public static String dup(char c, int num) {
        if (c == 0 || num < 1)
            return "";
        StringBuilder sb = new StringBuilder(num);
        for (int i = 0; i < num; i++)
            sb.append(c);
        return sb.toString();
    }
@Test
    public void testDup() {
        String str = StringKit.dup('c', 6);
        Assert.assertEquals(6, str.length());
        Assert.assertEquals("cccccc", str);
    }