[java] Update OptimizableToArrayCall rule for current JVMs

See https://sourceforge.net/p/pmd/bugs/1454/
and https://shipilev.net/blog/2016/arrays-wisdom-ancients/

docs/pages/release_notes.md

@@ -252,6 +252,11 @@ The rule reference documentation has been updated to reflect these changes.
 *   The Java rule `EmptyCatchBlock` (category `errorprone`, former ruleset `java-empty`) has been changed to ignore
     exceptions named `ignore` or `expected` by default. You can still override this behaviour by setting the `allowExceptionNameRegex` property.
 
+*   The Java rule `OptimizableToArrayCall` (category `performance`, former ruleset `design`) has been
+    modified to fit for the current JVM implementations: It basically detects now the opposite and suggests to
+    use `Collection.toArray(new E[0])` with a zero-sized array.
+    See [Arrays of Wisdom of the Ancients](https://shipilev.net/blog/2016/arrays-wisdom-ancients/).
+
 #### Deprecated Rules
 
 *   The Java rules `NcssConstructorCount`, `NcssMethodCount`, and `NcssTypeCount` (ruleset `java-codesize`) have been
@@ -339,6 +344,7 @@ a warning will now be produced suggesting users to adopt it for better performan
 *   cpp
     *   [#448](https://github.com/pmd/pmd/issues/448): \[cpp] Write custom CharStream to handle continuation characters
 *   java
+    *   [#1454](https://sourceforge.net/p/pmd/bugs/1454/): \[java] OptimizableToArrayCall is outdated and invalid in current JVMs
     *   [#1513](https://sourceforge.net/p/pmd/bugs/1513/): \[java] Remove deprecated rule UseSingleton
     *   [#328](https://github.com/pmd/pmd/issues/328): \[java] java.lang.ClassFormatError: Absent Code attribute in method that is not native or abstract in class file javax/servlet/jsp/PageContext
     *   [#487](https://github.com/pmd/pmd/pull/487): \[java] Fix typeresolution for anonymous extending object

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

@@ -428,9 +428,17 @@ public class Foo {
           class="net.sourceforge.pmd.lang.rule.XPathRule"
           externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_performance.html#optimizabletoarraycall">
         <description>
-Calls to a collection's toArray() method should specify target arrays sized to match the size of the
-collection. Initial arrays that are too small are discarded in favour of new ones that have to be created
-that are the proper size.
+Calls to a collection's `toArray(E[])` method should specify a target array of zero size. This allows the JVM
+to optimize the memory allocation and copying as much as possible.
+
+Previous versions of this rule (pre PMD 6.0.0) suggested the opposite, but current JVM implementations
+perform always better, when they have full control over the target array. And allocation an array via
+reflection is nowadays as fast as the direct allocation.
+
+See also [Arrays of Wisdom of the Ancients](https://shipilev.net/blog/2016/arrays-wisdom-ancients/)
+
+Note: If you don't need an array of the correct type, then the simple `toArray()` method without an array
+is faster, but returns only an array of type `Object[]`.
         </description>
         <priority>3</priority>
         <properties>
@@ -442,7 +450,7 @@ that are the proper size.
 [
 PrimarySuffix/Arguments/ArgumentList/Expression
  /PrimaryExpression/PrimaryPrefix/AllocationExpression
- /ArrayDimsAndInits/Expression/PrimaryExpression/PrimaryPrefix/Literal[@Image='0']
+ /ArrayDimsAndInits/Expression/PrimaryExpression/PrimaryPrefix[not(Literal[@Image='0'])]
 ]
 ]]>
                 </value>
@@ -450,13 +458,13 @@ PrimarySuffix/Arguments/ArgumentList/Expression
         </properties>
         <example>
 <![CDATA[
-List foos = getFoos();
+List<Foo> foos = getFoos();
 
-    // inefficient, the array will be discarded
+// much better; this one allows the jvm to allocate an array of the correct size and effectively skip
+// the zeroing, since each array element will be overridden anyways
 Foo[] fooArray = foos.toArray(new Foo[0]);
 
-    // much better; this one sizes the destination array,
-    // avoiding of a new one via reflection
+// inefficient, the array needs to be zeroed out by the jvm before it is handed over to the toArray method
 Foo[] fooArray = foos.toArray(new Foo[foos.size()]);
 ]]>
         </example>

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/performance/xml/OptimizableToArrayCall.xml

@@ -4,10 +4,8 @@
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://pmd.sourceforge.net/rule-tests http://pmd.sourceforge.net/rule-tests_1_0_0.xsd">
     <test-code>
-        <description><![CDATA[
-failure case
-     ]]></description>
-        <expected-problems>1</expected-problems>
+        <description>Preferred usage (sf #1454)</description>
+        <expected-problems>0</expected-problems>
         <code><![CDATA[
 public class Foo {
  {x.toArray(new Foo[0]);}
@@ -15,10 +13,8 @@ public class Foo {
      ]]></code>
     </test-code>
     <test-code>
-        <description><![CDATA[
-Array dimensioner uses method call, ok
-     ]]></description>
-        <expected-problems>0</expected-problems>
+        <description>Array dimensioner uses method call, performance issue</description>
+        <expected-problems>1</expected-problems>
         <code><![CDATA[
 public class Foo {
  {x.toArray(new Foo[x.size()]);}
@@ -26,10 +22,8 @@ public class Foo {
      ]]></code>
     </test-code>
     <test-code>
-        <description><![CDATA[
-Array dimensioner uses variable, ok
-     ]]></description>
-        <expected-problems>0</expected-problems>
+        <description>Array dimensioner uses variable, performance issue</description>
+        <expected-problems>1</expected-problems>
         <code><![CDATA[
 public class Foo {
  {x.toArray(new Foo[y]);}
@@ -39,12 +33,36 @@ public class Foo {
 
     <test-code>
         <description>#937 OptimizableToArrayCall does not catch multilevel method chains</description>
-        <expected-problems>1</expected-problems>
+        <expected-problems>0</expected-problems>
         <code><![CDATA[
 public class Test {
     public void foo() {
         x.y.toArray(new Foo[0]);
     }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Array with a literal dimension, zero still better</description>
+        <expected-problems>1</expected-problems>
+        <code><![CDATA[
+public class Foo {
+    public void foo() {
+        x.toArray(new Foo[10]);
+    }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>toArray call without an array should not be flagged</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class Foo {
+    public void foo() {
+        x.toArray();
+    }
 }
         ]]></code>
     </test-code>