public static String deleteWhitespace(String str) {
        if (isEmpty(str)) {
            return str;
        }
        int sz = str.length();
        char[] chs = new char[sz];
        int count = 0;
        for (int i = 0; i < sz; i++) {
            if (!Character.isWhitespace(str.charAt(i))) {
                chs[count++] = str.charAt(i);
            }
        }
        if (count == sz) {
            return str;
        }
        return new String(chs, 0, count);
    }
@Test
    void testDeleteWhitespace() {
        assertNull(StringUtils.deleteWhitespace(null));
        assertEquals(StringUtils.EMPTY, StringUtils.deleteWhitespace(""));
        assertEquals("abc", StringUtils.deleteWhitespace("abc"));
        assertEquals("abc", StringUtils.deleteWhitespace("   ab  c  "));
    }