Cleanup ChangeApi to simplify the code

Reuse a common block of code for creating the RestApi object that will
be used to handle the request. This consolidates the URI formatting
into a single location and makes the code a bit shorter to read.

Fix the formatting on the method Javadocs to be consistent and a
single line of code, these methods aren't complex and only need
one line to describe them.

Only set properties on the Input type if the value != null. This saves
a little bit of JSON formatting to the server. It isn't strictly
necessary, but it makes the example of how to make the REST API calls
as produced by the web UI more clear for tool authors that are trying
to perform the same action and learning by watching the requests.

Change-Id: Icac32a8a81399bd9c0a4d441795bb71c753917a2

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeApi.java

@@ -24,40 +24,31 @@ import com.google.gwtjsonrpc.common.AsyncCallback;
  * changes.
  */
 public class ChangeApi {
-  private static final String URI = "/changes/";
-
-  /**
-   * Sends a REST call to abandon a change and notify a callback. TODO: switch
-   * to use the new id triplet (project~branch~change) once that data is
-   * available to the UI.
-   */
-  public static void abandon(int changeId, String message,
-      AsyncCallback<ChangeInfo> callback) {
+  /** Abandon the change, ending its review. */
+  public static void abandon(int id, String msg, AsyncCallback<ChangeInfo> cb) {
     Input input = Input.create();
-    input.setMessage(emptyToNull(message));
-    new RestApi(URI + changeId + "/abandon").data(input).post(callback);
+    input.message(emptyToNull(msg));
+    api(id, "abandon").data(input).post(cb);
   }
 
-  /**
-   * Sends a REST call to revert a change.
-   */
-  public static void revert(int changeId, String message,
-      AsyncCallback<ChangeInfo> callback) {
+  /** Create a new change that reverts the delta caused by this change. */
+  public static void revert(int id, String msg, AsyncCallback<ChangeInfo> cb) {
     Input input = Input.create();
-    input.setMessage(emptyToNull(message));
-    new RestApi(URI + changeId + "/revert").data(input).post(callback);
+    input.message(emptyToNull(msg));
+    api(id, "revert").data(input).post(cb);
   }
 
+  /** Update the topic of a change. */
   public static void topic(int id, String topic, String msg, AsyncCallback<String> cb) {
     Input input = Input.create();
-    input.setTopic(emptyToNull(topic));
-    input.setMessage(emptyToNull(msg));
-    new RestApi(URI + id + "/topic").data(input).put(NativeString.unwrap(cb));
+    input.topic(emptyToNull(topic));
+    input.message(emptyToNull(msg));
+    api(id, "topic").data(input).put(NativeString.unwrap(cb));
   }
 
   private static class Input extends JavaScriptObject {
-    final native void setTopic(String t) /*-{ this.topic = t; }-*/;
-    final native void setMessage(String m) /*-{ this.message = m; }-*/;
+    final native void topic(String t) /*-{ if(t)this.topic=t; }-*/;
+    final native void message(String m) /*-{ if(m)this.message=m; }-*/;
 
     static Input create() {
       return (Input) JavaScriptObject.createObject();
@@ -67,6 +58,11 @@ public class ChangeApi {
     }
   }
 
+  private static RestApi api(int id, String action) {
+    // TODO Switch to triplet project~branch~id format in URI.
+    return new RestApi("/changes/" + id + "/" + action);
+  }
+
   private static String emptyToNull(String str) {
     return str == null || str.isEmpty() ? null : str;
   }