private static int evaluatePostfix(){
        StackInt stackInt = new StackInt(postfix.length());
        int num1=0,num2=0;
        for(int i=0;i<postfix.length();i++){
            char character = postfix.charAt(i);
            if(character>='0' && character<='9'){
                stackInt.push((int)(character-'0'));
            }else{
                num1=stackInt.pop();
                num2=stackInt.pop();
            }
            switch(character){
                case '+':
                    stackInt.push((char)(num1+num2));
                    break;
                case '-':
                    stackInt.push((char)(num1-num2));
                    break;
                case '*':
                    stackInt.push((char)(num1*num2));
                    break;
                case '/':
                    stackInt.push((char)(num1/num2));
                    break;
                default:
                    //stack.push(character);
            }
        }
        return stackInt.pop();
    }