I don't know how to resolve Java NullPointerException.

class GStack<T> {

    int top;
    Object[] stck;

    public GStack() {
        top = 0;
        stck = new Object[100];
    }

    public void push(T item) {
        if (top == 100)
            return;
        stck[top] = item;
        top++;
    }

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

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

}

public class postfixCalc {

    public static int priority(char op) {
        if(op == '+' || op == '-')
            return 1;
        else if(op == '*' || op == '/')
            return 2;
        else
            return 3;
    }

    public static void main(String[] args) {
        GStack<Character> opStk = new GStack<>(); // Stack to store operators
        ArrayListexp = newArrayList(); //ArrayList to store the intermediate notation formula
        exp.add(2.0); exp.add('+'); exp.add(3.0); exp.add('*'); exp.add(2.0); exp.add('('); exp.add('('); exp.add(3.0); exp.add('-');
        exp.add(2.0); exp.add(')'); exp.add('*'); exp.add(4.0); exp.add(')'); exp.add('+'); exp.add(5.0); exp.add('=');
        ArrayList expr = newArrayList(); //ArrayList to store backward notation formulas


        /********Middle patrol->Back patrol conversion**********/
        for(int i=0; i<exp.size(); i++) {
            If (exp.get(i) == "(") //Open is in parentheses '('), put it in the stack.
                opStk.push((Character) exp.get(i));

            else if(exp.get(i) == "+" || exp.get(i) == "-" || exp.get(i) == "*" || exp.get(i) == "/") {
                if(priority((char) exp.get(i)) >= priority(opStk.peak()) { // when the current operator has a higher or equal priority than the one in the stack
                    expr.add(opStk.pop()); // Subtract the contained operator
                    opStk.push((char)exp.get(i)); //put the operator of the current position
                }
                else
                    opStk.push((char)exp.get(i)); // Otherwise, just insert the operator.
            }

            else if(exp.get(i) == ")") { // If you meet a closing parenthesis,
                while(opStk.top != '(') 
                    expr.add(opStk.pop(); // output until parentheses meet           
            }

            else
                exp.add(exp.get(i)); // output immediately (if operand)
        }

        while(((opStk.top!=0)) //Take out all that is left in the stack at the end of the tour.
            expr.add(opStk.pop());


        for(int j=0; j<expr.size(); j++) {
            System.out.print(expr.get(j));
        }


        /*****************************************

        double result;
        GStack<Double> vstk = new GStack<>(); //stack to store operands

        for (int i=0; i<expr.size(); i++) {
            if(expr.get(i) instance of Double) { //push to operand surface stack
                vStk.push((Double) expr.get(i));
            }

            else if(expr.get(i) == "+" || expr.get(i) == "-" || expr.get(i) == "*" || expr.get(i) == "/") { // operator to compute two times and push each other to the stack
                double d1 = vStk.pop();
                double d2 = vStk.pop();
                if(expr.get(i) == "+")
                    vStk.push((d2+d1));
                if(expr.get(i) == "-")
                    vStk.push((d2-d1));
                if(expr.get(i) == "*")
                    vStk.push((d2*d1));
                if(expr.get(i) == "/")
                    vStk.push((d2/d1));
            }

            else
                System.out.print(" ");
        }

        result = vstk.pop(); // The last value remaining in the stack is the result, where NullPointerException

        System.out.print(result);

    }

}