Minor changes around ProtectionConstraint

- use factory
- implement toString

Change-Id: Ic30b2d3a7c2f6c65fac9b0971291d82af8ec23d7

@@ -25,7 +25,7 @@ import org.onosproject.net.intent.Constraint;
 import org.onosproject.net.intent.Intent;
 import org.onosproject.net.intent.IntentService;
 import org.onosproject.net.intent.PointToPointIntent;
-import org.onosproject.net.intent.constraint.ProtectionConstraint;
+import static org.onosproject.net.intent.constraint.ProtectionConstraint.protection;
 
 import java.util.List;
 
@@ -65,7 +65,7 @@ public class AddPointToPointIntentCommand extends ConnectivityIntentCommand {
 
         List<Constraint> constraints = buildConstraints();
         if (backup) {
-            constraints.add(new ProtectionConstraint());
+            constraints.add(protection());
         }
 
         Intent intent = PointToPointIntent.builder()

@@ -29,6 +29,8 @@ import org.onosproject.net.intent.ResourceContext;
  */
 @Beta
 public class ProtectionConstraint implements Constraint {
+    private static final ProtectionConstraint PROTECTION_CONSTRAINT = new ProtectionConstraint();
+
     // doesn't use LinkResourceService
     @Override
     public double cost(Link link, ResourceContext context) {
@@ -55,4 +57,21 @@ public class ProtectionConstraint implements Constraint {
         }
         return false;
     }
+
+    /**
+     * Returns protection constraint.
+     *
+     * @return
+     */
+    public static ProtectionConstraint protection() {
+        return PROTECTION_CONSTRAINT;
+    }
+
+    protected  ProtectionConstraint() {
+    }
+
+    @Override
+    public String toString() {
+        return "Protection";
+    }
 }