[doc] [java] SimplifyStartsWith: update description and example, fixes #1868

pmd-java/src/main/resources/category/java/performance.xml

@@ -699,7 +699,8 @@ public class C {
           externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#simplifystartswith">
         <description>
 Since it passes in a literal of length 1, calls to (string).startsWith can be rewritten using (string).charAt(0)
-at the expense of some readability.
+at the expense of some readability. To prevent IndexOutOfBoundsException being thrown by the charAt method,
+ensure that the (string) is not empty by making an additional check first.
         </description>
         <priority>3</priority>
         <properties>
@@ -730,7 +731,7 @@ public class Foo {
     }
 
     boolean fasterCheckIt(String x) {
-        return x.charAt(0) == 'a';  // faster approach
+        return !x.isEmpty() && x.charAt(0) == 'a';  // faster approach
     }
 }
 ]]>