fixed matrixk2 test

src/main/java/de/dhbwstuttgart/typeinference/unify/model/PlaceholderType.java

@@ -1,13 +1,10 @@
 package de.dhbwstuttgart.typeinference.unify.model;
 
-import java.io.File;
-import java.io.FileWriter;
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Random;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 import de.dhbwstuttgart.typeinference.unify.distributeVariance;
 import de.dhbwstuttgart.typeinference.unify.interfaces.IFiniteClosure;
@@ -23,7 +20,7 @@ public final class PlaceholderType extends UnifyType{
 	 * Static list containing the names of all existing placeholders.
 	 * Used for generating fresh placeholders.
 	 */
-	public static final ArrayList<String> EXISTING_PLACEHOLDERS = new ArrayList<String>();
+	public static final Set<String> EXISTING_PLACEHOLDERS = ConcurrentHashMap.newKeySet();
 	
 	/**
 	 * Prefix of auto-generated placeholder names.
@@ -96,7 +93,7 @@ public final class PlaceholderType extends UnifyType{
 	 * A user could later instantiate a type using the same name that is equivalent to this type.
 	 * @return A fresh placeholder type.
 	 */
-	public synchronized static PlaceholderType freshPlaceholder() {
+	public static PlaceholderType freshPlaceholder() {
 		String name = nextName + (char) (rnd.nextInt(22) + 97); // Returns random char between 'a' and 'z'
 		// Add random chars while the name is in use.
 		while(EXISTING_PLACEHOLDERS.contains(name)) {