The pattern Role object (#1031)

* init repo for role object

* add to init

*  add to init

*  add first impl

*  add pattern

*  add license

*  add changes

role-object/README.md

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+---
+layout: pattern
+title: Role object
+folder: Migration
+permalink: /patterns/role-object/
+categories: Structural
+tags:
+ - Java
+ - Difficulty-Medium
+ - Handle Body Pattern
+---
+
+## Also known as
+Post pattern, Extension Object pattern
+
+## Intent
+Adapt an object to different client’s needs through transparently attached role objects, each one representing a role
+the object has to play in that client’s context. The object manages its role set dynamically. By representing roles as
+individual objects, different contexts are kept separate and system configuration is simplified.
+
+## Applicability
+Use the Role Object pattern, if:
+- you want to handle a key abstraction in different contexts and you do not want to put the resulting context specific interfaces into the same class interface.
+- you want to handle the available roles dynamically so that they can be attached and removed on demand, that is at runtime, rather than fixing them statically at compile-time.
+- you want to treat the extensions transparently and need to preserve the logical object identity of the resultingobject conglomerate.
+- you want to keep role/client pairs independent from each other so that changes to a role do not affect clients that are not interested in that role.
+
+## Credits
+- [Hillside - Role object pattern](https://hillside.net/plop/plop97/Proceedings/riehle.pdf)
+- [Role object](http://wiki.c2.com/?RoleObject)
+- [Fowler - Dealing with roles](https://martinfowler.com/apsupp/roles.pdf)