Declare CompletableFuture.completedStage(value) to accept null values

CHANGELOG.md

@@ -8,6 +8,7 @@ Currently the versioning policy of this project follows [Semantic Versioning v2.
 
 ### Fixed
 * Invalid HTML in the description of `LI_LAZY_INIT_UPDATE_STATIC` bug pattern ([#1383](https://github.com/spotbugs/spotbugs/pull/1383))
+* NP_NONNULL_PARAM_VIOLATION false-positive in CompletableFuture.completedStage(value) ([#1397](https://github.com/spotbugs/spotbugs/issues/1397))
 
 ## 4.2.0 - 2020-11-28
 ### Fixed

spotbugs/src/main/java/edu/umd/cs/findbugs/ba/DefaultNullnessAnnotations.java

@@ -232,6 +232,8 @@ public static void addDefaultNullnessAnnotations(INullnessAnnotationDatabase dat
                 "(Ljava/lang/Object;)Z", false, 0, NullnessAnnotation.NULLABLE);
         database.addMethodParameterAnnotation("java.util.concurrent.CompletableFuture", "completedFuture",
                 "(Ljava/lang/Object;)Ljava/util/concurrent/CompletableFuture;", true, 0, NullnessAnnotation.NULLABLE);
+        database.addMethodParameterAnnotation("java.util.concurrent.CompletableFuture", "completedStage",
+                "(Ljava/lang/Object;)Ljava/util/concurrent/CompletionStage;", true, 0, NullnessAnnotation.NULLABLE);
 
         database.addMethodParameterAnnotation("java.util.concurrent.ExecutionException", "<init>", "(Ljava/lang/String;)V",
                 false, 0, NullnessAnnotation.CHECK_FOR_NULL);

spotbugsTestCases/src/java11/Issue1397.java

@@ -0,0 +1,32 @@
+/*
+ * Contributions to SpotBugs
+ * Copyright (C) 2021 Mikael Ståldal
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+package ghIssues;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+
+/**
+ * @see <a href="https://github.com/spotbugs/spotbugs/issues/1397">GitHub issue</a>
+ */
+public class Issue1397 {
+    CompletionStage<Void> completedFuture() {
+        // 1st argument of CompletableFuture#completedStage(U) should be nullable
+        return CompletableFuture.completedStage(null);
+    }
+}