static void urlEncode(String str, StringBuilder sb) {
        for (int idx = 0; idx < str.length(); ++idx) {
            char c = str.charAt(idx);
            if ('+' == c) {
                sb.append("%2B");
            } else if ('%' == c) {
                sb.append("%25");
            } else {
                sb.append(c);
            }
        }
    }
@Test
    void testUrlEncodePlus() {
        
        // Arrange
        final StringBuilder sb = new StringBuilder("????");
        
        // Act
        GroupKey.urlEncode("+", sb);
        
        // Assert side effects
        assertNotNull(sb);
        assertEquals("????%2B", sb.toString());
    }