Better generics

src/main/java/org/dataloader/impl/PromisedValues.java

@@ -35,7 +35,7 @@ public interface PromisedValues<T> {
      *
      * @return a new PromisedValues
      */
-    static <T> PromisedValues<T> allOf(List<CompletionStage<T>> cfs) {
+    static <T> PromisedValues<T> allOf(List<? extends CompletionStage<T>> cfs) {
         return PromisedValuesImpl.combineAllOf(cfs);
     }
 
@@ -68,7 +68,7 @@ static <T> PromisedValues<T> allOf(CompletionStage<T> f1, CompletionStage<T> f2)
      *
      * @return a new PromisedValues
      */
-    static <T> PromisedValues allOf(CompletionStage<T> f1, CompletionStage<T> f2, CompletionStage<T> f3) {
+    static <T> PromisedValues<T> allOf(CompletionStage<T> f1, CompletionStage<T> f2, CompletionStage<T> f3) {
         return PromisedValuesImpl.combineAllOf(asList(f1, f2, f3));
     }
 
@@ -87,7 +87,7 @@ static <T> PromisedValues allOf(CompletionStage<T> f1, CompletionStage<T> f2, Co
      *
      * @return a new PromisedValues
      */
-    static <T> PromisedValues allOf(CompletionStage<T> f1, CompletionStage<T> f2, CompletionStage<T> f3, CompletionStage<T> f4) {
+    static <T> PromisedValues<T> allOf(CompletionStage<T> f1, CompletionStage<T> f2, CompletionStage<T> f3, CompletionStage<T> f4) {
         return PromisedValuesImpl.combineAllOf(asList(f1, f2, f3, f4));
     }
 

src/main/java/org/dataloader/impl/PromisedValuesImpl.java

@@ -15,11 +15,11 @@
 
 public class PromisedValuesImpl<T> implements PromisedValues<T> {
 
-    private final List<CompletionStage<T>> futures;
+    private final List<? extends CompletionStage<T>> futures;
     private final CompletionStage<Void> controller;
     private AtomicReference<Throwable> cause;
 
-    private PromisedValuesImpl(List<CompletionStage<T>> cs) {
+    private PromisedValuesImpl(List<? extends CompletionStage<T>> cs) {
         this.futures = nonNull(cs);
         this.cause = new AtomicReference<>();
         List<CompletableFuture> cfs = cs.stream().map(CompletionStage::toCompletableFuture).collect(Collectors.toList());
@@ -36,7 +36,7 @@ private PromisedValuesImpl(PromisedValuesImpl<T> other, CompletionStage<Void> co
         this.controller = controller;
     }
 
-    public static <T> PromisedValues<T> combineAllOf(List<CompletionStage<T>> cfs) {
+    public static <T> PromisedValues<T> combineAllOf(List<? extends CompletionStage<T>> cfs) {
         return new PromisedValuesImpl<>(nonNull(cfs));
     }
 

src/test/java/org/dataloader/impl/PromisedValuesImplTest.java

@@ -2,9 +2,11 @@
 
 import org.junit.Test;
 
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import static java.util.Arrays.asList;
@@ -13,6 +15,7 @@
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.nullValue;
 import static org.junit.Assert.assertThat;
 
@@ -178,5 +181,35 @@ public void exceptions_are_captured_and_reported() throws Exception {
 
         assertThat(promisedValues.toList(), equalTo(asList(1, null)));
         assertThat(result, equalTo(asList(1, null)));
-   }
+    }
+
+    @Test
+    public void type_generics_compile_as_expected() throws Exception {
+
+        PromisedValues<String> pvList = PromisedValues.allOf(Collections.singletonList(new CompletableFuture<String>()));
+        PromisedValues<String> pvList2 = PromisedValues.allOf(Collections.<CompletionStage<String>>singletonList(new CompletableFuture<>()));
+
+        assertThat(pvList, notNullValue());
+        assertThat(pvList2, notNullValue());
+
+        PromisedValues<String> pv2args = PromisedValues.allOf(new CompletableFuture<>(), new CompletableFuture<>());
+        PromisedValues<String> pv3args = PromisedValues.allOf(new CompletableFuture<>(), new CompletableFuture<>(), new CompletableFuture<>());
+        PromisedValues<String> pv4args = PromisedValues.allOf(new CompletableFuture<>(), new CompletableFuture<>(), new CompletableFuture<>(), new CompletableFuture<>());
+
+        assertThat(pv2args, notNullValue());
+        assertThat(pv3args, notNullValue());
+        assertThat(pv4args, notNullValue());
+
+        PromisedValues<String> pvListOfPVs = PromisedValues.allPromisedValues(Arrays.asList(pv2args, pv3args));
+
+        assertThat(pvListOfPVs, notNullValue());
+
+        PromisedValues<String> pv2ArgsOfPVs = PromisedValues.allPromisedValues(pv2args, pv3args);
+        PromisedValues<String> pv3ArgsOfPVs = PromisedValues.allPromisedValues(pv2args, pv3args, pv4args);
+        PromisedValues<String> pv4ArgsOfPVs = PromisedValues.allPromisedValues(pv2args, pv3args, pv4args, pv2args);
+
+        assertThat(pv2ArgsOfPVs, notNullValue());
+        assertThat(pv3ArgsOfPVs, notNullValue());
+        assertThat(pv4ArgsOfPVs, notNullValue());
+    }
 }