Design pattern that Wrapper Classes use in Java?

Question

I have found an old post which does not clarify my understanding about the design patterns that are used by Wrapper Classes, Moreover, on reading from Wikipedia I'm not getting any clear information.

Does a Wrapper Class really use any design pattern or not?

If it is using a pattern, then which pattern is it out of these: Decorator Pattern, Facade Pattern or Adapter Pattern?

Solution 1

If you refer to wrapping primitive

Wrapper classes provide a way to use primitive types as objects

Adapter pattern is the most exact meaning:

A decorator makes it possible to add or alter behavior of an interface at run-time. Alternatively, the adapter can be used when the wrapper must respect a particular interface and must support polymorphic behavior, and the Facade when an easier or simpler interface to an underlying object is desired

We use Wrapper class ability to use primitive as Objects, meaning add support to a polymorphic behavior

Solution 2

All of the three design patterns in someway describe a wrapper:

• Decorator pattern. Wraps a component and potentially decorates it with some additional characteristics.
• Adapter pattern. Simply wraps a component to provide a suitable interface for consumers.
• Facade pattern. Wraps a component to facilitate the usage of otherwise complex external interface.

Solution 3

They don't follow any of the design patterns that you have mentioned.

Adapter converts the interface of a class to another interface. Primitives do not implement any interfaces.

Decorator adds behaviour of to a class implementing one interface by wrapping it in another that implements the same interface. Primitives do not implement any interfaces.

A facade's purpose is to mask the complex behaviour of the object that it's wrapping. There is nothing less complex in a programming language than a primitive. The clue's in the name. If anything, the wrapper classes are the opposite of this.

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Off the top of my here, here's a few design patterns that they do make use of:

Integer, Long and Byte make use of an object pool of flyweight objects, to avoid creating unnecessary instances.

Boolean somewhat tries to be a multiton (in that the constructor is deprecated) but in practice it isn't.

Solution 4

Wrapper classes use composition. The same composition as in the popular maxim, "Favor composition over inheritance." Composition is not a design pattern; however, most OO design patterns use composition as part of their implementation. This is one reason many people struggle to discriminate among different design patterns: the common use of composition makes them all appear the same to a certain degree.

There are two basic parts in a composition relationship: the composer and the composed. You can think of this generally as a part/whole relationship. It may be one-to-one or one-to-many. A wrapper is the composer, i.e. it is the whole. It may wrap one or more composed parts.

Many different design patterns make use of the general composition relationship for different purposes. Many of those different patterns are referred to collectively as "wrappers". The GoF book calls out at least two such patterns.

• ADAPTER Also Known As Wrapper page 139
• DECORATOR Also Known As Wrapper page 175

In summary, Wrapper is not any single design pattern; rather, it is a category of design patterns. Incidentally, we see the same dynamic with the term Factory. There is no single design pattern named Factory; rather, it is a category of design patterns.

Solution 5

Well, the answers seems like to indicate that you can wrap an object for many reasons and as such, many patterns. So I'll try to give a more general answer.

A Wrapper is basically an object whose sole purpose is to provide something without modifying the main object (add fonctionnalities, simplify API, serialisation, ... see other answers), that wrapper is generally tightly coupled to the "main" object. For exemples look others answers.

Another alternative for some usage of the wrapper is inheritance, but not for every case.

As such the wrapper is just a technical way of doing some stuff. It is not a pattern in itself.

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Comments

[Nordiii]

My take on it is that a wrapper class is no pattern, it just wraps existing classes. Patterns like Decorator, Facade and Adapter often make use of a wrapper class. But that's only my opinion!

[just.another.programmer]

Not a decorator. A decorator isolates behavior into separate classes which can be added to the "main" component independently. The structure requires wrapping the main component, but that's not the main point.

[just.another.programmer]

Not a facade. A facade is all about changing the public API of a class. That's a lot more than just "wrapping" it.

[just.another.programmer]

Might be worthwhile adding examples of wrappers as adapters - eg boxing of primitive types, COM component wrappers.

[user7294900]

@just.another.programmer the question already ask about boxing of primitive type,no ?

[NiVeR]

Have you checked wikipedia: en.wikipedia.org/wiki/Decorator_pattern?. A basic test is to look the occurences of the word wrapper. Otherwise you can read the Intent part...If you want to find a black dot in white wall you almost certainly always will, but this time is not the point IMO.

[NiVeR]

If you notice, I said in someway, it doesn't mean ===. It means to some extent. Your points are wrong in my opinion.

[Michael]

Your adapter pattern link goes to the decorator pattern

[user7294900]

@Michael true, but it has good description when to use either

[user7294900]

all Wrapper classes constructor are deprecated in java 9 as docs.oracle.com/javase/9/docs/api/?java/lang/Integer.html

[user7294900]

You wrote the note specific for Boolean

[Michael]

@user7294900 Because that's the only one which is (sort of) a multiton (Boolean.TRUE and Boolean.FALSE).

[just.another.programmer]

I didn't see anything explicit about boxing of primitives in the question. OP just says "Wrapper Classes".

[just.another.programmer]

Decorator classes wrap the original type as an implementation detail. The key difference is their purpose: to extend or alter behavior, not the interface. It's kind of like the "all squares are rectangles but not all rectangles are squares".

[user7294900]

@just.another.programmer you are right, updated my answer

[GhostCat]

Unrelated: it is always a bit sad when someone puts up a bounty, and then earns downvotes on the question as reward. I did a bit to compensate for that. And I appreciate the quick accept and awarding of the bounty, and I think that you picked a good answer there!