multithreading - Java CountDownLatch with Threads -

i looking learn using java countdownlatch control execution of thread.

i have 2 classes. 1 called poller , other referendum. threads created in referendum class , run() methods contained in poller class.

in poller , referendum classes have imported java countdown latch via import java.util.concurrent.countdownlatch.

i looking understand why , the *.countdown(); , *.await(); statements need applied , understand if have correctly initialised countdownlatch within poller constructor.

the complete code 2 classes are:

import java.util.concurrent.countdownlatch;  public class poller extends thread { private string id;                  // pollster id private int pollsize;               // number of samples private int numberofpolls;          // number of times perform poll private referendum referendum;      // referendum (implies voting population) private int sampledvotes[];         // counts of votes or against static countdownlatch pollsarecomplete; //the countdownlatch   /**  * constructor polling organisation.  * @param r referendum on poller gathering stats  * @param id name of polling organisation  * @param pollsize size of poll poller use  * @param polltimes number of times poller conduct poll  * @param alatch coutn down latch prevents referendum results being published  */  public poller(referendum r, string id, int pollsize, int polltimes, countdownlatch alatch) {     this.referendum = r;     this.id = id;     this.pollsize = pollsize;     this.numberofpolls = polltimes;     this.pollsarecomplete = alatch;     alatch = new countdownlatch(3);      // , against votes counted     sampledvotes = new int[2]; }   // getter numberofpolls public int getnumberofpolls() {     return numberofpolls; }  @override //to use countdown latch public void run() {           (int = 0; < getnumberofpolls(); i++)     {         resetvotes();         pollvotes();         publishpollresults();     } }  // make sure sampledvotes reset 0 protected void resetvotes() {     // initialise vote counts in poll     (int = 0; < sampledvotes.length; i++)     {         sampledvotes[i] = 0;     } }  // sampling way citizens vote in referendum protected void pollvotes() {     (int n = 0; n < pollsize; n++)     {         citizen c = referendum.pickrandomcitizen();          //as things stand, pickrandomcitizen can return null         //because haven't protected access collection         if (c != null)         {             sampledvotes[c.votefor()]++;         }     } }  protected void publishpollresults() {     int vfor = 100 * sampledvotes[referendum.for] / pollsize;      int vagainst = 100 * sampledvotes[referendum.against] / pollsize;      system.out.printf("according %-20s \t(", this.id + ":");      system.out.print("for " + vfor);      try     {         thread.sleep(1000);     } catch (exception e)     {         e.printstacktrace();     }      system.out.println(", against " + vagainst + ")");  } } 

and

import java.util.linkedlist; import java.util.list; import java.util.concurrent.countdownlatch;     public class referendum { private list<citizen> citizens;         //voters private list<poller> pollers;           //vote samplers public static final int = 0;        //index votes array public static final int against = 1;    //index votes array private int votes[];                    //for , against votes counters   public referendum(int population) {     citizens = new linkedlist<citizen>();     pollers = new linkedlist<poller>();      // initialise referendum population     (int = 0; < population; i++)     {         citizen c = new citizen(i % 4); //suppose equal party membership         citizens.add(c);     }      votes = new int[2]; //in example, or against }  public void addpoller(poller np) {     pollers.add(np); }  public citizen removecitizen(int i) {     return citizens.remove(i); }  public list<poller> getpollers() {     return pollers; }  public void startpollswithlatch() {    //create poller threads use latch     addpoller(new poller(this, "the daily day", 100, 3, poller.pollsarecomplete));     addpoller(new poller(this, "stats people", 100, 3, poller.pollsarecomplete));     addpoller(new poller(this, "tv pundits", 100, 3, poller.pollsarecomplete));      // start polls     (poller p : pollers)     {         p.start();      } }    // pick citizen randomly - access not controlled yet public citizen pickrandomcitizen() {     //todo add code method part (b)      citizen randomcitizen;     // first random index     int index = (int) (math.random() * getpopulationsize());     randomcitizen = citizens.remove(index);      return randomcitizen; }  // counting actual votes cast in referendum public void castvotes() {     (int h = 0; h < getpopulationsize(); h++)     {         citizen c = citizens.get(h);          votes[c.votefor()]++;     } }  // tell size of population public int getpopulationsize() {     return citizens.size(); }  // display referendum results public void revealresults() {     system.out.println(" **** referendum results out! ****");      system.out.println("for");     system.out.printf("\t %.2f %%\n", 100.0 * votes[for] / getpopulationsize());      system.out.println("against");     system.out.printf("\t %.2f %%\n", 100.0 * votes[against] / getpopulationsize()); }  public static void main(string[] args) {     // initialise referendum. number of people     // has been made smaller here reduce simulation time.     referendum r = new referendum(50000);       r.startpollswithlatch();      r.castvotes();      // reveal results of referendum     r.revealresults();  } } 

in nutshell... threads must execute publishpollresults(); statement before revealresults(); executed.

ok,

now, if publishpollresults must done before reavelresults, need wait proper count in reaveal method. so, latch must shared referendum object not pollers.

so, let referendum creates latch , pass pollers:

public class referendum {   countdownlatch pollsarecomplete;   ...      public void startpollswithlatch()     {         pollsarecomplete = new countdownlatch(3); //create new latch know when voting done        //create poller threads use latch         addpoller(new poller(this, "the daily day", 100, 3, pollsarecomplete)); //pass pollers         addpoller(new poller(this, "stats people", 100, 3, pollsarecomplete));         addpoller(new poller(this, "tv pundits", 100, 3, pollsarecomplete));          // start polls         (poller p : pollers)         {             p.start();         }     }      public void revealresults()     {      pollsarecomplete.await(); //we can pass line if latch count went 0      system.out.println(" **** referendum results out! ****");     ....     }  } 

so pollers should share latch. using static variable okish want able use pollers different referendums. betther, have instance field , pass in constructor (you kind of started constructor passed value static variable makes no sense (and actaully null).

public class poller extends thread {     ...     private countdownlatch pollsarecomplete; //the countdownlatch shared referendum      public poller(referendum r, string id, int pollsize, int polltimes, countdownlatch alatch)     {         ...         this.pollsarecomplete = alatch;     }      public void run()     {               (int = 0; < getnumberofpolls(); i++)         {             resetvotes();             pollvotes();             publishpollresults();         }         pollsarecomplete.countdown(); //voting finished, let referendum publish results.     }  } 

so once poller finished work lowers latch, , when referendum can continue , print results.

mind poller thread publish results 3 times (as have loop) , when 3 cycles down signal referendum.

if wanted 3 separate phases of referendum difficult achieve if latch cannot reset once it's been down 0.