How do I make my ArrayList Thread-Safe Another approach to problem in Java

Question

I have an ArrayList that I want to use to hold RaceCar objects that extend the Thread class as soon as they are finished executing  A class  called Race  handles this ArrayList using a callback method that the RaceCar object calls when it is finished executing  The callback method  addFinisher RaceCar finisher   adds the RaceCar object to the ArrayList  This is supposed to give the order in which the Threads finish executing   I know that ArrayList isn t synchronized and thus isn t thread-safe  I tried using the Collections synchronizedCollection c Collection  method by passing in a new ArrayList and assigning the returned Collection to an ArrayList  However  this gives me a compiler error   Race java 41  incompatible types found     java util Collection required  java util ArrayList finishingOrder   Collections synchronizedCollection new ArrayList numberOfRaceCars      Here is the relevant code   public class Race implements RaceListener       private Thread   racers      private ArrayList finishingOrder         Make an ArrayList to hold RaceCar objects to determine winners     finishingOrder   Collections synchronizedCollection new ArrayList numberOfRaceCars           Fill array with RaceCar objects     for int i 0  i lt numberOfRaceCars  i          racers i    new RaceCar laps  inputs i               Add this as a RaceListener to each RaceCar           RaceCar  racers i   addRaceListener this                Implement the one method in the RaceListener interface     public void addFinisher RaceCar finisher            finishingOrder add finisher           What I need to know is  am I using a correct approach and if not  what should I use to make my code thread-safe  Thanks for the help

User · Answer

Change   private ArrayList finishingOrder     Make an ArrayList to hold RaceCar objects to determine winners finishingOrder   Collections synchronizedCollection new ArrayList numberOfRaceCars    to  private List finishingOrder     Make an ArrayList to hold RaceCar objects to determine winners finishingOrder   Collections synchronizedList new ArrayList numberOfRaceCars    List is a supertype of ArrayList so you need to specify that   Otherwise  what you re doing seems fine  Other option is you can use Vector  which is synchronized  but this is probably what I would do

User · Answer

You can also use as Vector instead  as vectors are thread safe and arraylist are not  Though vectors are old but they can solve your purpose easily   But you can make your Arraylist synchronized like code given this   Collections synchronizedList new ArrayList numberOfRaceCars

User · Answer

You might be using the wrong approach  Just because one thread that simulates a car finishes before another car-simulation thread doesn t mean that the first thread should win the simulated race   It depends a lot on your application  but it might be better to have one thread that computes the state of all cars at small time intervals until the race is complete  Or  if you prefer to use multiple threads  you might have each car record the  simulated  time it took to complete the race  and choose the winner as the one with shortest time

User · Answer

Whenever you want to use ant thread safe version of ant collection object take help of java util concurrent   package  It has almost all concurrent version of unsynchronized collection objects        eg  for ArrayList  you have java util concurrent CopyOnWriteArrayList  You can do Collections synchronizedCollection any collection object  but remember this classical synchr  technique is expensive and comes with performence overhead  java util concurrent   package is less expensive and manage the performance in better way by using mechanisms like      copy-on-write compare-and-swap Lock snapshot iterators etc    So Prefer something from java util concurrent   package

User · Answer

You can change from ArrayList to Vector type  in which every method is synchronized    private Vector finishingOrder    Make a Vector to hold RaceCar objects to determine winners finishingOrder   new Vector numberOfRaceCars

User · Answer

You can also use synchronized keyword for addFinisher method like this        Implement the one method in the RaceListener interface     public synchronized void addFinisher RaceCar finisher            finishingOrder add finisher           So you can use ArrayList add method thread-safe with this way

User · Answer

CopyOnWriteArrayList  Use CopyOnWriteArrayList class  This is the thread safe version of ArrayList

User · Answer

Use Collections synchronizedList     Ex   Collections synchronizedList new ArrayList lt YourClassNameHere gt

[java] How do I make my ArrayList Thread-Safe? Another approach to problem in Java?

Examples related to java

Examples related to multithreading

Examples related to collections

Examples related to synchronization

Examples related to arraylist