Annotate CompletionService classes. (#124)

Compare
google/xplat@9de0f25
(which missed `@Nullable` on the `poll` methods, so I'll be fixing
that later).

Author: cpovirk

src/java.base/share/classes/java/util/concurrent/CompletionService.java

@@ -35,6 +35,9 @@
 
 package java.util.concurrent;
 
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
 /**
  * A service that decouples the production of new asynchronous tasks
  * from the consumption of the results of completed tasks.  Producers
@@ -62,7 +65,8 @@
  *
  * @since 1.5
  */
-public interface CompletionService<V> {
+@NullMarked
+public interface CompletionService<V extends @Nullable Object> {
     /**
      * Submits a value-returning task for execution and returns a Future
      * representing the pending results of the task.  Upon completion,
@@ -108,7 +112,7 @@ public interface CompletionService<V> {
      * @return the Future representing the next completed task, or
      *         {@code null} if none are present
      */
-    Future<V> poll();
+    @Nullable Future<V> poll();
 
     /**
      * Retrieves and removes the Future representing the next
@@ -124,5 +128,5 @@ public interface CompletionService<V> {
      *         before one is present
      * @throws InterruptedException if interrupted while waiting
      */
-    Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
+    @Nullable Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
 }

src/java.base/share/classes/java/util/concurrent/ExecutorCompletionService.java

@@ -35,6 +35,9 @@
 
 package java.util.concurrent;
 
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
 /**
  * A {@link CompletionService} that uses a supplied {@link Executor}
  * to execute tasks.  This class arranges that submitted tasks are,
@@ -102,7 +105,8 @@
  *
  * @since 1.5
  */
-public class ExecutorCompletionService<V> implements CompletionService<V> {
+@NullMarked
+public class ExecutorCompletionService<V extends @Nullable Object> implements CompletionService<V> {
     private final Executor executor;
     private final AbstractExecutorService aes;
     private final BlockingQueue<Future<V>> completionQueue;
@@ -202,11 +206,11 @@ public Future<V> take() throws InterruptedException {
         return completionQueue.take();
     }
 
-    public Future<V> poll() {
+    public @Nullable Future<V> poll() {
         return completionQueue.poll();
     }
 
-    public Future<V> poll(long timeout, TimeUnit unit)
+    public @Nullable Future<V> poll(long timeout, TimeUnit unit)
             throws InterruptedException {
         return completionQueue.poll(timeout, unit);
     }