#590 Add explanation for Chain of Responsibility

chain/README.md

@@ -16,7 +16,131 @@ Avoid coupling the sender of a request to its receiver by giving
 more than one object a chance to handle the request. Chain the receiving
 objects and pass the request along the chain until an object handles it.
 
-![alt text](./etc/chain_1.png "Chain of Responsibility")
+## Explanation
+
+Real world example
+
+> The Orc King gives loud orders to his army. The closest one to react is the commander, then officer and then soldier. The commander, officer and soldier here form a chain of responsibility.
+
+In plain words
+
+> It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.
+
+Wikipedia says
+
+> In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.
+
+**Programmatic Example**
+
+Translating our example with orcs from above. First we have the request class
+
+```
+public class Request {
+
+  private final RequestType requestType;
+  private final String requestDescription;
+  private boolean handled;
+
+  public Request(final RequestType requestType, final String requestDescription) {
+    this.requestType = Objects.requireNonNull(requestType);
+    this.requestDescription = Objects.requireNonNull(requestDescription);
+  }
+
+  public String getRequestDescription() { return requestDescription; }
+
+  public RequestType getRequestType() { return requestType; }
+
+  public void markHandled() { this.handled = true; }
+
+  public boolean isHandled() { return this.handled; }
+
+  @Override
+  public String toString() { return getRequestDescription(); }
+}
+
+public enum RequestType {
+  DEFEND_CASTLE, TORTURE_PRISONER, COLLECT_TAX
+}
+```
+
+Then the request handler hierarchy
+
+```
+public abstract class RequestHandler {
+  private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
+  private RequestHandler next;
+
+  public RequestHandler(RequestHandler next) {
+    this.next = next;
+  }
+
+  public void handleRequest(Request req) {
+    if (next != null) {
+      next.handleRequest(req);
+    }
+  }
+
+  protected void printHandling(Request req) {
+    LOGGER.info("{} handling request \"{}\"", this, req);
+  }
+
+  @Override
+  public abstract String toString();
+}
+
+public class OrcCommander extends RequestHandler {
+  public OrcCommander(RequestHandler handler) {
+    super(handler);
+  }
+
+  @Override
+  public void handleRequest(Request req) {
+    if (req.getRequestType().equals(RequestType.DEFEND_CASTLE)) {
+      printHandling(req);
+      req.markHandled();
+    } else {
+      super.handleRequest(req);
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "Orc commander";
+  }
+}
+
+// OrcOfficer and OrcSoldier are defined similarly as OrcCommander
+
+```
+
+Then we have the Orc King who gives the orders and forms the chain
+
+```
+public class OrcKing {
+  RequestHandler chain;
+
+  public OrcKing() {
+    buildChain();
+  }
+
+  private void buildChain() {
+    chain = new OrcCommander(new OrcOfficer(new OrcSoldier(null)));
+  }
+
+  public void makeRequest(Request req) {
+    chain.handleRequest(req);
+  }
+}
+```
+
+Then it is used as follows
+
+```
+OrcKing king = new OrcKing();
+king.makeRequest(new Request(RequestType.DEFEND_CASTLE, "defend castle")); // Orc commander handling request "defend castle"
+king.makeRequest(new Request(RequestType.TORTURE_PRISONER, "torture prisoner")); // Orc officer handling request "torture prisoner"
+king.makeRequest(new Request(RequestType.COLLECT_TAX, "collect tax")); // Orc soldier handling request "collect tax"
+```
 
 ## Applicability
 Use Chain of Responsibility when