Scanner(inputLine);

         // Get an ArrayList containing the tokens on the just-read input line.
         ArrayList<String> tokens = tokenList(lineScanner);

         // Reverse the order of the tokens in the list.
         reverseList(tokens);

         // Print the list's elements.
         for (int k = 0; k != tokens.size(); k++) {
            System.out.printf("%s ", tokens.get(k));
         }

         // Print prompt for next line of input, and then read it.
         System.out.print("\n\n> ");
         inputLine = input.nextLine();
      }
      System.out.println("Goodbye.");
   }

   /* Returns a new ArrayList containing all the tokens readable by
   ** the given Scanner, in the same order that the Scanner reads them.
   */
   private static ArrayList<String> tokenList(Scanner scanner)
   {
      // Create an ArrayList to store the tokens.  Note that specifying a
      // length (by passing an int parameter to the constructor) is optional
      // and is a good idea if you have a good idea as to the size to which 
      // the list will grow.  Here we have no idea.
      ArrayList<String> result = new ArrayList();

      // Read each token and store it in the ArrayList.  
      // The add() method adds the specified element onto the end of the list. 
      while (scanner.hasNext()) {
         String token = scanner.next();
         result.add(token);
      }
      return result;
   }

   /* Reverses the order of the elements in the given ArrayList.
   ** Note that there is no need to specify the type of elements in the
   ** list because it is irrelevant here.
   */
   private static void reverseList(ArrayList list) {
      int left = 0, right = list.size()-1;
      while (left < right) {
         // Swap the elements at positions left and right of the list.
         Object temp = list.get(left);
         list.set(left, list.get(right));
         list.set(right, temp);

         // Move 'left' and 'right' one place towards the middle of the list.
         left = left + 1;
         right = right - 1;
      }
   }

}