Excel column string to row number and vice versa
Your code is basically fine, and your unit tests are good.  All I have is nitpicks.

"Excel" in the method names are a bit redundant.

In getExcelColumnNumber(), group - 'A' + 1 with parentheses as - ('A' - 1).  Then the compiler can generate 64 as a constant.

In getExcelColumnName(), the similarly named variables number, num, and numChar are confusing.

The complexity O(log n) is correct.  (With Big-O notation, the base of the logarithm is an unimportant detail, since the base just scales the logarithm by a constant factor, and constant factors are conventionally discarded with Big-O.  For example, O(log_26 n) = O(ln n / ln 26) = O(ln n).)

public final class ExcelColumn {

    private ExcelColumn() {}

    public static int toNumber(String name) {
        int number = 0;
        for (int i = 0; i &lt; name.length(); i++) {
            number = number * 26 + (name.charAt(i) - ('A' - 1));
        }
        return number;
    }

    public static String toName(int number) {
        StringBuilder sb = new StringBuilder();
        while (number-- &gt; 0) {
            sb.append((char)('A' + (number % 26)));
            number /= 26;
        }
        return sb.reverse().toString();
    }
}