extracted distance cache logic into separate class

java/com.sap.sailing.domain.common/src/com/sap/sailing/domain/common/AbstractDistance.java

@@ -1,6 +1,7 @@
 package com.sap.sailing.domain.common;
 
 import com.sap.sailing.domain.common.impl.KilometersPerHourSpeedImpl;
+import com.sap.sailing.domain.common.impl.NauticalMileDistance;
 
 public abstract class AbstractDistance implements Distance {
 
@@ -65,4 +66,9 @@ public abstract class AbstractDistance implements Distance {
     public String toString() {
         return getMeters()+"m";
     }
+    
+    @Override
+    public Distance add(Distance d) {
+        return new NauticalMileDistance(getNauticalMiles()+d.getNauticalMiles());
+    }
 }

java/com.sap.sailing.domain.common/src/com/sap/sailing/domain/common/Distance.java

@@ -64,6 +64,10 @@ public interface Distance extends Comparable<Distance>, Serializable {
             return Speed.NULL;
         }
         
+        @Override
+        public Distance add(Distance d) {
+            return d;
+        }
     };
     
     double getGeographicalMiles();
@@ -88,4 +92,6 @@ public interface Distance extends Comparable<Distance>, Serializable {
      * then the resulting speed will be negative.
      */
     Speed inTime(long milliseconds);
+    
+    Distance add(Distance d);
 }