/**
 * <p> <code>class ParenChecker</code> provides an example of using 
 * a stack to determine if an EXPR has balanced parenthesis.  An 
 * EXPR is supplied as a command-line argument.  The EXPR should 
 * be specified enclosed in double quotes. </p>
 *
 * <p> If the EXPR contains an operand with the name P, then the content 
 * of the stack is printed.  If the EXPR contains an operand with the 
 * name T, then the top element of the stack is printed. </p>
 *
 * @author G.D.Thurman
 * @version 99.09.26
 */

public class ParenChecker {
   public static void main(String[] argv) {
      if (argv.length != 1) {
         System.err.println("Usage:  java ParenChecker \"some EXPR\"\n");
         System.exit(1);
      }

      Stack stack = new Stack();   

      for (int n = 0, len = argv[0].length(); n < len; n++) {
         char c = argv[0].charAt(n);
         switch (c) {
            case '(':
              stack.push(new Character(c));
              break;
            case ')':
              if (stack.size() == 0) printResult(true /*is bad*/);
              Character ch = (Character) stack.pop();
              break;
           case 'P':
              System.out.println(stack);
              break;
           case 'T':
              System.out.println("top() is: " + stack.top());
              break;
         }
      }
      printResult(stack.size() != 0);
   }
   /**
    * Print status message and exit the application.
    */
   private static void printResult(boolean isBad) {
      System.out.println("The parentheses are" + (isBad ? " NOT " : " ") +
                         "correct.");
      System.exit(isBad ? 1 : 0);
   }
}