double-checked-locking pattern is added.

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

@@ -0,0 +1,36 @@
+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;
+	}
+
+}