Update creating mechanism.

README.md

@@ -239,7 +239,14 @@
 * many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid "polluting" their classes with these operations. Visitor lets you keep related operations together by defining them in one class. When the object structure is shared by many applications, use Visitor to put operations in just those applications that need them
 * the classes defining the object structure rarely change, but you often want to define new operations over the structure. Changing the object structure classes requires redefining the interface to all visitors, which is potentially costly. If the object structure classes change often, then it's probably better to define the operations in those classes
 
+##Double Checked Locking
+**Intent:** Reduce the overhead of acquiring a lock by first testing the locking criterion (the "lock hint") without actually acquiring the lock. Only if the locking criterion check indicates that locking is required does the actual locking logic proceed.
 
+![alt text](https://github.com/yusufaytas/java-design-patterns/blob/master/double-checked-locking/etc/double_checked_locking.jpeg "Double Checked Locking")
+
+**Applicability:** Use the Double Checked Locking pattern when
+* there is a concurrent access in object creation, e.g. singleton, where you want to create single instance of the same class and checking if it's null or not maybe not be enough when there are two or more threads that checks if instance is null or not.
+* there is a concurrent access on a method where method's behaviour changes according to the some constraints and these constraint change within this method.
 
 # Frequently asked questions
 

double-checked-locking/pom.xml

@@ -0,0 +1,17 @@
+<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>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>3.8.1</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</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 +1,6 @@
+package com.iluwatar;
+
+public class Item {
+	String name;
+	int level;
+}

pom.xml

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

singleton/src/main/java/com/iluwatar/IvoryTower.java

@@ -9,8 +9,7 @@ public class IvoryTower {
 
 	private static IvoryTower instance = new IvoryTower();
 
-	private IvoryTower() {
-	}
+	private IvoryTower() {}
 
 	public static IvoryTower getInstance() {
 		return instance;