created a singleton to be used instead of using wrapper classes around the constants used by BreinEngine

Author: pmeisen

src/com/brein/api/Brein.java

@@ -10,18 +10,13 @@
 
 public class Brein {
     private BreinConfig config;
-    private BreinEngine engine;
 
     /**
      * Sets the configuration
      *
      * @param breinConfig config object
      */
     public Brein setConfig(final BreinConfig breinConfig) {
-        if (this.engine != null) {
-            shutdown();
-        }
-
         this.config = breinConfig;
         return this;
     }
@@ -33,7 +28,7 @@ public Brein setConfig(final BreinConfig breinConfig) {
      * This request is asynchronous.
      */
     public void activity(final BreinActivity data, final Consumer<BreinResult> callback) {
-        getEngine().invokeAsync(this.config, data, callback);
+        BreinEngine.instance().invokeAsync(this.config, data, callback);
     }
 
     /**
@@ -46,7 +41,7 @@ public void activity(final BreinActivity data, final Consumer<BreinResult> callb
      * @return response from request wrapped in an object called BreinResponse
      */
     public BreinResult lookup(final BreinLookup data) {
-        return getEngine().invoke(this.config, data);
+        return BreinEngine.instance().invoke(this.config, data);
     }
 
     /**
@@ -58,7 +53,7 @@ public BreinResult lookup(final BreinLookup data) {
      * @return result from the Breinify engine
      */
     public BreinTemporalDataResult temporalData(final BreinTemporalData data) {
-        final BreinResult result = getEngine().invoke(this.config, data);
+        final BreinResult result = BreinEngine.instance().invoke(this.config, data);
         return new BreinTemporalDataResult(result.getMap());
     }
 
@@ -73,25 +68,14 @@ public BreinTemporalDataResult temporalData(final BreinTemporalData data) {
      * @return the recommended items
      */
     public BreinRecommendationResult recommendation(final BreinRecommendation data) {
-        final BreinResult result = getEngine().invoke(this.config, data);
+        final BreinResult result = BreinEngine.instance().invoke(this.config, data);
         return new BreinRecommendationResult(result.getMap());
     }
 
     /**
      * Shutdown Breinify services
      */
     public void shutdown() {
-        if (this.engine != null) {
-            this.engine.terminate();
-            this.engine = null;
-        }
-    }
-
-    public BreinEngine getEngine() {
-        if (this.engine == null) {
-            this.engine = new BreinEngine();
-        }
-
-        return engine;
+        BreinEngine.instance().terminate();
     }
 }

src/com/brein/engine/BreinEngine.java

@@ -18,6 +18,21 @@ public class BreinEngine {
     private static final Map<BreinEngineType, IRestEngine> ENGINES = new ConcurrentHashMap<>();
     private static final Lock ENGINES_LOCK = new ReentrantLock();
 
+    private static class SingletonHolder {
+        static final BreinEngine INSTANCE = new BreinEngine();
+    }
+
+    /**
+     * The {@code BreinEngine} class is a singleton, which can actually be accessed by this method. Creating a new
+     * instance of a {@code BreinEngine} does not actually make any differences, since all attributes are constants,
+     * i.e., {@code static} and {@code final}.
+     *
+     * @return the singleton instance of the {@code BreinEngine}
+     */
+    public static BreinEngine instance() {
+        return SingletonHolder.INSTANCE;
+    }
+
     public BreinResult invoke(final BreinConfig config, final BreinBase data) {
         return getEngine(config).invokeRequest(config, data);
     }