add Strings#isDigits API

inspiration taken from [this SO answer][SO].

note that the stream is not parallelised to avoid the overhead of this
as the method is intended to be called primarily with shorter strings
where the time to set up would take longer than the actual check.

[SO]: https://stackoverflow.com/a/35150400

Signed-off-by: Ralph Ursprung <[email protected]>

libs/core/src/main/java/org/opensearch/core/common/Strings.java

@@ -815,4 +815,17 @@ public static String toLowercaseAscii(String in) {
  }
  return out.toString();
  }
+
+ /**
+ * Check whether every single character in the string is a digit.
+ *
+ * <p>An empty string returns {@code false}.</p>
+ *
+ * @param s the string, must not be null.
+ * @return {@code true} if the string only contains digits, {@code false} otherwise.
+ */
+ public static boolean isDigits(final String s) {
+ return !s.isEmpty() && s.chars().allMatch(Character::isDigit);
+ }
+
 }

libs/core/src/test/java/org/opensearch/core/common/StringsTests.java

@@ -114,4 +114,15 @@ public void testToStringToXContentWithOrWithoutParams() {
  containsString("\"color_from_param\":\"blue\"")
  );
  }
+
+ public void testIsDigits() {
+ assertTrue(Strings.isDigits("1"));
+ assertTrue(Strings.isDigits("123"));
+ assertFalse(Strings.isDigits(""));
+ assertFalse(Strings.isDigits("abc"));
+ assertFalse(Strings.isDigits("123a"));
+ assertFalse(Strings.isDigits("0x123"));
+ assertFalse(Strings.isDigits("123.4"));
+ assertFalse(Strings.isDigits("123f"));
+ }
 }