What are some situations in which the Decorator Pattern solves the problem really well?

Question

I think a classic example is that the Window class can have a Border decorator and a ScrollBar decorator in the GoF book.

What are some situations you think, know of, in which the Decorator Pattern solves the problem really well?

Answer 1

The java.io package.

Answer 2

Anything that is abstractable to an extendable root upon which you have to define different overlapping and interchangeable behaviors:

• Windows and borders, scrollbars and menubars
• Musical instruments (audio waves) and effects (flanger, wah-wah, and so on)
• NPC and weaponry

and so on...

Answer 3

I've used the Decorator pattern for managing complex roles.

Example off the top of my head:

public abstract class DecoratedUser : User
{
  protected User _decoratedUser;
  public DecoratedUser(User decoratedUser)
  {
    _decoratedUser = decoratedUser;
  }

  public abstract List<Permissions> GetPermissions;
}

public EditorUser : DecoratedUser
{
  public EditorUser(User decoratedUser)
    : base(decoratedUser)
  {}

  public override List<Permissions> GetPermissions
  {
    // return previous permissions + editor permissions
  }
}

public ModeratorUser : DecoratedUser
{
  public ModeratorUser(User decoratedUser)
    : base(decoratedUser)
  {}

  public override List<Permissions> GetPermissions
  {
    // return previous permissions + moderator permissions
  }
}