### Question:
StringUtil { public static boolean isSimpleLetterOrDigit(final char c) { return c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9'; } private  StringUtil(); static boolean isBlank(final String strValue); static String first2LowerCase(final String strValue); static String first2UpperCase(final String strValue); static String constToLowerCamelCase(final String str); static String constToUpperCamelCase(final String str); static String camelToConstCase(final String str); static String camelToSnakeCase(final String str); static boolean isUpperCamelCase(final String testString); static boolean isLowerCamelCase(final String testString); static boolean isSimpleLetterOrDigit(final char c); static String replace(final String str, final String oldStr, final String newStr); static void replace(final StringBuilder str, final String oldStr, final String newStr); static String format(final String msg, final Object... params);  }### Answer:
@Test public void testSimpleLetter() { assertTrue(StringUtil.isSimpleLetterOrDigit('a')); assertTrue(StringUtil.isSimpleLetterOrDigit('b')); assertTrue(StringUtil.isSimpleLetterOrDigit('c')); assertTrue(StringUtil.isSimpleLetterOrDigit('x')); assertTrue(StringUtil.isSimpleLetterOrDigit('y')); assertTrue(StringUtil.isSimpleLetterOrDigit('z')); assertTrue(StringUtil.isSimpleLetterOrDigit('A')); assertTrue(StringUtil.isSimpleLetterOrDigit('B')); assertTrue(StringUtil.isSimpleLetterOrDigit('C')); assertTrue(StringUtil.isSimpleLetterOrDigit('X')); assertTrue(StringUtil.isSimpleLetterOrDigit('Y')); assertTrue(StringUtil.isSimpleLetterOrDigit('Z')); assertTrue(StringUtil.isSimpleLetterOrDigit('0')); assertTrue(StringUtil.isSimpleLetterOrDigit('1')); assertTrue(StringUtil.isSimpleLetterOrDigit('2')); assertTrue(StringUtil.isSimpleLetterOrDigit('3')); assertTrue(StringUtil.isSimpleLetterOrDigit('4')); assertTrue(StringUtil.isSimpleLetterOrDigit('5')); assertTrue(StringUtil.isSimpleLetterOrDigit('6')); assertTrue(StringUtil.isSimpleLetterOrDigit('7')); assertTrue(StringUtil.isSimpleLetterOrDigit('8')); assertTrue(StringUtil.isSimpleLetterOrDigit('9')); }
@Test public void testNoSimpleLetter() { assertFalse(StringUtil.isSimpleLetterOrDigit('é')); assertFalse(StringUtil.isSimpleLetterOrDigit('à')); assertFalse(StringUtil.isSimpleLetterOrDigit('ù')); assertFalse(StringUtil.isSimpleLetterOrDigit('â')); assertFalse(StringUtil.isSimpleLetterOrDigit('"')); assertFalse(StringUtil.isSimpleLetterOrDigit('+')); assertFalse(StringUtil.isSimpleLetterOrDigit('-')); assertFalse(StringUtil.isSimpleLetterOrDigit('&')); }