Oh no, Java - rextester

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//'main' method must be in a class 'Rextester'.
//openjdk version '11.0.5'

import java.util.*;
import java.lang.*;

class Rextester
{  
    // get number before "(" like 123( ... 2( ... 3( ... 12(
    public static int getnum(String s,int i)
    {
        int index = i-1;
        while (index >= 0 && Character.isDigit(s.charAt(index)))
            index--;
        return (i - index - 1);
    }
    public static void main(String args[])
    {
        String s ="2(245(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)2(2(hi)2(co))x2(bo)";   
        int first,second =0;
        while(true)
        {
            first = second = -1;
            for (int i=0; i< s.length(); i++)
            {
                if (s.charAt(i) == '(') 
                    first = i;
                if (s.charAt(i) == ')')
                {
                    second = i;
                    break;
                }
            }
            if (second != -1 && first != -1)
            {
                int num = 0;
                String s1;
                int total_num = getnum(s,first);
                total_num = first - total_num;
                for (int i = total_num; i < first; i++)
                    num = num * 10 + Character.getNumericValue(s.charAt(i));
                s1 = s.substring(0,total_num);
                for (int i=1; i<= num; i++)
                    s1 += s.substring(first + 1,second);
                s = s1 + s.substring(second + 1, s.length());
            }
             else break;
        }
       System.out.println(s);
    }
}

[ - ] Show input
Compilation time: 0.72 sec, absolute running time: 0.27 sec, cpu time: 0.24 sec, memory peak: 39 Mb, absolute service time: 1,03 sec