I'm happy to submit a PR for this
Missing synchronized keyword
Hi,

https://github.com/iluwatar/java-design-patterns/blob/master/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java#L58 - the method should also be `synchronized`.

Best regards,
Martin
object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
diff --git a/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java b/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
index df6e887b86f1..e76d003521a5 100644
--- a/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
+++ b/object-pool/src/main/java/com/iluwatar/object/pool/ObjectPool.java
@@ -55,7 +55,7 @@ public synchronized void checkIn(T instance) {
   }
 
   @Override
-  public String toString() {
+  public synchronized String toString() {
     return String.format("Pool available=%d inUse=%d", available.size(), inUse.size());
   }
 }
