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()));
+    			}
+    		});
+		}
+    }
+}

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

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+package com.iluwatar;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+
+public class Inventory {
+	
+	private int inventorySize;
+	private List<Item> items;
+	private Lock lock = new ReentrantLock();
+
+	public Inventory(int inventorySize) {
+		this.inventorySize = inventorySize;
+		this.items = new ArrayList<Item>(inventorySize);
+	}
+	
+	public boolean addItem(Item item){
+		if(items.size()<inventorySize){
+			lock.lock();
+			try{
+				if(items.size()<inventorySize){
+					items.add(item);
+					System.out.println(Thread.currentThread());
+					return true;
+				}
+			}finally{
+				lock.unlock();
+			}
+		}
+		return false;
+	}
+
+}

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

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+package com.iluwatar;
+
+public class Item {
+
+}