Decorator

About 2 min

Also known as

Wrapper

Intent

Attach additional responsibilities to an object dynamically. Decorators provide a flexible
alternative to subclassing for extending functionality.

Explanation

Real-world example

There is an angry troll living in the nearby hills. Usually, it goes bare-handed but sometimes it
has a weapon. To arm the troll it's not necessary to create a new troll but to decorate it
dynamically with a suitable weapon.

In plain words

Decorator pattern lets you dynamically change the behavior of an object at run time by wrapping
them in an object of a decorator class.

Wikipedia says

In object-oriented programming, the decorator pattern is a design pattern that allows behavior to
be added to an individual object, either statically or dynamically, without affecting the behavior
of other objects from the same class. The decorator pattern is often useful for adhering to the
Single Responsibility Principle, as it allows functionality to be divided between classes with
unique areas of concern as well as to the Open-Closed Principle, by allowing the functionality
of a class to be extended without being modified.

Programmatic Example

Let's take the troll example. First of all we have a SimpleTroll implementing the Troll
interface:

public interface Troll {
  void attack();
  int getAttackPower();
  void fleeBattle();
}

@Slf4j
public class SimpleTroll implements Troll {

  @Override
  public void attack() {
    LOGGER.info("The troll tries to grab you!");
  }

  @Override
  public int getAttackPower() {
    return 10;
  }

  @Override
  public void fleeBattle() {
    LOGGER.info("The troll shrieks in horror and runs away!");
  }
}

Next, we want to add a club for the troll. We can do it dynamically by using a decorator:

@Slf4j
public class ClubbedTroll implements Troll {

  private final Troll decorated;

  public ClubbedTroll(Troll decorated) {
    this.decorated = decorated;
  }

  @Override
  public void attack() {
    decorated.attack();
    LOGGER.info("The troll swings at you with a club!");
  }

  @Override
  public int getAttackPower() {
    return decorated.getAttackPower() + 10;
  }

  @Override
  public void fleeBattle() {
    decorated.fleeBattle();
  }
}

Here's the troll in action:

// simple troll
LOGGER.info("A simple looking troll approaches.");
var troll = new SimpleTroll();
troll.attack();
troll.fleeBattle();
LOGGER.info("Simple troll power: {}.\n", troll.getAttackPower());

// change the behavior of the simple troll by adding a decorator
LOGGER.info("A troll with huge club surprises you.");
var clubbedTroll = new ClubbedTroll(troll);
clubbedTroll.attack();
clubbedTroll.fleeBattle();
LOGGER.info("Clubbed troll power: {}.\n", clubbedTroll.getAttackPower());

Program output:

A simple looking troll approaches.
The troll tries to grab you!
The troll shrieks in horror and runs away!
Simple troll power: 10.

A troll with huge club surprises you.
The troll tries to grab you!
The troll swings at you with a club!
The troll shrieks in horror and runs away!
Clubbed troll power: 20.

Class diagram

alt text — Decorator pattern class diagram

Applicability

Decorator is used to:

Add responsibilities to individual objects dynamically and transparently, that is, without
affecting other objects.
For responsibilities that can be withdrawn.
When extension by subclassing is impractical. Sometimes a large number of independent extensions
are possible and would produce an explosion of subclasses to support every combination. Or a class
definition may be hidden or otherwise unavailable for subclassing.

Tutorials

Decorator Pattern Tutorialopen in new window