What are behavioral design patterns?

Question

Accepted Answer

**Behavioral patterns** deal with how **objects interact and communicate** — the assignment of responsibilities and the flow of communication between objects. They include Observer, Strategy, Command, Iterator, State, Template Method, and more.

## What behavioral patterns address

```text
BEHAVIORAL patterns concern how objects INTERACT, communicate, and distribute
responsibilities:
  → how objects collaborate and assign responsibilities to achieve behavior
  → manage algorithms, communication, and control flow between objects
→ about BEHAVIOR and interaction, not creation or structure
```

## The main behavioral patterns

```text
OBSERVER → notify many objects of state changes (covered separately) — very common
STRATEGY → interchangeable algorithms, swappable at runtime (covered separately) — common
COMMAND → encapsulate a request as an object (queue, log, undo operations)
ITERATOR → traverse a collection without exposing its internal structure
STATE → change behavior based on internal state (an object behaving like different "states")
TEMPLATE METHOD → define an algorithm's skeleton, let subclasses fill in steps
CHAIN OF RESPONSIBILITY → pass a request along a chain of handlers until one handles it
MEDIATOR → centralize complex communication between objects (reduce direct coupling)
```

## Practical, common ones

```text
✓ OBSERVER → event/reactive systems (everywhere — UI, state management)
✓ STRATEGY → swappable algorithms/behaviors (very common, eliminates conditionals)
✓ ITERATOR → traversing collections (built into many languages — for-of, iterators)
✓ COMMAND → undo/redo, queuing operations, actions (e.g. Redux actions)
✓ STATE → state machines (workflow, game states, UI states)
→ use the one that fits the interaction/behavior problem
```

## Why it matters

Understanding behavioral patterns is valuable because they address **how objects interact and communicate**, a common design concern, so it's useful design knowledge.

Behavioral patterns deal with object interaction, communication, and the distribution of responsibilities — how objects collaborate to achieve behavior and manage algorithms and control flow.

Understanding the **main behavioral patterns** — **Observer** (notifying many objects of changes), **Strategy** (interchangeable algorithms), **Command** (encapsulating requests as objects for queuing, logging, and undo), **Iterator** (traversing collections without exposing structure), **State** (changing behavior based on internal state), **Template Method** (defining an algorithm skeleton with subclass-filled steps), **Chain of Responsibility** (passing requests along handlers), and **Mediator** (centralizing communication) — covers the toolkit for object interaction.

Understanding the **practical, common ones** is especially valuable: **Observer** (underlying event and reactive systems everywhere — UIs, state management), **Strategy** (swappable behaviors, eliminating conditionals — very common), **Iterator** (traversing collections, built into many languages), **Command** (undo/redo and queuing operations, like Redux actions), and **State** (state machines for workflows, games, and UI states).

These patterns solve real, recurring interaction problems — notifying observers, swapping behaviors, encapsulating actions, traversing collections, and managing state.

Understanding behavioral patterns as a category and how each addresses different interaction scenarios helps you design object communication and responsibility distribution well.

Since how objects interact and communicate is a common design concern and behavioral patterns address it (Observer for notifications, Strategy for swappable behavior, Command for actions, State for state machines, etc.), and since understanding the patterns and the practical common ones is useful for designing object interactions, understanding behavioral patterns is valuable, practically-relevant design knowledge — a useful category of patterns for object interaction and communication, with Observer, Strategy, Iterator, Command, and State being especially practical and common, and important design knowledge for designing how objects collaborate.