Add Optional version of projecting Behaviour

Change-Id: I3d3411763be1a75a2bdb04fb59c70d9779f1b02a

apps/optical-model/src/main/java/org/onosproject/net/optical/device/OpticalDeviceServiceView.java

@@ -73,13 +73,7 @@ public class OpticalDeviceServiceView
         = CacheBuilder.newBuilder()
             .weakKeys() // == for Key comparison
             .maximumSize(100)
-            .build(CacheLoader.from(elm -> {
+            .build(CacheLoader.from(elm -> elm.project(OpticalDevice.class)));
-                if (elm.is(OpticalDevice.class)) {
-                    return Optional.of(elm.as(OpticalDevice.class));
-                } else {
-                    return Optional.empty();
-                }
-            }));
 
     // Not intended to be instantiated directly
     protected OpticalDeviceServiceView(DeviceService base) {

core/api/src/main/java/org/onosproject/net/driver/Projectable.java

@@ -16,6 +16,8 @@
 
 package org.onosproject.net.driver;
 
+import java.util.Optional;
+
 import com.google.common.annotations.Beta;
 
 /**
@@ -46,4 +48,20 @@ public interface Projectable {
      */
     <B extends Behaviour> boolean is(Class<B> projectionClass);
 
+    /**
+     * Returns the specified projection of this entity if such projection
+     * is supported.
+     *
+     * @param projectionClass requested projection class
+     * @param <B> type of behaviour
+     * @return projection instance
+     */
+    default <B extends Behaviour> Optional<B> project(Class<B> projectionClass) {
+        if (is(projectionClass)) {
+            return Optional.of(as(projectionClass));
+        } else {
+            return Optional.empty();
+        }
+    }
+
 }