Unlocking Behavioral Design Patterns: When and How to Use Them

Behavioral Patterns Overview

Behavioral patterns describe complex runtime flow control, i.e., how multiple classes or objects cooperate to accomplish tasks that a single object cannot, involving algorithm and responsibility distribution.

According to their presentation, behavioral patterns are divided into class behavioral patterns (using inheritance) and object behavioral patterns (using composition or aggregation). Because composition/aggregation has lower coupling and satisfies the Composite Reuse Principle, object behavioral patterns are more flexible than class behavioral patterns.

List of Behavioral Patterns

Chain of Responsibility

Command

Interpreter

Iterator

Mediator

Memento

Observer

State

Strategy

Template Method

Visitor

1. Chain of Responsibility Pattern

Motivation : The chain can be a straight line, a loop, or a tree; the most common is a linear chain where a request travels along a one‑way chain.

Class Diagram :

Roles: Handler (abstract handler), ConcreteHandler (concrete handler), Client.

Advantages :

Reduces coupling.

Simplifies object connections.

Increases flexibility in assigning responsibilities.

Adding new request handlers is easy.

Disadvantages :

Cannot guarantee a request will be handled.

Potential performance impact and debugging difficulty; possible circular calls.

Scenarios : Multiple objects can handle the same request; the runtime decides which object processes it. Examples include early Java AWT event model (JDK 1.0) and Java exception handling.

2. Command Pattern

Motivation : Decouples sender and receiver completely; the sender does not need to know how the request is executed.

Class Diagram :

Roles: Command (abstract command), ConcreteCommand, Invoker, Receiver, Client.

Advantages :

Reduces system coupling.

New commands can be added easily.

Facilitates command queues, macro commands, and undo/redo operations.

Disadvantages :

May lead to many concrete command classes, increasing memory usage.

Scenarios : Decoupling request sender and receiver, queuing and executing requests, supporting undo/redo and macro commands (e.g., delegated event models).

3. Interpreter Pattern

Provides a way to evaluate language grammar or expressions.

4. Iterator Pattern

Provides a way to access elements of a collection sequentially without exposing its underlying representation.

5. Mediator Pattern

Motivation : To reduce complex inter‑object references and achieve loose coupling by centralizing communication in a mediator.

Class Diagram :

Roles: Mediator (abstract mediator), ConcreteMediator, Colleague (abstract colleague), ConcreteColleague.

Advantages :

Simplifies object interactions.

Decouples colleagues.

Reduces subclass proliferation.

Simplifies colleague design and implementation.

Disadvantages :

Concrete mediator can become complex, making the system hard to maintain.

Scenarios : Complex reference relationships among objects; need to encapsulate multiple class behaviors without generating many subclasses.

6. Memento Pattern

Motivation : Record an object's internal state to allow undo or rollback.

Class Diagram :

Roles: Originator, Memento, Caretaker.

Advantages :

Provides state restoration mechanism.

Encapsulates information; multiple mementos can be stored for multi‑step undo.

Disadvantages :

High resource consumption if many state snapshots are needed.

Scenarios : Saving an object's state at a point in time for later restoration.

7. Observer Pattern

Motivation : Define a one‑to‑many dependency so that when a subject changes state, all its observers are automatically notified and updated.

Class Diagram :

Roles: Subject, ConcreteSubject, Observer, ConcreteObserver.

Advantages :

Separates presentation and data logic layers.

Provides a stable message‑update mechanism.

Supports broadcast communication and adheres to the Open/Closed Principle.

Disadvantages :

Notifying many observers can be time‑consuming.

Potential circular dependencies may cause system crashes.

Observers do not know the exact cause of the subject's change.

Scenarios : When an object's change should trigger updates in multiple dependent objects without the subject knowing who they are.

8. State Pattern

Motivation : Allows an object to change its behavior when its internal state changes, appearing as if the object changed its class.

Class Diagram :

Roles: Context, State (abstract), ConcreteState.

Advantages :

Encapsulates state transition rules.

Enumerates possible states.

Easy to add new states.

Combines state logic with state objects, reducing conditional blocks.

Disadvantages :

Increases number of classes and objects.

Complex structure and implementation; may lead to messy code if misused.

Poor support for the Open/Closed Principle when adding new states.

Scenarios : When an object's behavior depends on its state and needs to change dynamically.

9. Strategy Pattern

Motivation : A task can be performed by multiple algorithms (strategies); the client can choose an appropriate strategy at runtime.

Class Diagram :

Roles: Context, Strategy (abstract), ConcreteStrategy.

Advantages :

Perfectly supports the Open/Closed Principle.

Manages families of algorithms.

Avoids multiple conditional statements.

Disadvantages :

Client must know all strategy classes.

May lead to many concrete strategy classes; can be mitigated with Flyweight.

Scenarios : When a system needs to select among several algorithms dynamically, such as Java SE layout managers.

10. Template Method Pattern

Motivation : Define the skeleton of an algorithm in a base class, letting subclasses implement specific steps.

Class Diagram :

Roles: AbstractClass (defines template method), ConcreteClass (overrides hook methods).

Advantages :

Formalizes algorithm in a parent class while allowing subclass variations.

Supports the Open/Closed Principle.

Improves code reuse and cohesion.

Disadvantages :

Each variation requires a new subclass, increasing class count.

Scenarios : When an algorithm has invariant parts and variable parts that should be separated.

11. Visitor Pattern

Motivation : Encapsulate operations on elements of an object structure so that new operations can be added without changing the element classes.

Class Diagram :

Roles: Visitor (abstract), ConcreteVisitor, Element (abstract), ConcreteElement.

Advantages :

Adding new operations is easy.

Centralizes related operations in visitor classes.

Allows crossing class hierarchies.

Disadvantages :

Adding new element classes is difficult and violates the Open/Closed Principle.

May break encapsulation by exposing element internals to visitors.

Scenarios : When an object structure contains many types of objects and you need to perform many unrelated operations on them without polluting the object classes.

Conclusion

Behavioral patterns provide powerful mechanisms for managing object interactions, encapsulating algorithms, and improving flexibility and maintainability in object‑oriented designs.