Issue with (my potential misuse of) the Decorator Pattern

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I am attempting to modify the behavior of my objects using the decorator pattern but have hit a bit of a snag where the decorator pattern seems to fail to be able to change the functionality of my objects the way I want.

Here is a simple example of what I am trying to do:

I have a class with a getter for an int and some other "complex" that does some computation using the getter

public class MyClass implements MyInterface {
    private int value = 5;

    @Override
    public int getValue() {
        return value;
    }

    @Override
    public int complexStuff() {
        return 50 + getValue();
    }
}

I have an abstract decorator that just passes all the calls defined by the interface to a MyClass instance

public abstract class MyDecorator implements MyInterface {
    private MyClass decorated;

    public MyDecorator(MyClass decorated) {
        this.decorated = decorated;
    }

    @Override
    public int getValue() {
        return decorated.getValue();
    }

    @Override
    public int complexStuff() {
        return decorated.complexStuff();
    }
}

I want to be able to decorate a MyClass instance to modify the behaviour of the getValue() method in such a way that complexStuff() is also affected.

For example if I decorate a MyClass instance like this:

MyDecorator myDecorator = new MyDecorator(myClassInstance) {
    @Override
    public int getValue() {
        return 100;
    }
};

The way I have currently implemented this a call to myDecorator.getValue() would return 100, but a call to myDecorator.complexStuff() would return 55 as if the MyClass instance had not been decorated. What I want is for the call to myDecorator.complexStuff() to return 150.

Is there a way I can modify my use of the decorator pattern to achieve my desired result? Or some other pattern/solution I can use to get this to work how I want?

Thanks

If you can't modify MyClass, maybe you can derive from it and use the subclass instead? Then you can override getValue.

When to Use the Decorator Pattern?, for adding additional functionality to a particular object as opposed to a class of objects. It is easy to add functionality to an entire class of objects by subclassing an object, but it is impossible to extend a single object this way. The decorator design pattern is one of the twenty-three well-known GoF design patterns; these describe how to solve recurring design problems and design flexible and reusable object-oriented software—that is, objects which are easier to implement, change, test, and reuse.

I don't think you can achieve this goal using a decorator pattern. You are trying to modify the behaviour of an existing instance, which isn't going to be trivial (if even possible).

Something like the following would work:

public class MyClass implements MyInterface {
  private ValueProvider provider = new ValueProvider() {    
    @Override
    public int getValue() {
      return 5;
    }
  };

  // (You may wish to include this method in MyInterface)
  public void setValueProvider(ValueProvider provider) {
    this.provider = provider;
  }

  @Override
  public int getValue() {
      return provider.getValue();
  }

  @Override
  public int complexStuff() {
      return 50 + getValue();
  }
}

You can then substitute a different ValueProvider at runtime. A decorator class could do the substitution for you, if you want.

Decorator Design Pattern in Java, allow a user to add new functionality to an existing object without altering its structure. Design Patterns points out two disadvantages of the Decorator pattern. One is that a Decorator and its enclosed component are not identical. Thus, tests for object types will fail. The second is that Decorators can lead to a system with “lots of little objects” that all look alike to the programmer trying to maintain the code.

Your mistake is obvious. When you call MyDecorator.complexStuff() is delegate this call directly to the MyClass decorated delegate, where getValue() method is not overridden, because it is overridden in MyDecorator which is not MyClass. In other words, MyDecorator HAS-A MyClass, but NOT IS-A MyClass.

To fix your problem, just modify MyDecorator.complexStuff() method. Using this, you're free to override getValue() method, but no complexStuff() method:

abstract class MyDecorator implements MyInterface {

    private final MyClass decorated;

    public MyDecorator(MyClass decorated) {
        this.decorated = decorated;
    }

    // this method is overridden, and returns 100 always
    @Override
    public int getValue() {
        return decorated.getValue();
    }

    @Override
    public final int complexStuff() {
        return decorated.complexStuff() - decorated.getValue() + getValue();
    }
}

Decorator Design Pattern - C#, provides an alternative way to inheritance for modifying the behavior of an object. Here comes the decorator pattern. As per the decorator pattern, you will implement toppings as decorators and pizzas will be decorated by those toppings' decorators. Practically each customer would want toppings of his desire and final bill-amount will be composed of the base pizzas and additionally ordered toppings.

Understanding the Decorator Pattern, We smile and reply that it's not a problem and can be done easily (because we know we have a class,  Design classes which are easily extended to implement new behaviours without having to resort to modifying the behaviour of existing code. Many design patterns are designed to do just that … an ideal example in this respect is the Decorator Pattern. The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to sub-classing for extending functionality.

Decorator pattern, In object-oriented programming, the decorator pattern is a design pattern that allows behavior Please help improve it or discuss these issues on the talk page. example demonstrates a static Decorator implementation, which is possible due  I post this issue because I have a doubt about your example for the Decorator pattern. The main feature of this pattern is to be an alternative to inheritance, but in your example your are just implementing inheritance, you are not using any decorator. Using your coffee situation, a decorator example would seems like this (in C#, sorry) :

That's Not a Decorator!, The Decorator Pattern is one of the most abused and misnamed patterns in all of Then, elsewhere in the application, you can call your new method like this: This pattern abuse causes a significant design problem. In order  A Decorator can be viewed as a degenerate Composite with only one component. However, a Decorator adds additional responsibilities - it isn't intended for object aggregation. Decorator is designed to let you add responsibilities to objects without subclassing. Composite's focus is not on embellishment but on representation.

Comments
  • I can't fathom a pleasant way to do this without coding the MyClass class to support a potential switch later. Is this an acceptable approach?
  • Im not sure what you mean, can you elaborate?
  • I've edited my answer below. It involves coding MyClass to support this kind of behavioural change.
  • I want to do this at runtime to an existing MyClass instance
  • Sorry, not happening. The class is clearly not designed to give you an opportunity to dynamically intercept getValue, so you can't do it (without getting into really nasty things like BCEL).
  • As I commented in another answer I want to do this with a MyClass instance I already have not one I construct, I corrected my code in my question
  • I see. That might be troublesome - I'll give it some thought.
  • unfortunately the actual objects I'm using are far more complex than this example and I want to be able to override any of their methods in this way so I don't think this solution would work very well as I would need a "Provider" for every method on the object which seems pretty awkward.
  • @CarlMinden So there are definitely times where the method will be called normally on that instance, but later you wish to decorate and alter the behaviour at runtime?
  • Also I want to be able to apply potentially multiple decorators to the same object affecting the same method. In the decorator pattern I can have 2 decorators that caller super.method() then do whatever they do and they will stack nicely but I don't see a way to do that with this method.