[github-405] more performant codepoint util code. Thanks to StanBertrand. This closes #405

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1905932 13f79535-47bb-0310-9956-ffa450edef68

poi/src/main/java/org/apache/poi/util/CodepointsUtil.java

@@ -23,7 +23,7 @@ import java.util.Iterator;
 public class CodepointsUtil {
     public static Iterator<String> iteratorFor(String text) {
         return text.codePoints()
-                .mapToObj(codePoint -> new StringBuilder().appendCodePoint(codePoint).toString())
+                .mapToObj(codePoint -> new String(Character.toChars(codePoint)))
                 .iterator();
     }
 }

poi/src/test/java/org/apache/poi/util/TestCodepointsUtil.java

@@ -35,7 +35,6 @@ class TestCodepointsUtil {
     void testIterator() {
         final String unicodeSurrogates = "\uD835\uDF4A\uD835\uDF4B\uD835\uDF4C\uD835\uDF4D\uD835\uDF4E"
                 + "abcdef123456";
-        Iterator<String> sci = CodepointsUtil.iteratorFor(unicodeSurrogates);
         List<String> codePoints = new ArrayList<>();
         CodepointsUtil.iteratorFor(unicodeSurrogates).forEachRemaining(codePoints::add);
         assertEquals(17, codePoints.size());