Hello community,
I have an issued converting a arithmetic string called the infix, into a prefix output. In one of my methods i have an infinite loop. An example would be (a + b) * (c - d)===> * + a b - c d Here is my code:

public static String inToPrefix(String s){
        Stack<Character> operator = new Stack<>();
        Stack<String> operand = new Stack<>();
        String output = "";
        for (int i = 0; i < s.length(); i++)
        {
            char chValue = s.charAt(i);
            if((isDigit(chValue))||(isAlpha(chValue))){
                operand.push("" + chValue);
            }
            else if(chValue == '('){
                operator.push(chValue);
            }
            else if(isOperator(chValue)){
                while(!operator.isEmpty())
                {
                   System.out.print("1");
                   if(priority(operator.peek())> priority(chValue))
                   {
                      String op = "" + operator.pop();
                      String Righthand = operand.pop();
                      String Lefthand = operand.pop();
                      operand.push(op + Lefthand + Righthand);
                   }
                   else{
                       operator.push(chValue);
                   }
                   
               }
               operator.push(chValue);
            }
            else if(chValue ==')'){
                while((operator.peek()!='(')&&(!operator.isEmpty())){
                    String op = ""+ operator.pop();
                    String Righthand = operand.pop();
                    String Lefthand = operand.pop();
                    operand.push(op + Lefthand + Righthand);
                }
                if(operator.peek() != '('){
                    operator.pop();
                }
                
            }
        }
        while(!operator.isEmpty()){
            String op =  "" + operator.pop();
            String Righthand = operand.pop();
            String Lefthand = operand. pop();
            operand.push(op+ Lefthand + Righthand);
            output = operand.pop();
            System.out.print("1");
        }
        return output;
    }
    public static boolean isDigit(char ch){
        
        String operators = "0123456789";
	if (operators.indexOf(ch) != -1)
	    return true;
	else
	    return false;
    }
    public static boolean isAlpha(char ch){
        String operators = "abcdefghijklmnopqrstuvwxyz";
        String Cap_oper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	if ((operators.indexOf(ch) != -1)||(Cap_oper.indexOf(ch)!= -1)){
            return true;
        }
        return false;
    }
    public static boolean isOperator(char ch){
        String operators = "+-*/%";
	if (operators.indexOf(ch) != -1){
            return true;
        }
        return false;
    }
    public static int priority(char op){
        int x;
        switch(op){
            case '+':
            case '-':
                x = 1;
                break;
            case '*':
            case '/':
                x = 2;
                break;
            default:
                x = 0;
                break;
        }
        return x;
    }
    
}
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