Added Promise.then method for handling cases where the value is Void …

src/main/java/com/ekotrope/gwt/emul/java/util/concurrent/CompletableFuture.java

@@ -91,7 +91,7 @@ public static CompletableFuture<Void> allOf(CompletableFuture<?>... futures) {
     }
 
     CompletableFuture<Void> future = new CompletableFuture<>();
-    and(futures).then((value, reason) -> {
+    and(futures).then((reason) -> {
       if (reason != null) {
         future.tryCompleteThrowable(reason);
       } else {

src/main/java/com/ekotrope/gwt/emul/java/util/concurrent/impl/NativePromiseImpl.java

@@ -16,6 +16,7 @@
 package java.util.concurrent.impl;
 
 import java.util.function.BiConsumer;
+import java.util.function.Consumer;
 
 /**
  *
@@ -64,4 +65,11 @@ public void then(Runnable callback) {
     JsPromise.OnSettledCallback func = value -> callback.run();
     jsPromise.then(func, func);
   }
+
+  public void then(final Consumer<? super Throwable> callback) {
+    assert callback != null;
+    jsPromise.then(
+            value -> callback.accept(null),
+            reason -> callback.accept((Throwable) reason));
+  }
 }

src/main/java/com/ekotrope/gwt/emul/java/util/concurrent/impl/Promise.java

@@ -16,6 +16,7 @@
 package java.util.concurrent.impl;
 
 import java.util.function.BiConsumer;
+import java.util.function.Consumer;
 
 /**
  *
@@ -29,4 +30,6 @@ public interface Promise<V> {
   void then(BiConsumer<? super V, ? super Throwable> callback);
 
   void then(Runnable callback);
+
+  void then(Consumer<? super Throwable> callback);
 }

src/main/java/com/ekotrope/gwt/emul/java/util/concurrent/impl/PromiseImpl.java

@@ -22,6 +22,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.function.BiConsumer;
+import java.util.function.Consumer;
 
 /**
  *
@@ -58,6 +59,12 @@ public void then(Runnable callback) {
     }
   }
 
+  @Override
+  public void then(Consumer<? super Throwable> callback) {
+    assert callback != null;
+    then(() -> callback.accept(reason));
+  }
+
   private void complete(V value, Throwable reason) {
     if (!done) {
       this.value = value;