double-checked-locking pattern is added.

double-checked-locking/src/main/java/com/iluwatar/App.java

@@ -0,0 +1,30 @@
+package com.iluwatar;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+/**
+ * 
+ * In Inventory we store the items with a given size. However,
+ * we do not store more items than the inventory size. To address
+ * concurrent access problems we use double checked locking to add
+ * item to inventory. In this method, the thread which gets the lock
+ * first adds the item. 
+ */
+
+public class App 
+{
+    public static void main( String[] args )
+    {
+    	final Inventory inventory = new Inventory(1000);
+    	ExecutorService executorService = Executors.newFixedThreadPool(3);
+    	for (int i = 0; i < 3; i++) {
+    		executorService.execute(new Runnable() {
+    			@Override
+    			public void run() {
+    				while(inventory.addItem(new Item()));
+    			}
+    		});
+		}
+    }
+}