Done. Thanks for the kata!

For some reason test cases 7 and 8 fail for me, even though my solution works locally. Oddly, test case 8 works on some attempts but not others.
I had to forfeit eligibility for the points to see the cases but haven't changed my solution since. If it is correct, is there a way to reinstate eligibility?

private static final String DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

public static String converter(double n, int decimals, double base) {
    if (n == 0) return "0";

    boolean isNeg = n < 0;
    n = Math.abs(n);

    StringBuilder sb = new StringBuilder();
    for (int exponent = (int) (Math.log(n) / Math.log(base)); exponent > -decimals - 1; exponent--) {
        if (exponent == -1) {
            sb.append(".");
        }
        sb.append(DIGITS.charAt((int) (n / Math.pow(base, exponent))));
        n %= Math.pow(base, exponent);
    }
    return (isNeg ? "-" : "") + sb.toString();
}