@Test
    public void testFlipCaseDigit() {
        testNoChangeOnFlip("0123456789");
    }

    @Test
    public void testFlipCaseWhitespace() {
        testNoChangeOnFlip(" \r\t\n");
    }

    @Test
    public void testFlipCaseOtherChars() {
        testNoChangeOnFlip("!@#$%^");
    }

    private static void testNoChangeOnFlip(CharSequence chars) {
        for (int index = 0; index < chars.length(); index++) {
            char expected = chars.charAt(index);
            char actual = CharacterUtils.flipCase(expected);
            assertEquals("Unexpected flipped value", expected, actual);
        }
    }

public class ExceptionUtils {
    private static final class ExceptionThrower {
        private static synchronized void spit(Throwable t) {
             ...
        }
   }

    @SuppressWarnings("synthetic-access")
    public static void rethrowException(Throwable t) {
        ExceptionThrower.spit(t);
    }
}