java-design-patterns/decorator/README.md

title, category, language, tags

title	category	language	tags
Decorator	Structural	en	Gang of Four Extensibility

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.

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

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 Tutorial

Known uses

• java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer
• java.util.Collections#synchronizedXXX()
• java.util.Collections#unmodifiableXXX()
• java.util.Collections#checkedXXX()

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software
• Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions
• J2EE Design Patterns
• Head First Design Patterns: A Brain-Friendly Guide
• Refactoring to Patterns
• J2EE Design Patterns