java: Add CompletableFuture.completedFuture()

Matches java.util.concurrent.CompletableFuture's method.

Author: jrose-signal

RELEASE_NOTES.md

@@ -1,3 +1,5 @@
 v0.76.5
 
 - Added HPKE operations to (EC)PublicKey---`seal`---and PrivateKey---`open`.
+
+- Java: CompletableFuture now has a convencience factory method `completedFuture`, matching its namesake in `java.util.concurrent`.

java/shared/java/org/signal/libsignal/internal/CompletableFuture.java

@@ -38,6 +38,12 @@ public CompletableFuture() {
     this.consumers = new ArrayList<>();
   }
 
+  public static <U> CompletableFuture<U> completedFuture(U value) {
+    final var result = new CompletableFuture<U>();
+    result.complete(value);
+    return result;
+  }
+
   @CalledFromNative
   void setCancellationId(long cancellationId) {
     this.cancellationId = Optional.of(cancellationId);

java/shared/test/java/org/signal/libsignal/internal/CompletableFutureTest.java

@@ -58,6 +58,15 @@ public void testSuccess() throws Exception {
     assertEquals(42, (int) future.get(1, TimeUnit.MILLISECONDS));
   }
 
+  @Test
+  public void testCompletedFutureFactoryMethod() throws Exception {
+    CompletableFuture<Integer> future = CompletableFuture.completedFuture(42);
+    assertTrue(future.isDone());
+    assertFalse(future.isCancelled());
+    assertEquals(42, (int) future.get());
+    assertEquals(42, (int) future.get(1, TimeUnit.MILLISECONDS));
+  }
+
   @Test
   public void testFailure() throws Exception {
     CompletableFuture<Integer> future = new CompletableFuture<>();