Top 20 Design Heuristics

from Arthur Riel's 61 Object Oriented Design Heuristics

What are design heuristics?

The OO Design Heuristics are a set of guidelines for good object oriented designs. As Bjarne Stroustrup (inventor of the C++ programming language) pointed out many years ago, just making a design "object oriented" doesn't always mean that you are making the design "good". In fact, many inexperienced designers have a very weak understanding of object oriented design -- and there are a number of common design mistakes that they make.

Here is one example:

• Some designers will define a single big central class (a "god class") that is responsible for controlling all of the principal system behavior. What is wrong with this?
• It is a disaster for evolution and maintenance: "God classes impede evolution because they achieve only a low level of procedural abstraction, so changes may affect many parts of a god class, its data containers, and its clients. By splitting a god class up into object oriented abstractions, changes will tend to be more localized and therefore easier to implement." [from Object-Oriented Reengineering Patterns by Serge Demeyer, Stephane Ducasse, and Oscar Nierstrasz (Morgan Kaufmann, 2003): http://scg.unibe.ch/download/oorp].

How should I use the heuristics?

The design heuristics are a good "checklist for designers":

• Use the design heuristics to help evaluate your design alternatives early in the design process. This can save a lot of design rework later.
• Use the design heuristics as part of a design review or code review process. Design reviewers should look for potential violations of the design heuristics, and they should suggest possible restructuring steps to restore a good system structure.

Heuristic 2.2: Users of a class must be dependent on its public interface, but a class should not be dependent on its users.

Interpretation:  The "users" of a class are the other functions and classes that call upon the class.  If the class is going to be as reusable as possible, the class implementor should make only minimal assumptions about the external functions and classes that will be calling on the public operations of the class.  A LinkedList class will provide insert(), remove(), search_for(), and length_of() operations the same way no matter what class is requesting these services.  A Timer class can time PhoneCall or TrackMeet objects.

Example:  Some class developers might write one or more public constructors for an abstract class (a class that contains declarations of "pure virtual functions" [in C++] or abstract functions [in Java]).  It is a better idea to make these constructors "protected" because they will only really be callable in the definition of the constructors of derived classes.

Interpretation:  The process of object oriented design is the process of discovering the important abstractions in the system and mapping them to implementable classes.  The "abstractions" that the classes will implement might come from several different places:
• an object oriented analysis of the specific problem
• the domain engineering process applied to a set of related software systems in the same problem domain
• the proposed "architecture" of the system
• low-level design abstractions that are used to help implement interfaces to hardware devices, external databases, and other subsystems
The main job of class design is to make sure that each class is "just the right size".  If a class contains multiple abstractions, there will be more cross-checking required later when a class needs to be modified or extended.

Interpretation:  This heuristic is an extension of the previous one.  A god class is a class that is either
• a class that makes all of the policy decisions for a significant part of the system after collecting the necessary information from its peers (a behavioral god class), or
• a class that encapsulates all of the important data in the system, giving it out to any of the other classes when they ask for it (a data god class).

A god class is a symptom of poor distribution of system intelligence.  Behavioral god classes are often created when a designer creates a "controller" class to combine all of the policy decisions in a single place.  Data god classes may result from naive conversion of an existing data-oriented system to an object oriented structure.

God classes are a problem because they increase the effort needed to change the design later. Most designs need to expand to handle new behaviors and new usage scenarios. In a system with a god class, that god class is often involved in most of the usage scenarios that will be modified as new features are added to the system. The designers will have the burden of verifying that a large number existing scenarios are unaffected by each change to the god class.

Exceptions:  This heuristic might be violated when designing an interface to legacy code:

• You are creating a Facade class that acts as a wrapper for a large legacy subsystem. This kind of wrapper class might need to be a behavioral god class -- which reflects the design decision that the legacy subsystem is taking the responsibility for specific behavior and scenarios.
• The Facade class serves an important purpose. It "hides" the details of the legacy implementation, and it provides the writers of the rest of the application with a simpler and stable "application programmer interface".

Interpretation:  An accessor method is a function in a class that gives access to one data attribute in the class.

Interpretation:  Non-communicating behavior is considered to be a sign that a class is responsible for multiple abstractions.

Example:  A poorly-designed CustomerOrder class might contain some attributes that contain information about the customer (with a set of operations that access and modify the customer attributes) plus some attributes that contain information about the items that are being ordered (with other operations that access and modify the item information).
 
class CustomerOrder {
private:
    std::string customer_name;
    std::string customer_billing_address;
    std::string customer_shipping_address;
    Money order_cost;
    std::vector<std::string> order_item_names;
    std::vector<Money> order_item_costs;
public:
    void set_customer_info(std::string name,
        std::string addr1, std::string addr2);
    void add_new_item(std::string item_name, Money item_cost);
    void clear_all_items();
    Money get_cost() const;
};
This CustomerOrder class is a good candidate for being refactored:  the customer attributes and operations can be moved to a Customer class, the item attributes and operations can be moved to an Item class, and the new CustomerOrder class will "contain" a reference to a Customer and a list of references to Items.  In the new design, the Customer class can now be used in other places (in the billing or marketing subsystems, for example).

Exceptions:  This heuristic might be violated for a couple of reasons:
• Some classes just have a large number of attributes (that can't easily be divided into meaningful functional groups), and many of the simple operations on the class only need to access a small number of attributes.
• In some programming languages, there may be a performance penalty in breaking up a large class into several smaller classes.  For example, a Java applet that is constructed from a large number of simple classes may take more time to download to a workstation than an applet constructed from a small number of more complex classes.

Interpretation:  This is a sure sign of procedural thinking.  Any procedural design can be turned into a "superficially object oriented" design by converting each procedure into a class.

Interpretation:  An agent class is an intermediary class that is mainly concerned with forwarding requests for operations to other classes.  These classes often find their way into a design because the designer wants to "model the real world".

Exceptions:  Some object oriented methodologies (such as Ivar Jacobson's Object Oriented Software Engineering [Objectory]) emphasize the creation of agent classes to decouple the behavior of major entities in the system.  This decoupling can make individual classes more reusable.

Interpretation:  This heuristic attempts to make the design simpler by dividing the attributes among lower-level classes that each contain groups of related attributes.

If all of the intelligence of the system is in the big "domain-level" classes, then each of those classes may need to access many of the individual primitive data fields that are contained either directly in the class or indirectly in other classes that it contains.  If much of the system intelligence is delegated to the subcomponents of the "domain-level" classes, then these top-level classes will avoid being god classes.

Interpretation:  This heuristic and Heuristic 4.12 give two alternative ways to organize the state information which needs to be accessed to enforce a semantic constraint.  If the constraint conditions change frequently, it makes sense to create a separate class to contain all of the constraint limits, so they don't clutter the other "real" classes.

Interpretation:  If a constraint is relatively stable, it doesn't have to be implemented with a third-party class.  It can be implemented in the classes involved instead, eliminating an "unnecessary class".

Interpretation:  This means that a subclass must follow the "is-a" rule:  each operation in the superclass must make sense in the subclass (although it is permissible to have a different implementation of any superclass operation in a subclass).

Note:  The most common violation of this rule is to use inheritance to model a "has-a" relationship.  For example, maybe my C++ application class CourseRoster needs to hold a list of names, so I think about defining my class as a subclass of list.


class CourseRoster : public list<string> {
public:
   CourseRoster(const char *coursename);
   void addStudent(const char *sname);
   ...
};

This is a bad design strategy, because there are now two ways to add a new student to the CourseRoster: by calling the public addStudent() function or by calling the list insert function in the list<string> base class.

You don't want to do this, because the "is-a" relationship isn't valid -- CourseRoster is not a list, it has very specific behavior for performing updates of its internal data structures. There are much better ways to implement the "has-a" relationship -- for example, define an attrbute within the CourseRoster class:

class CourseRoster {
public:
   CourseRoster(const char *coursename);
   void addStudent(const char *sname);
   ...
private:
   list<string> registeredStudents;
};

Interpretation:  This heuristic makes it "illegal" to create a concrete subclass of another concrete class.  The inheritance tree should only have concrete classes at the leaves, and all of the interior nodes of the inheritance tree should be abstract. This is intended to keep separate abstractions separate: if a new operation is added to a concrete class that is a base class, there may be unintended changes to derived classes that inherit from the base class.

Exceptions:  This heuristic is often violated.  If this heuristic is followed blindly, the designer might create a large number of artificial classes as part of the design.

Simple extensions of concrete classes are often OK in a design. If future modifications begin to make this part of the design more complicated, you can consider at that point doing some refactoring to introduce an abstract base class.

Interpretation:  You should minimize the amount of duplicated code in an inheritance hierarchy.  If two leaf classes contain the same function with identical implementations, the function should be moved to the common parent class of the two classes.  If two leaf classes contain the same function with different implementations, the base class might define an abstract function (pure virtual function in C++) so that other classes can invoke this function polymorphically.

Interpretation:  This is a situation where inheritance should be avoided.  It doesn't make sense to create a common base class for two classes that only have some common data but no common operations.  The common data can be encapsulated in a class and contained by both classes.

Interpretation:  This is the most common reason for reorganizing a design to use inheritance.

Interpretation:  The "messages, not methods" part of this sentence means that the two classes have one or more operations that have the same name and take the same arguments (the "messages" are the same), even though the implementations of the operation are different in the two classes (the "methods" are different).

Example:  If there is a cancel() operation in both the CustomerOrder class and the Subscription class, it is not necessary to have CustomerOrder and Subscription derived from the same superclass, especially if there are no parts of the system that contain functions that work on both CustomerOrders and Subscriptions.  The name of the operation in the two classes is just accidentally the same.

On the other hand, if there are different implementations of the edit() operation for the classes BookList and CDList, but several other classes need to operate on both BookList and CDList objects, then it makes sense to create a common base class for BookList and CDList (and to make edit() an abstract function in that common base class).

Interpretation:  Object oriented software tries to eliminate explicit case analysis by putting variations of system behavior into different subclasses of the same superclass.

Exceptions:  Programming languages like C++ and Java have support for "runtime type identification".  This feature can be misused to create code that is ugly and difficult to maintain.  But runtime type identification may be necessary when trying to extend certain kinds of library classes.

Interpretation:  You should avoid defining classes with names that look like "state names" – names that are modified by adjectives.  Names like EmptyStack, DelinquentCustomerAccount, LostPackage, and CallInProgress are "classes" that are probably just states of the classes Stack, CustomerAccount, Package, and Call.

Interpretation:  This is a special case of Heuristic 5.1.  If a function from the base class is overridden with a NOP method in a derived class, then that derived class doesn't really satisfy the "is-a" relationship to the base class. This heuristic is often violated for pragmatic reasons, but for long-term maintenance, it is preferable to use delegation instead of inheritance. The following example is from Martin Fowler's Refactoring book (p. 352):


/* don't make Stack a subclass of Vector -
   Stack will need to cancel all of the non-Stack
   Vector operations */
/* This example shows the wrong way to define a Stack */
class MyStack extends Vector {
   public void push(Object element) {
     super.insertElementAt(element,0);
   }
   public Object pop() {
      Object result = super.firstElement();
      super.removeElementAt(0);
      return result;
   }
   /* size() and isEmpty() are inherited from Vector */
   /* Unfortunately, we now have to "cancel" the other
      Vector operations, to prevent users from inadvertently
      modifying the Stack internals */
   private void insertElementAt(Object,int) {}
   private Object firstElement() { return nil; }
   private void removeElementAt(int) {}
}

/* A better way - use delegation */
class MyStack {
   private Vector _vector = new Vector();
   public void push(Object element) {
      _vector.insertElementAt(element,0);
   }
   public Object pop() {
      Object result = _vector.firstElement();
      _vector.removeElementAt(0);
      return result;
   }
   public int size() {
      return _vector.size();
   }
   public Boolean isEmpty() {
      return _vector.isEmpty();
   }
}