Message Chains

Description (What)

Message chains are simply multiple chained calls to functions to retrieve data. Message chains create a chain of dependency throughout the program which can be harder to keep track of as the chain increases and any changes made at any part of the chain might require refactoring in other parts of the chain

How to Locate It (Where)

Look for calls resembling a().b().c().d() where data is being retrieved through a chain of function calls where each function in the call calls another function and retrieves data from it.

How It Manifests (Why)

While writing code in silos, it comes naturally that when you need to interact between Classes or Methods, it will require using chains to bring information to the client surface.

How to Fix It (Possible Treatments)

In Example 1, Hide Delegate is used to push the chain to an inner class which leads the message chain being hidden behind a single public method which itself contains a chain of private or internal methods. Hide Delegate simplifies the end call for the client.

In Example 2, Move Methods is used when the message chain comes into form due to the placements of methods.

We could address the code smell by moving the methods back to which they belong to.

Other treatments are also possible based on the specific scenario, they can be found here

Examples

Example 1

Before:

For the vehicle to get the address for the latest order to be delivered, it needs to go through a message chain of going to the Distributor object which returns a Warehouse object which returns an Order object from which the address can be retrieved.

Observed Code Smells:
- Message Chains (line 117)

After:

Applied Hide Delegate by adding a new public method getLatestOrderDestination to Distributor class to make the calls internal

Removed getWarehouse() method from Distributor so it remains private and reduces its public exposure.

Refactoring Applied:
- Message Chains
    - Hide Delegate (getWarehouse())

Observed Code Smells After Refactoring:
- None

Example 2

Before:

Here the message chains arise when, an attendance manager wants to know if an employee is on shift at a particular

point in time. The attendance manager has to get the employee instance first, and then get the schedule of that employee

finally arrives at the boolean if the employee is on shift: AttendanceManager -> Employee -> Schedule, where the class

Employee functions like an agent, but it should not be.

Attendance Manager should have this method directly inside its own class.

In other words, Attendance Manager should not go through an employee to know if they are on shift.

Observed Code Smells:
- Message Chains (line 78, line 82)

After:

The two methods of isOnShift() should sit in the class Attendance Manager because to know if an employee is on shift

is the duty of the attendance manager.


Refactoring Applied:
- Message Chains:
    - Move Method (lines 30-33, lines 35-38)

Observed Code Smells After Refactoring:
- None

When to Ignore

In cases where solving this code smell might inadvertently cause the Middle Man code smell.

More about Message Chains

Example 1
Example 2

Before
After

001  import java.util.LinkedList;
002  import java.util.Queue;
003  
004  class Order {
005      private String item;
006      private String destination;
007      private boolean isDelivered;
008  
009      public Order(String item, String destination) {
010          this.item = item;
011          this.destination = destination;
012          this.isDelivered = false;
013      }
014  
015      public String getOrderDetails() {
016          return "Item: " + this.item + " to: " + this.destination + "\nDelivered: " + this.isDelivered;
017      }
018  
019      public String getDestination() {
020          return destination;
021      }
022  
023      public void markOrderDelivered() {
024          this.isDelivered = true;
025      }
026  
027      public boolean getDeliveredStatus() {
028          return this.isDelivered;
029      }
030  }
031  
032  class Warehouse {
033      private String address;
034      private String owningEntity;
035      private Queue<Order> orders;
036  
037      public Warehouse(String address, String owningEntity) {
038          this.address = address;
039          this.owningEntity = owningEntity;
040          this.orders = new LinkedList<>();
041      }
042  
043      public void addOrder(int orderId, Order newOrder) {
044          orders.add(newOrder);
045      }
046  
047      public Order getLatestOrder() {
048          return orders.peek();
049      }
050  
051      public String getWarehouseDetails() {
052          return "Owned by " + this.owningEntity + "located at " + this.address;
053      }
054  
055      public int getPendingOrderSize() {
056          return orders.size();
057      }
058  
059      public void checkAndEmptyWarehouse() throws Error {
060          for (Order order : orders) {
061              if (!order.getDeliveredStatus()) {
062                  throw new Error("Warehouse is not empty!");
063              }
064          }
065          orders.clear();
066      }
067  }
068  
069  class Distributor {
070      private String owner;
071      private String address;
072      private Warehouse warehouse;
073  
074      public Distributor(String owner, String address, Warehouse warehouse) {
075          this.owner = owner;
076          this.address = address;
077          this.warehouse = warehouse;
078      }
079  
080      public Warehouse getWarehouse() {
081          return this.warehouse;
082      }
083  
084      public Order getLatestOrder() {
085          return this.warehouse.getLatestOrder();
086      }
087  
088      public String getDistributorDetails() {
089          return "Owned by " + this.owner + " located at " + this.address;
090      }
091  
092  }
093  
094  class Vehicle {
095      private String vehicleNumber;
096      private String model;
097      private Distributor distributor;
098  
099      public Vehicle(Distributor distributor, String vehicleNumber, String model) {
100          this.vehicleNumber = vehicleNumber;
101          this.model = model;
102          this.distributor = distributor;
103      }
104  
105      public String getVehicleDetails() {
106          return this.vehicleNumber + "\n" + this.model;
107      }
108  
109      public void stationVehicle() {
110          System.out.println(this.vehicleNumber + " is now stationed");
111      }
112  
113      public void startVehicle() {
114          System.out.println("Vehicle has been started");
115      }
116  
117      public void getOrderRoute() {
118          String destination = distributor.getWarehouse().getLatestOrder().getDestination();
119          System.out.println(vehicleNumber + " inbound to " + destination);
120      }
121  
122  }
123  
124  public class MCBE1 {
125  
126      public static void main(String[] args) {
127          Order boots = new Order("Timberlands", "1191 Boylston St");
128          Order hat = new Order("Red Sox Cap", "1193 Boylston St");
129          Warehouse fenway = new Warehouse("1203 Akron St", "Fenway Goods Storage");
130          fenway.addOrder(1, boots);
131          fenway.addOrder(2, hat);
132          Distributor redSox = new Distributor("Red Sox Co.", "42 Chanice Blvd", fenway);
133          Vehicle jeep = new Vehicle(redSox, "9XJ3F", "Mercedes GLX");
134          jeep.getOrderRoute();
135      }
136  }
137