multithreading - How to execute dependent tasks in Java 8 without any blocking -

not long ago answered question: executing dependent tasks in parallel in java using future.get() blocking current thread, , there possibility thread pool runs out of threads if many gets() called @ 1 time. how 1 compose futures futures in java?

i thought answer question myself, 1 can use completablefutures in java instead of futures. completablefutures allow composition via thencombine method, similiar scalas flatmap. there no blocking happening , 3 threads needed achieve fastest time.

import java.util.concurrent.completablefuture; import java.util.concurrent.executorservice; import java.util.concurrent.executors; import java.util.function.bifunction; import java.util.function.supplier;  public class barrista {      // number of threads used in executor     static final int nothreads = 3;      // time of each task     static final int heatwater = 1000;     static final int grindbeans = 1000;     static final int frothmilk = 1000;     static final int brewing = 1000;     static final int combine = 1000;      // method simulate work (pause current thread without throwing checked exception)     public static void pause(long t)     {         try         {             thread.sleep(t);         }         catch(exception e)         {             throw new error(e.tostring());         }     }      // task heat water     static class heatwater implements supplier<string>     {         @override         public string get()         {             system.out.println("heating water");             pause(heatwater);             return "hot water";         }     }      // task grind beans     static class grindbeans implements supplier<string>     {         @override         public string get()         {             system.out.println("grinding beans");             pause(grindbeans);             return "grinded beans";         }     }      // task froth milk     static class frothmilk implements supplier<string>     {         @override         public string get()         {             system.out.println("frothing milk");             pause(frothmilk);             return "some milk";         }     }      // task brew coffee     static class brew implements bifunction<string,string, string>     {         @override         public string apply(string groundbeans, string heatedwater)         {             system.out.println("brewing coffee " + groundbeans + " , " + heatedwater);             pause(brewing);             return "brewed coffee";         }     }      // task combine brewed coffee , milk     static class combine implements bifunction<string,string, string>     {         @override         public string apply(string frothedmilk, string brewedcoffee)         {             system.out.println("combining " + frothedmilk + " "+ brewedcoffee);             pause(combine);             return "final coffee";         }     }      public static void main(string[] args)     {         executorservice executor = executors.newfixedthreadpool(nothreads);          long starttime = system.currenttimemillis();          try         {             // create tasks , let executor handle execution order             completablefuture<string> frothmilk =       completablefuture.supplyasync(new frothmilk(), executor);             completablefuture<string> heatwaterfuture = completablefuture.supplyasync(new heatwater(), executor);             completablefuture<string> grindbeans =      completablefuture.supplyasync(new grindbeans(), executor);              completablefuture<string> brew = heatwaterfuture.thencombine(grindbeans, new brew());             completablefuture<string> coffee =          brew.thencombine(frothmilk,  new combine());              // final coffee             system.out.println("here coffee:" + coffee.get());              // analyzing times:             system.out.println("\n\n");             system.out.println("actual time: \t\t\t\t" + (system.currenttimemillis() - starttime)/1000.0);              // compute quickest possible time:             long path1 = math.max(grindbeans, heatwater)+ brewing + combine;             long path2 = frothmilk + combine;             system.out.println("quickest time multi-threaded:\t\t" + math.max(path1, path2)/1000.0);              // compute longest possible time:             long longesttime = heatwater + grindbeans + frothmilk + brewing + combine;             system.out.println("quickest time single-threaded thread:\t" + longesttime/1000.0);         }         catch (exception e)         {             e.printstacktrace();         }                 {             executor.shutdown();         }      } }