Parameter mismatch in insert method in StringBuilder

public String fractionToDecimal(int numerator, int denominator) {
    if (numerator==0)
        return "0";
    long a = numerator,b = denominator;
    StringBuilder ans = new StringBuilder();
    if (a*b<0)
        ans.append('-');
    a=Math.abs(a);
    b=Math.abs(b);
    ans.append(a/b);
    if (a%b==0)
        return ans.toString();
    ans.append('.');
    Map<Long,Integer> flag = new HashMap<>();
    while ((a=(a%b)*10)!=0&&!flag.containsKey(a)){
        flag.put(a,ans.length());
        ans.append(a/b);
    }
    if (a%b==0)
        return ans.toString();
    return ans.insert(flag.get(a),'(').append(")").toString();
}

This is the code in question. An error message appears in the last returnIf you use the intvalue method after get (a), there will be no compilation error, or if you change the following char type to string "(", there will be no error. Why?