add mapPayload(Fn<A,B>) to Response

apollo-api/pom.xml

@@ -69,6 +69,11 @@
             <artifactId>hamcrest-core</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.hamcrest</groupId>
+            <artifactId>hamcrest-library</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>uk.org.lidalia</groupId>
             <artifactId>slf4j-test</artifactId>

apollo-api/src/main/java/com/spotify/apollo/Response.java

@@ -21,6 +21,7 @@
 
 import java.util.Map;
 import java.util.Optional;
+import java.util.function.Function;
 
 import javax.annotation.Nullable;
 
@@ -70,9 +71,25 @@ public interface Response<T> {
    * headers, etc., are copied over. To clear out the payload, one can pass in {@code null}.
    *
    * @param newPayload the new payload
+   * @return A response containing the new payload
    */
   <P> Response<P> withPayload(@Nullable P newPayload);
 
+  /**
+   * Maps the contained payload if it exists, otherwise does nothing except changing the type
+   * of the response.
+   *
+   * @param mapFunction  The function for mapping the payload
+   * @param <P>          The type of the mapped payload
+   * @return A response with a converted payload
+   */
+  default <P> Response<P> mapPayload(Function<? super T, ? extends P> mapFunction) {
+    //noinspection unchecked
+    return !payload().isPresent()
+        ? (Response<P>) this
+        : withPayload(mapFunction.apply(payload().get()));
+  }
+
   /**
    * Returns a typed 200 OK {@link Response}.
    *

apollo-api/src/test/java/com/spotify/apollo/ResponsePayloadMapTest.java

@@ -0,0 +1,67 @@
+/*
+ * -\-\-
+ * Spotify Apollo API Interfaces
+ * --
+ * Copyright (C) 2013 - 2015 Spotify AB
+ * --
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * -/-/-
+ */
+
+package com.spotify.apollo;
+
+import org.junit.experimental.theories.DataPoints;
+import org.junit.experimental.theories.Theories;
+import org.junit.experimental.theories.Theory;
+import org.junit.runner.RunWith;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.Matchers.hasEntry;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
+
+@RunWith(Theories.class)
+public class ResponsePayloadMapTest {
+
+  @DataPoints
+  public static Response<String>[] responses() {
+    Response<String> response = Response.<String>forStatus(Status.IM_A_TEAPOT)
+        .withHeader("foo", "bar");
+
+    //noinspection unchecked
+    return new Response[] {
+        response,
+        response.withPayload("hello")
+
+    };
+  }
+
+  @Theory
+  public void mapsPayload(Response<String> input) throws Exception {
+    Response<Integer> intResponse = input.mapPayload(String::length);
+
+    if (input.payload().isPresent()) {
+      assertThat(intResponse.payload().get().intValue(), is(5));
+    } else {
+      assertFalse(intResponse.payload().isPresent());
+    }
+  }
+
+  @Theory
+  public void preserveStatusAndHeaderOnMap(Response<String> input) throws Exception {
+    Response<Integer> intResponse = input.mapPayload(String::length);
+
+    assertThat(intResponse.status(), is(Status.IM_A_TEAPOT));
+    assertThat(intResponse.headers(), hasEntry("foo", "bar"));
+  }
+}