kandi X-RAY | Design-Pattern-in-java Summary
kandi X-RAY | Design-Pattern-in-java Summary
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Following is stretagy design pattern example take from here. First of all we will create the interface for our strategy, in our case to pay the amount passed as argument....
ANSWERAnswered 2021-Mar-23 at 07:03
The answer is the user who has the ShoppingCart (
ShoppingCart cart = new ShoppingCart();)
I don't know exactly you mean3. Why do we need interface , all which does is call a method?
We need the interface
PaymentStrategy because we need use Polymorphism to implement many way to pay (paypal or credit card), let's change the source a bit and you can see clearer:
Whatever Chain of responsibility patterns accomplishes can be accomplished with a simple for loop. Consider the currency notes dispenser example in this article. https://www.journaldev.com/1617/chain-of-responsibility-design-pattern-in-java We can easily do it using a for loop and if conditions. Is n;t COR pattern simply complicating things here?...
ANSWERAnswered 2020-Jul-02 at 13:51
It's not so much about: This is the only way we can do this!
But more: Hey, check out this crazy alternative way to do this!
With design patterns in general, I would strongly encourage you to just look at them as ideas of how to do certain things and not as solutions to specific problems.
It's more like... they will teach you interesting ways to approach certain circumstances, that you might not have considered before. So when you get to a point where you actually need something similar, your brain doesn't say 'Nope, never thought that way, leave me alone.", but make connections it might not make otherwise.
I've never seen anyone go: "Let's use the XYZ Design Pattern to do this!" However, when people have to explain their code, I have sometimes heard them refer to certain design patterns. It means they don't have to explain that specific part of their train of thought and still be understood.
Like "Well, it's kind of like a Chain of Responsibility Pattern, except ..."
So don't spend too much time wondering why and use your time to explore and understand the how. Talking about design choices takes way too much time anyway. There are pros and cons to everything.
Finally, let's actually talk about the why for a moment, as much as I don't want to. I'll keep it brief though and I'm sure there are other aspects to consider as well.
(Note: When I say 'part of the chain' I mean a lower part ending at the same end point. Like A->B->C->D->E->F. I would be talking about C->D->E->F or D->E->F, but not about B->C->D)
- If you want to switch out part of the chain, you only need to change one variable. In your for, there'll be a whole lot of ifs to either remove, comment out or put somewhere else because you don't need them anymore. Also, you can easily keep your old chain around without cluttering up the code that is being used.
- It is very easy to use part of the same chain at different places. If you only use part of the if-statements in your loop but replace others, that means you'll need to create a new loop where the parts you do still use get copied over. Better to avoid duplicate code.
- Well... testing and having multiple developers working on different parts at once would be easier with the chain, but only if your loop is very simple. If you start calling methods to do parts (ideally from different classes), it should be just as easy to do it without the pattern.
- The person that created the design pattern might nod at you approvingly if you ever meet them and tell them you used their pattern.
Honestly though, except for the last point, the others will also be achieved by just structuring the code surrounding your loop differently. Then again, that is pretty much exactly what you do by using this design pattern anyway ;)
Above posts are good but still I could not get answer to my confusion, hence I am putting it as a new post here.
MY Questions based on the GOF's Elements of Reusable Object-Oriented Software book content about Pluggable Adapters (mentioned after questions below), hence I would appreciate if the discussions/answers/comments are more focused on the existing examples from GOF regarding the pluggable Adapters rather than other examples
Q1) What do we mean by built-in interface adaptation ?
Q2) How is Pluggable Interface special as compared to usual Adapters ? Usual Adapters also adapt one interface to another.
Q3) Even in the both the use cases, we see both the methods of the Extracted "Narrow Interface"
Node is an internal to Toolkit. Is Node same as GraphicNode and is the parameter passed in
CreateGraphicNode only for populating the states like (name, parentID, etc) of an already created Node object ?
As per the GOF (I have marked few words/sentences as bold to emphasis the content related to my Questions)
ObjectWorks\Smalltalk [Par90] uses the term pluggable adapter to describe classes with built-in interface adaptation.
Consider a TreeDisplay widget that can display tree structures graphically. If this were a special-purpose widget for use in just one application, then we might require the objects that it displays to have a specific interface; that is, all must descend from a Tree abstract class. But if we wanted to make TreeDisplay more reusable (say we wanted to make it part of a toolkit of useful widgets), then that requirement would be unreasonable. Applications will define their own classes for tree structures. They shouldn't be forced to use our Tree abstract class. Different tree structures will have different interfaces.
Pluggable adapters. Let's look at three ways to implement pluggable adapters for the TreeDisplay widget described earlier, which can lay out and display a hierarchical structure automatically. The first step, which is common to all three of the implementations discussed here, is to find a "narrow" interface for Adaptee, that is, the smallest subset of operations that lets us do the adaptation. A narrow interface consisting of only a couple of operations is easier to adapt than an interface with dozens of operations. For TreeDisplay, the adaptee is any hierarchical structure. A minimalist interface might include two operations, one that defines how to present a node in the hierarchical structure graphically, and another that retrieves the node's children.
Then there are two use cases
(There is a 3rd approach of Parameterized adapter also but skipping it for simplicity, Also this 3rd one is I guess more specific to Small talk)...
ANSWERAnswered 2020-Apr-19 at 22:25
Q1) Interface adaptation just means adapting one interface to implement another, i.e., what adapters are for. I'm not sure what they mean by "built-in", but it sounds like a specific feature of Smalltalk, with which I'm not familiar.
Q2) A "Pluggable Adapter" is an adapter class that implements the target interface by accepting implementations for its individual methods as constructor arguments. The purpose is to allow adapters to be expressed succinctly. In all cases, this requires the target interface to be small, and it usually requires some kind of language facility for succinctly providing a computation - a lambda or delegate or similar. In Java, the facility for inline classes and functional interfaces means that a specific adapter class that accepts lambda arguments is unnecessary.
Pluggable adapters are a convenience. They are not important beyond that. However...
Q3) The quoted text isn't about pluggable adapters, and neither of the two use cases has a pluggable adapter in it. That part is about the Interface Segregation Principle, and it is important.
In the first example,
TreeDisplay is subclassed. The actual adapter interface is the subset of methods in
TreeDisplay that require implementation. This is less than ideal, because there is no concise definition of the interface that the adapter must implement, and the
DirectoryTreeDisplay cannot simultaneously implement another similar target interface. Also such implementations tend interact with the subclass in complex ways.
In the second example,
TreeDisplay comes with a
TreeAccessorDelegate interface that captures the requirements for things it can display. This is a small interface that that can be easily implemented in a variety of ways, including by a pluggable adapter. (although the example
DirectoryBrowser is not pluggable). Also, interface adaptation does not have to be the sole purpose of the adapter class. You see that
DirectoryBrowser class implements methods that have nothing to do with tree display.
Node type in these examples would be an empty/small interface, i.e., another adapter target, or even a generic type argument so that no adaptation is required. I think this design could be improved, actually, by making
Node the only adaptation target.
I need help understanding this example for a decorator:...
ANSWERAnswered 2020-Jan-27 at 20:30
You have to implement them because in this case
ShapeDecorator is only an abstract class implements
Shape: it simply provide a uniform way to store the decorated
I create decorator pattern example:
ANSWERAnswered 2018-Jul-16 at 11:39
BufferedInputStream is implemented in exactly the same way as your
Car example. It decorates an
InputStream which, although it's an abstract class, still provides a contract, just as an interface would. An abstract class was chosen in this case because there are methods with default behaviour (
skip) and, at the time the class was written, interfaces were not able to support this (they can now, but default methods
in interfaces were added much later).
Because an abstract class was used for the contract,
FilterInputStream, just as
Car. Both are just simple holders for a delegate. They contain a single
protected field (an
InputStream and a
Car) and delegate all method calls to the field. The reason for this is because, if you have a large number of methods in your contract, delegating all of the methods in every decorator might result in a lot of code duplication. When your interface only has a single method, as yours does, then it provides very little benefit.
This is certainly not a necessary component of the decorator pattern; it's just a slightly different way of implementing it. Your implementation is still a 100% correct implementation of the decorator pattern.
I have read this article about the decorator pattern and I have one question.
This class inhertis from the abstract class
ANSWERAnswered 2017-Oct-30 at 16:30
Your suggestion is fine IMHO. It also matches the GOF original: decorator
When discussing patterns in general, it's important to remember that variants are acceptable, there's no "Pattern Police" to reprimand you for every tiny deviation (as long as it's well considered)... If it were me, I'd change some of the article's AbstractClasses to interfaces, and I still believe my design intent would be clear and acceptable.
Perhaps you can ask the author in the comments (though it's an old post). If I had to guess, I'd assume the author wanted to leave some flexibility in the "currentSandwich" declaration/location (like taking it from a threadLocal, putting together 2 sandwitches, etc), but such 'creativity' would ruin the generality and modularity - namely the freedom to wrap several decorators one over the other. I'd prefer my decorators to stick to a well-defined "currentSandwitch", in which case it's fine to adopt your idea.
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