How do i find NZEC error

Below is my code getting NZEC can anyone help me???

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;

public class InfixToPostfix {

public static void main(String[] args) throws Exception {

	int noOfTestCases;
	int top = -1;
	char[] items = new char[500];
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
	PrintWriter pw = new PrintWriter(System.out);
	noOfTestCases = Integer.parseInt(in.readLine());

	while (noOfTestCases > 0) {

		String str;
		str = in.readLine();

		int j = 0;
		char[] postfix = new char[500];

		for (int i = 0; i < str.length(); i++) {
			if (Character.isLetter(str.charAt(i))) {
				postfix[j++] = str.charAt(i);
			}

			else if (str.charAt(i) == '+' || str.charAt(i) == '-'
					|| str.charAt(i) == '*' || str.charAt(i) == '/'
					|| str.charAt(i) == '^') {
				items[++top] = str.charAt(i);
			}

			else if (str.charAt(i) == ')') {
				postfix[j++] = items[top--];
			} else if (str.charAt(i) == '(') {

			}

		}

		pw.println(postfix);
		pw.flush();
		pw.close();
	}

	noOfTestCases--;
}

}

nzec is the run time error may be your code gives correct output for some sample test cases but throw an exception for some other test cases try to find those cases in which your code throw exception in your case it may be ArrayIndexOutofBound Exception…

//