#554: Subclass Sandbox pattern (#998)

* Fix issue #761: ThreadSafeDoubleCheckLocking.java: Instantiating by Reflection call will be successful if you do that firstly

* Create leader election module

* Create Interface of Instance and MessageManager

* Create implementations with token ring algorithm

* Change package structure.
Create basic message system.

* Implement heartbeat and heartbeat invoking message system

* Implement election message handler

* Add leader message handler

* Add main entry point

* Add comments

* Update README.md

* Fix checkstyle issue

* Add Unit Tests

* Add Unit Tests

* Add bully leader selection

* Change System.out to log print.
Add MIT license in each file.

* Add More java doc comments

* Add unit test

* Add unit tests

* Add subclass-sandbox

* Add Unit Test

* Add Unit Test

* Fix Typo

* Move dependency into parent pom.xml

* Change local valuable reference to be var

subclass-sandbox/README.md

@@ -0,0 +1,29 @@
+
+---  
+layout: pattern  
+title: Subclass Sandbox 
+folder:  subclass-sandbox  
+permalink: /patterns/subclass-sandbox/  
+categories: Other  
+tags:  
+ - Java  
+ - Difficulty-Beginner  
+---  
+  
+## Intent  
+The subclass sandbox pattern describes a basic idea, while not having a lot of detailed mechanics. You will need the pattern when you have several similar subclasses. If you have to make a tiny change, then change the base class, while all subclasses shouldn't have to be touched. So the base class has to be able to provide all of the operations a derived class needs to perform.
+  
+## Applicability  
+The Subclass Sandbox pattern is a very simple, common pattern lurking in lots of codebases, even outside of games. If you have a non-virtual protected method laying around, you’re probably already using something like this. Subclass Sandbox is a good fit when:
+
+-   You have a base class with a number of derived classes.
+    
+-   The base class is able to provide all of the operations that a derived class may need to perform.
+    
+-   There is behavioral overlap in the subclasses and you want to make it easier to share code between them.
+    
+-   You want to minimize coupling between those derived classes and the rest of the program.
+  
+## Credits  
+  
+* [Game Programming Patterns - Subclass Sandbox]([http://gameprogrammingpatterns.com/subclass-sandbox.html](http://gameprogrammingpatterns.com/subclass-sandbox.html))