public static boolean validate(Subscription current,Subscription next){
  Objects.requireNonNull(next,"Subscription cannot be null");
  if (current != null) {
    next.cancel();
    return false;
  }
  return true;
}

public static boolean validate ( Subscription current , Subscription next ) { Objects . requireNonNull ( next , "Subscription cannot be null" ) ; if ( current != null ) { next . cancel ( ) ; return false ; } return true ; }
check subscription current state and cancel new subscription if different null , returning true if ready to subscribe .
