Update performance.xml

Added note about Arrays.asList()

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

@@ -790,6 +790,11 @@ public class SimpleTest extends TestCase {
         <description>
 The java.util.Arrays class has a "asList" method that should be used when you want to create a new List from
 an array of objects. It is faster than executing a loop to copy all the elements of the array one by one.
+
+Note that the result of Arrays.asList() is backed by the specified array,
+changes in the returned list will result in the array to be modified.
+For that reason, it is not possible to add new elements to the returned list of Arrays.asList() (UnsupportedOperationException).
+You must use new ArrayList<>(Arrays.asList(...)) if that is inconvenient for you (e.g. because of concurrent access).
         </description>
         <priority>3</priority>
         <properties>
@@ -834,7 +839,7 @@ an array of objects. It is faster than executing a loop to copy all the elements
 public class Test {
     public void foo(Integer[] ints) {
         // could just use Arrays.asList(ints)
-        List l= new ArrayList(10);
+        List<Integer> l= new ArrayList<>(100);
         for (int i=0; i< 100; i++) {
             l.add(ints[i]);
         }