Fix for older jdk versions

src/main/java/fr/ntr/Main.java

@@ -1,5 +1,7 @@
 package fr.ntr;
 
+import fr.ntr.scheduler.MaxSNR;
+import fr.ntr.scheduler.ProportionalFair;
 import fr.ntr.scheduler.RoundRobin;
 import fr.ntr.scheduler.Scheduler;
 
@@ -13,6 +15,7 @@ import java.util.Objects;
 import java.util.Random;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
 
 public class Main {
 
@@ -100,7 +103,15 @@ public class Main {
                     System.exit(1);
                 }
             }
-            executor.close();
+            executor.shutdown();
+            try {
+                executor.awaitTermination(2, TimeUnit.MINUTES);
+            }
+            catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            executor.shutdownNow();
+
             System.out.println("Executor closed");
         }
         else {