public void addDependency(String swcLocation1,String swcLocation2){
  if (swcLocation1 == null || swcLocation2 == null)   throw new NullPointerException();
  String head=swcLocation1;
  String tail=swcLocation2;
  if (!head.equals(tail) && dependencies.containsKey(head) && dependencies.containsKey(tail)&& !dependencies.dependencyExists(head,tail)) {
    dependencies.addDependency(head,tail);
  }
}

public void addDependency ( String swcLocation1 , String swcLocation2 ) { if ( swcLocation1 == null || swcLocation2 == null ) throw new NullPointerException ( ) ; String head = swcLocation1 ; String tail = swcLocation2 ; if ( ! head . equals ( tail ) && dependencies . containsKey ( head ) && dependencies . containsKey ( tail ) && ! dependencies . dependencyExists ( head , tail ) ) { dependencies . addDependency ( head , tail ) ; } }
add dependency that swc1 depends on swc2 .
