using interface type in variable definition

src/main/java/it/unibz/inf/isochrone/algorithm/Isochrone.java

@@ -12,6 +12,7 @@ import it.unibz.inf.isochrone.network.NodeConnection;
 import it.unibz.inf.isochrone.util.EnumContainer.Direction;
 import it.unibz.inf.isochrone.util.Query;
 
+import java.util.AbstractQueue;
 import java.util.Calendar;
 import java.util.Collection;
 import java.util.Collections;
@@ -33,7 +34,7 @@ import java.util.Set;
 public abstract class Isochrone {
 	private static final double COMPARE_PRECISION = 0.0000001d;
 	private final Collection<Integer> dateCodes;
-	private final PriorityQueue<Node> nodeQueue;
+	private final AbstractQueue<Node> nodeQueue;
 	private final long qDuration;
 	private final boolean qIsExpiring;
 	private final boolean qIsIncoming;