### Question:
StringUtil { public static boolean isDigitOnly(String chkStr) { if (chkStr == null) return false; for (char c : chkStr.toCharArray()) if (!Character.isDigit(c)) return false; return true; }  static boolean isEmpty(String str); static String getLeft(String src, int length); static String getRight(String src, int length); static boolean isDigitOnly(String chkStr); static String SQLEscape(String str); static String fillString(String str, int length, char fillChar, boolean fillLeft); static String cEscape(String str); static String cEscape(String str, char quoteChar); static String cEscape(String str, char quoteChar, boolean isWchar); static String lowerFirst(String str); static String upperFirst(String str); static boolean isJavaIdentifier(String str); static int[] splitAsIntArray(String str, String delimiter); static String join(byte[] items, String delimiter); static String join(String[] items, String delimiter); static String join(List<String> items, String delimiter); static StringBuilder appendRepeatedly(StringBuilder result, String str, int times); static StringBuilder appendRepeatedly(StringBuilder result, char c, int times); static String toTrimmedStr(Object o, int len);  }### Answer:
@Test public void testIsDigitOnly() { assertTrue("1234 contains only digits", StringUtil.isDigitOnly("1234")); assertFalse("1234a doesn't contain only digits", StringUtil.isDigitOnly("1234a")); assertFalse("null string is not digits only", StringUtil.isDigitOnly(null)); }