You had some excellent suggestions here. However, I've got some as well ...

**

Use toCharArray () to make iterating over the String cleaner looking.

**

I would keep the ArrayDeque in the method's scope instead that of the class since that's the only place where it's used.

**

Instead of using size () == 0, use isEmpty () it's more readable.

**

Finally, once you fail, there is no longer a reason to continue the routine, just exit with false immediately.

**

Finally, you could also ignore characters other than the 6 we're looking for instead of failing. I've kept the failure logic below.

private static boolean isBalanced (String s) {

   final ArrayDeque<Character> stack = new ArrayDeque<> ();
   final char[] chars = s.toCharArray ();

   for (char c : chars) {

       // Matching character, . means not initialized
       char mc = '.';

       switch (c) {

       case '(':
       case '{':
       case '[':
         stack.addFirst (c);
         break;

       case ')':
         mc = '(';
       case '}':
         if (mc == '.') mc = '{';
       case ']':
         if (mc == '.') mc = '[';

         if (stack.isEmpty () || 
             stack.removeFirst () != mc) {
             return false;
         }
         break;

       default:
         // Any other character is bad input data
         return false;
       }
   }

   return stack.isEmpty ();
}