Smell Detail

Message Chains

过长的消息链

Couplers

The client needs to obtain the required object through a long chain of method calls.

Define

Introduction

Message Chains occur when a client must navigate through a sequence of method calls to get the final object it needs. This increases coupling to multiple classes and violates the Law of Demeter.

Key Identifiers

  • Long chains of successive method calls
  • The client must understand internal object structure
  • Changes to intermediate objects ripple into client code
  • Violates the "talk only to your immediate friends" principle

Refactoring Suggestions

  1. Hide Delegation: Provide convenience methods on the intermediate class
  2. Extract Method: Wrap the long navigation chain inside a single method
  3. Move Method: Relocate behavior to a class that can directly supply the needed data
  4. Introduce Local Extension: Add helper/convenience methods (e.g., via wrapper or extension) to shorten access

Explanation

The client traverses a long chain to obtain the office location. A direct method such as getManagerOfficeLocation() should be added to the Company class to shorten the chain and reduce coupling.

Code Example
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import java.util.List;
import java.util.ArrayList;


class Company {
    private List<Department> departments;
    private String name;
    
    public Company() {
        this.departments = new ArrayList<>();
    }
    
    public Department getDepartment(int index) { 
        return departments.get(index); 
    }
    
    public String getName() { return name; }
    public int getDepartmentCount() { return departments.size(); }
    
    public void addDepartment(Department department) {
        departments.add(department);
    }
}


class Department {
    private List<Employee> employees;
    private Manager manager;
    private String name;
    
    public Department() {
        this.employees = new ArrayList<>();
    }
    
    public Employee getEmployee(int index) { 
        return employees.get(index); 
    }
    
    public Manager getManager() { return manager; }
    public String getName() { return name; }
    
    public void setManager(Manager manager) { this.manager = manager; }
    public void addEmployee(Employee employee) { employees.add(employee); }
}


class Manager {
    private String name;
    private Office office;
    private Employee assistant;
    
    public String getName() { return name; }
    public Office getOffice() { return office; }
    public Employee getAssistant() { return assistant; }
    
    public void setName(String name) { this.name = name; }
    public void setOffice(Office office) { this.office = office; }
    public void setAssistant(Employee assistant) { this.assistant = assistant; }
}


class Office {
    private String location;
    private String building;
    private int floor;
    
    public String getLocation() { return location; }
    public String getBuilding() { return building; }
    public int getFloor() { return floor; }
    
    public void setLocation(String location) { this.location = location; }
    public void setBuilding(String building) { this.building = building; }
    public void setFloor(int floor) { this.floor = floor; }
}


class Employee {
    private String name;
    private String position;
    
    public String getName() { return name; }
    public String getPosition() { return position; }
    
    public void setName(String name) { this.name = name; }
    public void setPosition(String position) { this.position = position; }
}


// Client code that requires long call chains
class CompanyService {
    
    // Long message chain example 1
    public void printManagerOffice(Company company) {
        String location = company.getDepartment(0).getManager().getOffice().getLocation();
        System.out.println("Manager office: " + location);
    }
    
    // Long message chain example 2
    public void printAssistantName(Company company) {
        String assistantName = company.getDepartment(0).getManager().getAssistant().getName();
        System.out.println("Assistant: " + assistantName);
    }
    
    // Even longer message chain example
    public void printManagerOfficeFloor(Company company) {
        int floor = company.getDepartment(0).getManager().getOffice().getFloor();
        System.out.println("Manager office floor: " + floor);
    }
    
    // Multiple chained calls in one method
    public void printDepartmentSummary(Company company) {
        Department dept = company.getDepartment(0);
        String managerName = dept.getManager().getName();
        String officeLocation = dept.getManager().getOffice().getLocation();
        String assistantName = dept.getManager().getAssistant().getName();
        String assistantPosition = dept.getManager().getAssistant().getPosition();
        
        System.out.println("Department Manager: " + managerName);
        System.out.println("Office: " + officeLocation);
        System.out.println("Assistant: " + assistantName + " (" + assistantPosition + ")");
    }
    
    // Another example showing violation of Law of Demeter
    public boolean isManagerOfficeOnTopFloor(Company company) {
        return company.getDepartment(0).getManager().getOffice().getFloor() > 10;
    }
    
    // Complex chain with conditional logic
    public String getManagerContactInfo(Company company) {
        Manager manager = company.getDepartment(0).getManager();
        if (manager.getAssistant() != null) {
            return "Contact " + manager.getAssistant().getName() + 
                   " at " + manager.getOffice().getLocation();
        } else {
            return "Contact " + manager.getName() + 
                   " directly at " + manager.getOffice().getLocation();
        }
    }
}