Creating a postfix calculator with java stack generic I don't know where the error is coming from.

package myfirstjava;

import java.util.ArrayList;
import java.util.Scanner;
import java.util.StringTokenizer;

class GStack<T> {
    static int top;
    Object[] stck;

    public GStack() {
        top = 0;
        stck = new Object[200];
    }

    public T peek() {
        if (top == 0)
            return null;
        else
            return (T) stck[top];
    }

    public void push(T item) {
        if (top == 200)
            return;
        stck[top] = item;
        top++;
    }

    public T pop() {
        if (top == 0)
            return null;
        top--;
        return (T) stck[top];
    }
}

public class Postfix {
    private static int PIS(String temp) {

        if (("^").equals(temp))
            return 4;
        if (("*").equals(temp) || ("/").equals(temp))
            return 3;
        if (("+").equals(temp) || ("-").equals(temp))
            return 2;
        if ((")").equals(temp))
            return 1;
        if (("(").equals(temp))
            return 0;
        return 0;
    }

    private static int PIE(String temp) {
        if (("(").equals(temp))
            return 5;
        if (("^").equals(temp))
            return 4;
        if (("*").equals(temp) || ("/").equals(temp))
            return 3;
        if (("+").equals(temp) || ("-").equals(temp))
            return 2;
        if ((")").equals(temp))
            return 1;
        if (("=").equals(temp) || ("    ").equals(temp))
            return -1;
        return 0;
    }

    static String IntoPost(StringTokenizer st) {
        GStack<String> stringStack = new GStack<String>();
        int n = st.countTokens();
        ArrayList<String> a = new ArrayList<String>(n);
        for (int i = 0; i < n; i++) {
            String token = st.nextToken();
            try {
                Double.parseDouble(token);
                a.add(token);
                System.out.print(token + " ");

            } } catch (NumberFormatException e) {
            }

            switch (token) {
            case ")":
                while (!("(").equals(stringStack)) {
                    a.add(stringStack.peek());

                    System.out.print(stringStack.pop() + " ");
                }
                stringStack.pop();
                break;

            case "+":
            case "*":
            case "-":
            case "/":
            case "(":
                if (PIS(stringStack.peek()) >= (PIE(token))) {
                    while (stringStack.top != 0) {
                        a.add(stringStack.peek());

                        System.out.print(stringStack.pop() + " ");
                    }
                }
                stringStack.push(token);
                break;

            case "=":
                while (stringStack.top != 0) {
                    a.add(stringStack.peek());
                    System.out.print(stringStack.pop() + " ");
                }
            }
        }

        String Postfix = String.join("    ", a);
        return Postfix;
    }

    static double Evaluation(String s) {
        GStack<Double> DoubleStack = new GStack<Double>();
        StringTokenizer st = new StringTokenizer(s, " ");
        int n = st.countTokens();
        double temp = 0;
        double op1, op2;
        for (int i = 0; i < n; i++) {
            String token = st.nextToken();
            if ((token.equals("+") == false) && (token.equals("-") == false) && (token.equals("*") == false)
                    && (token.equals("/") == false)) {
                try {
                    temp = Double.parseDouble(token);
                    DoubleStack.push(temp);
                } } catch (NumberFormatException e) {

                }
            } } else {
                op2 = DoubleStack.pop();
                op1 = DoubleStack.pop();

                switch (token) {
                case "+":
                    DoubleStack.push(op1 + op2);
                    break;
                case "-":
                    DoubleStack.push(op1 - op2);
                    break;
                case "*":
                    DoubleStack.push(op1 * op2);
                    break;
                case "/":
                    DoubleStack.push(op1 / op2);
                    break;
                }
            }
        }
        return temp;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print ("Formula: ");
        String Postfix = in.nextLine();
        StringTokenizer st = new StringTokenizer(Postfix, "+ *(    /-)^=", true);
        String postFix = IntoPost(st);
        double result = Evaluation(postFix);
        System.out.print ("Answer: " + result");
        in.close();
    }
}