double-checked-locking pattern is added.

double-checked-locking/pom.xml

@@ -0,0 +1,9 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>double-checked-locking</artifactId>
+</project>

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

@@ -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;
+	}
+
+}

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

@@ -0,0 +1,5 @@
+package com.iluwatar;
+
+public class Item {
+
+}

pom.xml

@@ -41,6 +41,7 @@
     <module>strategy</module>
     <module>template-method</module>
     <module>visitor</module>
+    <module>double-checked-locking</module>
   </modules>
   
 <build>