flatMap to the rescue and some javadocs

core/metrics-core-api/src/main/java/org/hawkular/metrics/core/api/Aggregate.java

@@ -28,19 +28,29 @@
  * @author jay shaughnessy
  */
 public interface Aggregate {
-    Func1<Observable<DataPoint<Double>>, Observable<Double>> Average = data -> {
-        return MathObservable.averageDouble(data.map(DataPoint::getValue));
-    };
 
-    Func1<Observable<DataPoint<Double>>, Observable<Double>> Max = data -> {
-        return MathObservable.max(data.map(DataPoint::getValue));
-    };
+    /**
+     * A function that emits a single item, the average of {@link DataPoint data points} as a double.
+     */
+    Func1<Observable<DataPoint<Double>>, Observable<Double>> Average = data ->
+        MathObservable.averageDouble(data.map(DataPoint::getValue));
 
-    Func1<Observable<DataPoint<Double>>, Observable<Double>> Min = data -> {
-        return MathObservable.min(data.map(DataPoint::getValue));
-    };
+    /**
+     * A function that emits a single item, the max of {@link DataPoint data points} as a double.
+     */
+    Func1<Observable<DataPoint<Double>>, Observable<Double>> Max = data ->
+        MathObservable.max(data.map(DataPoint::getValue));
+
+    /**
+     * A function that emits a single item, the min of {@link DataPoint data points} as a double.
+     */
+    Func1<Observable<DataPoint<Double>>, Observable<Double>> Min = data ->
+        MathObservable.min(data.map(DataPoint::getValue));
+
+    /**
+     * A function that emits a single item, the sum of {@link DataPoint data points} as a double.
+     */
+    Func1<Observable<DataPoint<Double>>, Observable<Double>> Sum = data ->
+        MathObservable.sumDouble(data.map(DataPoint::getValue));
 
-    Func1<Observable<DataPoint<Double>>, Observable<Double>> Sum = data -> {
-        return MathObservable.sumDouble(data.map(DataPoint::getValue));
-    };
 }

core/metrics-core-api/src/main/java/org/hawkular/metrics/core/api/MetricsService.java

@@ -90,15 +90,27 @@ public interface MetricsService {
 
     Observable<Void> addGaugeData(Observable<Metric<Double>> gaugeObservable);
 
+    /**
+     * Fetches data points for a gauge metric.
+     *
+     * @param tenantId The tenant to which the metric belongs
+     * @param id The metric name
+     * @param start The start time inclusive as  aUnix timestamp in milliseconds
+     * @param end The end time exclusive as a Unix timestamp in milliseconds
+     * @return an {@link Observable} that emits {@link DataPoint data points}
+     */
     Observable<DataPoint<Double>> findGaugeData(String tenantId, MetricId id, Long start, Long end);
 
     /**
-     * @param tenantId the tenantId
-     * @param id the metricId
-     * @param start time window start, in ms
-     * @param end time window end, in ms
+     * This method applies one or more functions to an Observable that emits data points of a gauge metric. The data
+     * points Observable is asynchronous. The functions however, are applied serially in the order specified.
+     *
+     * @param tenantId The tenant to which the metric belongs
+     * @param id The metric name
+     * @param start The start time inclusive as a Unix timestamp in milliseconds
+     * @param end The end time exclusive as a Unix timestamp in milliseconds
      * @param funcs one or more functions to operate on the fetched gauge data
-     * @return the <code>Observable</code> emits in the same ordering as <code>funcs<funcs>
+     * @return An {@link Observable} that emits the results with the same ordering as <code>funcs<code>
      * @see Aggregate
      */
     <T> Observable<T> findGaugeData(String tenantId, MetricId id, Long start, Long end,

core/metrics-core-impl/src/main/java/org/hawkular/metrics/core/impl/MetricsServiceImpl.java

@@ -597,15 +597,7 @@ public <T> Observable<T> findGaugeData(String tenantId, MetricId id, Long start,
             Func1<Observable<DataPoint<Double>>, Observable<T>>... funcs) {
 
         Observable<DataPoint<Double>> dataCache = findGaugeData(tenantId, id, start, end).cache();
-        Observable<T> result = null;
-
-        for (Func1<Observable<DataPoint<Double>>, Observable<T>> func : funcs) {
-            result = (null == result) ?
-                    func.call(dataCache) :
-                    result.concatWith(func.call(dataCache));
-        }
-
-        return result;
+        return Observable.from(funcs).flatMap(fn -> fn.call(dataCache));
     }
 
     @Override