From 36654ed52ec9b8b5e9087ac523563dff6a2ea09f Mon Sep 17 00:00:00 2001
From: Andreas Dangel <andreas.dangel@pmd-code.org>
Date: Thu, 24 Oct 2024 13:08:09 +0200
Subject: [PATCH] [java] UselessOperationOnImmutable: false positive on void
 results

---
 .../UselessOperationOnImmutableRule.java           |  3 ++-
 .../errorprone/xml/UselessOperationOnImmutable.xml | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/errorprone/UselessOperationOnImmutableRule.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/errorprone/UselessOperationOnImmutableRule.java
index c326446c5e..89434b830d 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/errorprone/UselessOperationOnImmutableRule.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/errorprone/UselessOperationOnImmutableRule.java
@@ -30,7 +30,8 @@ public class UselessOperationOnImmutableRule extends AbstractJavaRulechainRule {
     @Override
     public Object visit(ASTMethodCall node, Object data) {
         ASTExpression qualifier = node.getQualifier();
-        if (node.getParent() instanceof ASTExpressionStatement && qualifier != null) {
+        boolean returnsVoid = node.getTypeMirror().isVoid();
+        if (node.getParent() instanceof ASTExpressionStatement && qualifier != null && !returnsVoid) {
 
             // these types are immutable, so any method of those whose
             // result is ignored is a violation
diff --git a/pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/errorprone/xml/UselessOperationOnImmutable.xml b/pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/errorprone/xml/UselessOperationOnImmutable.xml
index b311af77f0..35ee48f112 100644
--- a/pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/errorprone/xml/UselessOperationOnImmutable.xml
+++ b/pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/errorprone/xml/UselessOperationOnImmutable.xml
@@ -318,6 +318,20 @@ class Tester {
         period = period.plusWeeks(1); // ok
     }
 }
+]]></code>
+    </test-code>
+
+    <test-code>
+        <description>False positive for package private method calls in openjdk</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+package java.time;
+import java.io.ObjectOutput;
+class Tester {
+    static void writeExternal(ZonedDateTime zonedDateTime, ObjectOutput out) {
+        zonedDateTime.writeExternal(out); // false positive
+    }
+}
 ]]></code>
     </test-code>
 </test-data>