changed states-set to ConcurrentHashSet in JavaTemplate

src/main/resources/de/hhu/stups/codegenerator/JavaTemplate.stg

@@ -15,6 +15,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.Future;
 import java.util.concurrent.Executors;
@@ -1187,7 +1188,7 @@ private static class ModelChecker {
     private final boolean isDebug;
 
     private final LinkedList\<<machine>\> unvisitedStates = new LinkedList\<>();
-    private final Set\<<machine>\> states = new HashSet\<>();
+    private final Set\<<machine>\> states = ConcurrentHashMap.newKeySet();
     private AtomicInteger transitions = new AtomicInteger(0);
     private ThreadPoolExecutor threadPool;
     private Object waitLock = new Object();
@@ -1485,9 +1486,7 @@ private void modelCheckMultiThreaded() {
             Set\<<machine>\> nextStates = generateNextStates(state);
 
             nextStates.forEach(nextState -> {
-                synchronized (states) {
-                    if(!states.contains(nextState)) {
-                        states.add(nextState);
+                if(states.add(nextState)) {
                     synchronized (unvisitedStates) {
                         unvisitedStates.add(nextState);
                     }
@@ -1497,7 +1496,6 @@ private void modelCheckMultiThreaded() {
                         System.out.println("-------------------");
                     }
                 }
-                }
             });
 
             synchronized (unvisitedStates) {