A very long Java method

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: lawrence@krubner.com

A common rule in programming is to keep methods as small as possible, so I am surprised to see this method in a seemingly well-run project such as ZooKeeper:

       private void processEvent(Object event) {
          try {
              if (event instanceof WatcherSetEventPair) {
                  // each watcher will process the event
                  WatcherSetEventPair pair = (WatcherSetEventPair) event;
                  for (Watcher watcher : pair.watchers) {
                      try {
                          watcher.process(pair.event);
                      } catch (Throwable t) {
                          LOG.error("Error while calling watcher ", t);
                      }
                  }
                } else if (event instanceof LocalCallback) {
                    LocalCallback lcb = (LocalCallback) event;
                    if (lcb.cb instanceof StatCallback) {
                        ((StatCallback) lcb.cb).processResult(lcb.rc, lcb.path,
                                lcb.ctx, null);
                    } else if (lcb.cb instanceof DataCallback) {
                        ((DataCallback) lcb.cb).processResult(lcb.rc, lcb.path,
                                lcb.ctx, null, null);
                    } else if (lcb.cb instanceof ACLCallback) {
                        ((ACLCallback) lcb.cb).processResult(lcb.rc, lcb.path,
                                lcb.ctx, null, null);
                    } else if (lcb.cb instanceof ChildrenCallback) {
                        ((ChildrenCallback) lcb.cb).processResult(lcb.rc,
                                lcb.path, lcb.ctx, null);
                    } else if (lcb.cb instanceof Children2Callback) {
                        ((Children2Callback) lcb.cb).processResult(lcb.rc,
                                lcb.path, lcb.ctx, null, null);
                    } else if (lcb.cb instanceof StringCallback) {
                        ((StringCallback) lcb.cb).processResult(lcb.rc,
                                lcb.path, lcb.ctx, null);
                    } else {
                        ((VoidCallback) lcb.cb).processResult(lcb.rc, lcb.path,
                                lcb.ctx);
                    }
                } else {
                  Packet p = (Packet) event;
                  int rc = 0;
                  String clientPath = p.clientPath;
                  if (p.replyHeader.getErr() != 0) {
                      rc = p.replyHeader.getErr();
                  }
                  if (p.cb == null) {
                      LOG.warn("Somehow a null cb got to EventThread!");
                  } else if (p.response instanceof ExistsResponse
                          || p.response instanceof SetDataResponse
                          || p.response instanceof SetACLResponse) {
                      StatCallback cb = (StatCallback) p.cb;
                      if (rc == 0) {
                          if (p.response instanceof ExistsResponse) {
                              cb.processResult(rc, clientPath, p.ctx,
                                      ((ExistsResponse) p.response)
                                              .getStat());
                          } else if (p.response instanceof SetDataResponse) {
                              cb.processResult(rc, clientPath, p.ctx,
                                      ((SetDataResponse) p.response)
                                              .getStat());
                          } else if (p.response instanceof SetACLResponse) {
                              cb.processResult(rc, clientPath, p.ctx,
                                      ((SetACLResponse) p.response)
                                              .getStat());
                          }
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null);
                      }
                  } else if (p.response instanceof GetDataResponse) {
                      DataCallback cb = (DataCallback) p.cb;
                      GetDataResponse rsp = (GetDataResponse) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx, rsp
                                  .getData(), rsp.getStat());
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null,
                                  null);
                      }
                  } else if (p.response instanceof GetACLResponse) {
                      ACLCallback cb = (ACLCallback) p.cb;
                      GetACLResponse rsp = (GetACLResponse) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx, rsp
                                  .getAcl(), rsp.getStat());
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null,
                                  null);
                      }
                  } else if (p.response instanceof GetChildrenResponse) {
                      ChildrenCallback cb = (ChildrenCallback) p.cb;
                      GetChildrenResponse rsp = (GetChildrenResponse) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx, rsp
                                  .getChildren());
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null);
                      }
                  } else if (p.response instanceof GetChildren2Response) {
                      Children2Callback cb = (Children2Callback) p.cb;
                      GetChildren2Response rsp = (GetChildren2Response) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx, rsp
                                  .getChildren(), rsp.getStat());
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null, null);
                      }
                  } else if (p.response instanceof CreateResponse) {
                      StringCallback cb = (StringCallback) p.cb;
                      CreateResponse rsp = (CreateResponse) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx,
                                  (chrootPath == null
                                          ? rsp.getPath()
                                          : rsp.getPath()
                                    .substring(chrootPath.length())));
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null);
                      }
                  } else if (p.response instanceof Create2Response) {
                	  Create2Callback cb = (Create2Callback) p.cb;
                      Create2Response rsp = (Create2Response) p.response;
                      if (rc == 0) {
                          cb.processResult(rc, clientPath, p.ctx,
                                  (chrootPath == null
                                          ? rsp.getPath()
                                          : rsp.getPath()
                                    .substring(chrootPath.length())), rsp.getStat());
                      } else {
                          cb.processResult(rc, clientPath, p.ctx, null, null);
                      }
                  } else if (p.response instanceof MultiResponse) {
                	  MultiCallback cb = (MultiCallback) p.cb;
                	  MultiResponse rsp = (MultiResponse) p.response;
                	  if (rc == 0) {
                		  List results = rsp.getResultList();
                		  int newRc = rc;
                		  for (OpResult result : results) {
                			  if (result instanceof ErrorResult
                					  && KeeperException.Code.OK.intValue() != (newRc = ((ErrorResult) result)
                					  .getErr())) {
                				  break;
                			  }
                		  }
                		  cb.processResult(newRc, clientPath, p.ctx, results);
                	  } else {
                		  cb.processResult(rc, clientPath, p.ctx, null);
                	  }
                  }  else if (p.cb instanceof VoidCallback) {
                      VoidCallback cb = (VoidCallback) p.cb;
                      cb.processResult(rc, clientPath, p.ctx);
                  }
              }
          } catch (Throwable t) {
              LOG.error("Caught unexpected throwable", t);
          }
       }
    }
Source