检查（表达式中的[和{是否正确？

Question

I am trying to write a program for an assignment for my java class. The program is supposed to use Tokenization and the Stack class to check for correct balance of brackets, braces, and parenthesis in expressions.

What am i doing wrong?

My code

   import java.util.*;
   public class Lab10 {
   public static void main(String[] args) throws Exception {
        Stack<String> stack = new Stack<>();        
        Scanner input = new Scanner(System.in);
        System.out.print("Enter string: ");
        String s = input.nextLine();
        input.close();      
        StringTokenizer tokens = new StringTokenizer(s, "[] () {}", true);
        while (tokens.hasMoreTokens()) {
            String token = tokens.nextToken().trim();
            if (token.compareTo("(") == 0 || token.compareTo("{") == 0 ||
                 token.compareTo("[") == 0) {
                stack.push(token);
            }

            else if (token.compareTo(")") == 0 || token.compareTo("}") == 0        ||
                token.compareTo("]") == 0) {
            String popped = stack.pop();

            if (token.compareTo(")") == 0 && popped.compareTo("(") != 0 ||
                token.compareTo("}") == 0 && popped.compareTo("{") != 0 ||
                token.compareTo("]") == 0 && popped.compareTo("[") != 0) {
                System.out.println("Error");
            }
        }

    }

    }
    }

Answer 1

As suggested by @Eran , these could be some good changes to the code :

if(stack.empty()) {
    System.out.println("Error");
    break;
}
String popped = stack.pop();
if (token.compareTo(")") == 0 && popped.compareTo("(") != 0 || token.compareTo("}") == 0 && popped.compareTo("{") != 0 || token.compareTo("]") == 0 && popped.compareTo("[") != 0) {
    System.out.println("Error");
    break;
}