#1355 NullPointerException in a java file having a single comment line

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

@@ -47,8 +47,11 @@ public class ASTCompilationUnit extends AbstractJavaTypeNode implements RootNode
     }
 
     public ASTPackageDeclaration getPackageDeclaration() {
-        Node n = jjtGetChild(0);
-        return n instanceof ASTPackageDeclaration ? (ASTPackageDeclaration) n : null;
+        if (jjtGetNumChildren() > 0) {
+            Node n = jjtGetChild(0);
+            return n instanceof ASTPackageDeclaration ? (ASTPackageDeclaration) n : null;
+        }
+        return null;
     }
 
     public ClassTypeResolver getClassTypeResolver() {

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

@@ -79,6 +79,14 @@ public class ParserCornersTest extends ParserTst {
                 + "    private void deleteDirectory(String path) {\n" + "        LOG.debug(path);\n" + "    }\n" + "}");
     }
 
+    /**
+     * Test for https://sourceforge.net/p/pmd/bugs/1355/
+     */
+    @Test
+    public void emptyFileJustComment() {
+        parseJava18("// just a comment");
+    }
+
     @Test
     public void testMultipleExceptionCatching() {
         String code = "public class Foo { public void bar() { "

src/site/markdown/overview/changelog.md

@@ -19,5 +19,6 @@
 *   [#1345](https://sourceforge.net/p/pmd/bugs/1345/): UseCollectionIsEmpty throws NullPointerException
 *   [#1353](https://sourceforge.net/p/pmd/bugs/1353/): False positive "Only One Return" with lambda
 *   [#1354](https://sourceforge.net/p/pmd/bugs/1354/): Complex FieldDeclarationsShouldBeAtStartOfClass false positive with Spring annotations
+*   [#1355](https://sourceforge.net/p/pmd/bugs/1355/): NullPointerException in a java file having a single comment line
 
 **API Changes:**