Check for diamond operator for anonymous classes

pmd-java/etc/grammar/Java.jjt

@@ -1,7 +1,8 @@
 /**
- * Add support for Java 9 changes
- * private interface methods are only allowed with java9,
- * a single underscore "_" is an invalid identifier in java9.
+ * Add support for Java 9 changes:
+ * Private interface methods are only allowed with java9.
+ * A single underscore "_" is an invalid identifier in java9.
+ * Diamond operator for anonymous classes is only allowed with java9.
  * Andreas Dangel 09/2017
  *====================================================================
  * Add support for new Java 8 annotation locations.
@@ -324,6 +325,15 @@ public class JavaParser {
       throwParseException("Cannot use explicit receiver parameters when running in JDK inferior to 1.8 mode!");
     }
   }
+  private void checkForBadAnonymousDiamondUsage() {
+    if (jdkVersion < 9) {
+      ASTAllocationExpression node = (ASTAllocationExpression)jjtree.peekNode();
+      ASTTypeArguments types = node.getFirstDescendantOfType(ASTTypeArguments.class);
+      if (node.isAnonymousClass() && types != null && types.isDiamond()) {
+          throwParseException("Cannot use '<>' with anonymous inner classes when running in JDK inferior to 9 mode!");
+      }
+    }
+  }
   /**
    * Keeps track whether we are dealing with an interface or not. Needed since the tree is
    * is not fully constructed yet, when we check for private interface methods.
@@ -1960,6 +1970,7 @@ void AllocationExpression():
       Arguments() [ ClassOrInterfaceBody() ]
      )
   )
+  { checkForBadAnonymousDiamondUsage(); }
 }
 
 /*

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/ast/ASTAllocationExpression.java

@@ -5,6 +5,8 @@
 
 package net.sourceforge.pmd.lang.java.ast;
 
+import net.sourceforge.pmd.lang.ast.Node;
+
 public class ASTAllocationExpression extends AbstractJavaTypeNode {
     public ASTAllocationExpression(int id) {
         super(id);
@@ -20,4 +22,12 @@ public class ASTAllocationExpression extends AbstractJavaTypeNode {
     public Object jjtAccept(JavaParserVisitor visitor, Object data) {
         return visitor.visit(this, data);
     }
+
+    public boolean isAnonymousClass() {
+        if (jjtGetNumChildren() > 1) {
+            Node lastChild = jjtGetChild(jjtGetNumChildren() - 1);
+            return lastChild instanceof ASTClassOrInterfaceBody;
+        }
+        return false;
+    }
 }

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/ast/ASTTypeArguments.java

@@ -20,4 +20,8 @@ public class ASTTypeArguments extends AbstractJavaNode {
     public Object jjtAccept(JavaParserVisitor visitor, Object data) {
         return visitor.visit(this, data);
     }
+
+    public boolean isDiamond() {
+        return jjtGetNumChildren() == 0;
+    }
 }

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/ast/JDKVersionTest.java

@@ -237,4 +237,14 @@ public class JDKVersionTest {
     public final void jdk9InvalidIdentifier() {
         parseJava9(loadSource("jdk9_invalid_identifier.java"));
     }
+
+    @Test(expected = ParseException.class)
+    public final void jdk9AnonymousDiamondInJava8() {
+        parseJava18(loadSource("jdk9_anonymous_diamond.java"));
+    }
+
+    @Test
+    public final void jdk9AnonymousDiamond() {
+        parseJava9(loadSource("jdk9_anonymous_diamond.java"));
+    }
 }

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/ast/jdkversiontests/jdk9_anonymous_diamond.java

@@ -0,0 +1,8 @@
+import java.util.HashSet;
+import java.util.Set;
+
+public class Java9AnonymousDiamond {
+    public static void main(String ... args) {
+        Set<String> set = new HashSet<>() { };
+    }
+}