all index.md files renamed to README.md for more compatibility with github

callback/README.md

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+---
+layout: pattern
+title: Callback
+folder: callback
+permalink: /patterns/callback/
+categories: Other
+tags:
+ - Java
+ - Difficulty-Beginner
+ - Functional
+ - Idiom
+---
+
+## Intent
+Callback is a piece of executable code that is passed as an
+argument to other code, which is expected to call back (execute) the argument
+at some convenient time.
+
+![alt text](./etc/callback.png "Callback")
+
+## Applicability
+Use the Callback pattern when
+
+* when some arbitrary synchronous or asynchronous action must be performed after execution of some defined activity.
+
+## Real world examples
+
+* [CyclicBarrier] (http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept callback that will be triggered every time when barrier is tripped.