Problem 19 What wrong with my code? (with code)(java)(javalangNullPointerException)

import java.util.Scanner;

public class Main{
public static void main (String[] args){

    Scanner keyboard = new Scanner(System.in);
    // numb of strings
    int numOfTest = Integer.parseInt(keyboard.nextLine());

    // create to spec arr, for "open char" and "closer char"
    char specArrOp[] = { '<', '(', '[', '{' };
    char specArrCl[] = { '>', '}', ']', '}' };

    // condition of check.
    boolean cond;


    // for each string
    for(int t = 0; t < numOfTest; t++) {
        int stopper = 1;
    // read string and create stack
        String str = keyboard.nextLine();
        Stack stack = new Stack(str.length());
    //transform string into char[]
        char strArr[] = str.toCharArray();
    //for each char
        stackloop:
        for (int i = 0; i < str.length() - 1; i++) {
            // check: Is char special simbol
            for (int j = 0; j < specArrOp.length - 1; j++) {
                //if it is openSymb push it into stack
                if (strArr[i] == specArrOp[j]) {
                    stack.push(strArr[i]);
                    break;
                }
                //if it is closeSymb check last openSymb in stack
                if (strArr[i] == specArrCl[j]) {
                    //is it his openSymb?
                    cond = stack.pop(specArrCl[j]);
                    //if it isnt, so result is 0, end with that string;
                    if (!cond) {
                        stopper = 0;
                        break stackloop;
                    }
                    break;
                }
            }
        }
        // output 0 or 1, when string ended.
        System.out.print(stopper);
    }
}
}


class Stack{
char Arr[];
int tos; // pointer

//Constructor
Stack (int size){
    char Arr[] = new char[size*2];
    tos = -1;
}
// to add char into stack
void push (char ch){
    Arr[++tos] = ch;
}
// to get char from stack
boolean pop (char ch){
    // if stack is empty it is impossible to take anything from it ;)
    if(tos == -1){
        System.out.print("Stack is empty");
        return false;
    }

    // for each special symbol define his openSymb pair, if last char in stack is his openSymb return true and pointer - 1;
    if((ch == '>' && Arr[tos]=='<') || (ch == ')' && Arr[tos]=='(') ||
       (ch == '[' && Arr[tos]==']') || (ch == '}' && Arr[tos]=='{') ){
        Arr[tos] = '0';
        tos--;
        return true;
    }
    // if it is not his openSymb, return false
    else
        return false;
}
}

So this programm doesnt work: Exception in thread "main" java.lang.NullPointerException at Stack.push(Main.java:67) at Main.main(Main.java:33)

I didnt understand why. Look like it is something wrong method push. // to add char into stack void push (char ch){ Arr[++tos] = ch; } where tos is pointer. and tos = -1; so first ch should be into Arr[0], isnt it? or it maybe smth wrong with char strArr[] = str.toCharArray(); because it is first time when i called strArr[i];

Thanks for attentionю

P.S. Sorry, for bad english. And i didnt know how to decribe my prob without pasting code.

if (strArr[i] == specArrOp[j]) {

                    System.out.print("Null here! "+  i + " " + stack + " " + strArr[i] + "\n");
                    // Null here! 0 Stack@18be83e4 (
                    if (stack == null || strArr == null)   // stack isnt null, strArr isnt null.
                        System.out.print("Null here! "+ i + " " + strArr[i]);

                    stack.push(strArr[i]);
                    break;
}

Thanks for your hints!

I tryed to output some info about i, stack and strArr[i] if(stack == null || strArr == null) always true, so i doesnt help And System.out.print("Null here! " + i + " " + stack + " " + strArr[i]); i = 0; stack = Stack@18be83e4; so he isnt emplty, because i initialized it (23 line) or it is? Stack stack = new Stack(str.length()); Constructor: Stack (int size){ char Arr[] = new char[size]; tos = -1; }

Look like problem in this part, but i cant understand why? Method in (class Stack): void push (char ch){ tos++; Arr[tos] = ch;
} I cant understand why Arr[0] = ch; had NullPointerException I created Stack for string which included Arr[string.length] and tos = -1; So that method received ch. ch isnt null. ch = < { [ (; next tos++. so tos = 0 first time; Arr[0] = ch;

So stack.Arr[0] is null (35 line)but should I initialize each element of the array when I create an instance of the class stack?

Thanks for you help)