Design patterns aren't necessarily a programming style but rather a template for solving a problem in a number of different situations.
But how do they differ from other programming styles?
Thanks
Edit:
"(a) What are design patterns? How do these differ from other programming styles? [7]"
A design pattern provides a known solution to a common design problem, and is given a name to allow programmers to communicate more efficiently.
However, I have no idea what you mean by "programming styles".
Edit: I think the question is simply very badly worded. While "design pattern" has a relatively clear definition, "programming style" is, in my experience, not a term with a generally agreed clear definition. The question seems to assume that design patterns are a kind of programming style, and I don't think that's true.
If we take "programming style" to mean this, then the primary difference between programming styles and design patterns boils down to form vs. function: programming styles merely suggest how code is supposed to look, but dont have anything to say about how the code works; while design patterns are all about how code is programmatically structured and executes, but say nothing about how the code looks.
Design patterns are common solutions to object-oriented programming problems, so they differ from other styles in exactly the same way that object-oriented programming does.
As I see it, programming styles refer to the "specifc" way in which you'd go about implementing something (Design patterns included). Although Design Patterns have traditionally been linked to OOP many design patterns still exist in other programming paradigms. The question, i think, is trying to prove whether or not you've got the difference between a "generic" and proven way of solving a problem, and an specific way of implementing a solution to "that" problem.
Regards,
I agree with other answers and in additional to that I think some program specific properties integrated some design pattern philosophy. Like the "delegate" keyword in C#, which implements the delegate pattern but it is absolutely supported by the language itself.
Related
I've been looking across the web for a simple explanation about the differences between the two.
I understand composition is "bottom-up" design while decomposition is "top-down" design.
However, aside from that - are there any further differences?
If a program implements the "composability" principle, does it necessarily also implement the "decomposability" principle, and vice-versa?
It's obvious how these two can lead to different designs, but all in all, it seems they represent exactly the same thing from different point of views.
Clarifications will be highly appreciated.
Cheers!
Some reference links:
YorkU
Blog about Modular Composability
Blog about Modular Decomposability
As the first link you've provided shows it, these two approaches are not incompatible. You just need to know when to use one or the other.
From my experience, top-down design is a good approach when you start designing, as you need to discover your system, understand the requirements and making something works quickly. As you add more and more features, specific responsibilities start to emerge and this is where decomposing your problem is required. This will prevent to duplicate code from one feature to another, and lower the efforts required to compose new ones.
Choosing between one approach or the other is just a matter of figuring out the right design decision to take at a proper time. If you feel that some aspects of a problem are still unclear, there is no reason to decompose it. Just wait until your module cries out for modularization (for example, having a hard time to understand what you wrote some days ago would be a good sign, same for duplicated code).
Is this answering your question ?
Overusing Inheritance ?
Java Swing and Java2D rely a lot on inheritance. Most people have told me I should avoid inheritance as much as possible and only use it when necessary. So is the extensive use of Inheritance in Java2D and Java Swing justified?
Adapter Pattern
I have heard a lot of praise for the adapter pattern, and I've heard a lot of criticism. What I gathered from all that, though, is that the adapter pattern is only considered good design if used at the right place. Irrelevant use of the adapter pattern causes people reading your code to scowl. Is the adapter pattern correctly and relevantly used in the two Java APIs?
Singletons
Both APIs also tend to use a considerable number of Singletons. Is this good?
The Question
So, Is The Java2D Api and Java Swing a good example of an Object Oriented Programming Interface? Should I use their techniques in my code?
I have no certainty on this, but I can offer some thought:
inheritance: suppose you preffered to use interfaces and composition instead of reuse-by-inheritance in Swing, then you would need to do an awful lot of forwarding (Component and JComponent have dozens of methods)
adapter pattern: see above
singletons: they are appropriate to model parts of a system which are genuinely unique (I would agree that java.awt.Desktop could be implemented as a singleton -- even though it uses a factory method)
overall design: you might opt to judge things by results -- Swing is robust, extensible, and widely used.
A friend of mine who works with Swing complains constantly that it is an abstract morass. Determining the behavior of an object at run-time involves piecing together a bizarre collage of inherited functionality. From what I've seen of it I'm inclined to agree.
Inheritance is a useful programming concept, but it is easy to use
inappropriately.
Inheritance is a good choice when:
Your inheritance hierarchy represents an "is-a" relationship and not a "has-a" relationship.
You can reuse code from the base classes.
You need to apply the same class and methods to different data
types.
The class hierarchy is reasonably shallow, and other developers are
not likely to add many more levels.
You want to make global changes to derived classes by changing a
base class.
A microsoft article here and another good one here
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I find my self writing again and again the same programming patterns in many new projects.
I have been thinking about creating my own reusable library of typical implementations of such patterns -not trying to cover all possible design patterns, but only them that experience has shown that it makes sense to put such typical implementations in a library (e.g., adapter, factory, etc ...)- but before I would like to know if there is not an existing library for this purpose already available for Java?.
I know that it is very difficult to completely generalize programming patterns in a way that they could be reused across different implementations with complex requirements (e.g., composition of patterns, classes participating in more than one pattern, etc ...). However, most of the time the pattern instantiations I need are quite simple and standard, and in many situations the implementation work could be sped up a bit with the use of such a library.
Thanks for your feedback.!
Design pattern are just... patterns. They aren't classes ready to use for anyone, but common concepts found across several projects. That's why you won't find a Design Pattern API.
This is precisely the reason why I created PerfectJPattern. Just make sure to check out and understand the code examples. They are available for download as well as in the site documentation pages for each of the Pattern implementations. If you read the GoF book, then you will understand the examples more easily.
For instance, if you want to use the Composite Pattern in your code, and if you use PerfectJPattern you only need to specify which interface (generic parameter and class instance) you would like to use as Composite and the rest is provided to you, see PerfectJPattern Composite. At the bottom of that page, a working example is provided that shows how to accomplish that.
Another aspect you should also take into account, is that in PerfectJPattern you do not necessarily need to reuse the generic Pattern implementations (e.g. perfectjpattern-core Maven submodule), you also do have the choice to only reuse the pure abstract level (perfectjpattern-api Maven submodule) and provide the implementation yourself. In PerfectJPattern you have the flexibility of reuse at different levels of abstraction since there is a fine-grained layered design reflected also in the Maven project structure. Reusing the perfectjpattern-api gives you an abstract template guideline if you wish, that will help you speed up your own Design Pattern implementations. However, ideally you should reuse as much as possible.
Update: following up on a comment below, it is worth noting that not all Patterns can be fully compotentized see From Patterns to Components. Some Patterns can be only partially componentized and some others not at all like the case of the Singleton. The Singleton depends too much on the context and that's why you can only find an interface in PerfectJPattern. However in PerfectJPattern the following Patterns are fully componentized e.g. Observer, Command, Adapter, Decorator, Composite, Proxy, Visitor, DAO, etc.
I disagree with the other answers that no reuseable implementations can be created for design patterns. However, it might not always be straightforward, and involves a lot of abstract programming.
Coming from C# I was missing the simplicity of the observer pattern in Java. (events in C#) After finishing a reusable generic observer for Java I came across the PerfectJPattern library.
It might be worthwhile to check it out.
A componentized pattern is in essence a context-independent, reusable
and type-safe variation of the original pattern that covers at least
as many use-cases as the original pattern and that does not require
developers to re-implement the same boilerplate code in every
different context. Design Patterns are reusable in terms of design,
componentized patterns are reusable in terms of design and code.
Kudos for wanting to reduce any form of duplication possible. "Don't repeat yourself" is one of the most important principles in programming.
As some extra arguments design patterns can be centralized in a library I give you some further examples:
Lazy initialization in C#.
Entire LINQ is based on IEnumerable.
Java's attempt at a reusable Observer pattern. (I didn't say all are good.)
Several patterns integrated in the Spring framework.
As in Pangea's answer: Java JT framework.
What you are looking for is PerfectJPattern.
Checkout dp4j. It allows you to implement the Singleton pattern with #Singleton and lazy initialize it with #Singleton(lazy=true). It is a "collection of reusable componentized Design Patterns implemented in Java".
For the Singleton recommend dp4j. To implement a Singleton you annotate your class with #Singleton and to lazy initialize it you annotate with #Singleton(lazy=true).
If you find yourself repeating similar code in multiple projects, it might be a good idea to extract the portions that are repeated into a library of reusable code in hopes of avoiding repeating yourself in the future.
This is unlikely to lead to fully general reusable implementations of design patterns.
JT Design Pattern Framework for Java/j2EE
A lot of the patterns are built into the Java SE - you may not notice.
Decorator is all over the java.io package.
The java.sql package is AbstractFactory.
All the wrapper classes are Flyweights.
I stand with those who say don't look for a pattern library. They should be discovered as you write your code.
It is possible in a languange which offers higher order functions and some other stuff. Here are some slides which references a paper which discusses 'the origami pattern' library. It shows 'library code' for the following patterns: Composite, Iterator, Visitor, Builder. So if you want to try that out on the jvm you can today, just use scala. http://www.slideshare.net/remeniuk/algebraic-data-types-and-origami-patterns
Do you see a forest or trees?
Design patterns should become apparent rather than be selected up front. By the time they become apparent, you don't need the pattern because the code exists. Then you marvel at how the solution solved itself and your brain subconciously selected the right approach. It gives you that warm fuzzy "i trust that code" feeling.
If you build a design pattern library, you have built yourself a big hammer (or hammer factory, or hammer factory factory [1]) and everything becomes a convenient nail (including screws) resulting in lots of sore thumbs and bowls of spaghetti in your development team.
[1] http://discuss.joelonsoftware.com/default.asp?joel.3.219431.12
I am student striving to improve my Java and general programming skills. I am familiar with some of the basic design patterns.
I have two books on my disposal "Head First to Design Patterns" and "Effective Java"(Josh Bloch), both aiming to promote good programming practices.
However I am confused as to "how should I read the texts so that it makes the most sense?"
From what I understand, design patterns are basics and effective java has some addon practices in addition to general design patterns.
What is the right way to look on the entire scenario?
You just have to build your knowledge up. Some of that is by reading good books, of which the Josh Bloch book is a very good book. Some of it is by taking on programming tasks in your spare time and keeping an eye on blogs and other websites such as stackoverflow. While some of it you will get through other practical experience i.e. at work.
These things take time and come with experience. Just enjoy the language and immerse yourself in it. Patterns aren't the be all and end all of the language - they're part of an effective toolkit though. It's better to understand the language fully, then you will see how design patterns can help solve common problems and often solve them better. But definitely don't get obsessed by patterns, until you know the language well enough. If fact don't get obsessed by patterns at all!
I'd say the Josh Bloch book is the best book to concentrate on 1st - it's excellent and introduces you to all the key concepts (common methods such as equals and hashcode, enums, concurrency) and also introduces patterns which you'll get day to day benefits from e.g. Builder pattern.
However I am confused as to "how should I read the texts so that it makes the most sense?"
My only advice is to avoid overkill and over-engineering. Some experienced OOP (and Java) programmers that learn design patters taking them as they were the Bible often create fuzzy and useless code. That seems contradictory, but if you think about it, it isn't.
When the goal of programming is no longer resolving a problem, producing useful tools, and having measurable results, but becomes the exposition of one's great sophistication, cleverness, and sometimes smartass egotism, then the code becomes just a bunch of tricky brainwave that is useless to anybody, excluding the one who wrote the code and feels good looking at his crazy "white elephant".
Many times I've seen useless applications of them. The most abused is the factory pattern, that in this moment is quite trendy and in fact put everywhere like a food dressing.
Many times I've seen proliferation of classes and interfaces, useless generalizations, which are pointless: in some cases I've seen packages with 2 useful classes, and 10 interfaces and/or abstract classes. I too feel the need of making the code "generic" and "reusable" but there has to be a limit.
Many times I've seen "religious wars" about these topics.
Read all the books you want, and take from them what you need. But after that, write reasonable code. Write useful stuff, that's the original reason why you started programming.
The two books complement each other. Effective Java is about best practices in writing java code. Design Patters describe a set of standard solutions to common software problems. The design patterns are language independent really, while Effective Java is obviously focused on the java language.
But my general approach is "always agree with Josh Bloch" ( unless Jon Skeet disagrees with him )
Programming patterns are best practices.
They should be used sparingly.
Every design pattern tries to handle some form of variation.
For example strategy pattern tackles a variation in implementation
Which is basically a fancy way of saying it is a plug and play method.
Patterns result in flexible code, but they create a large amount of extra complexity in the code.
Maybe you want to take the book Applied Java Patterns next to those books you already have.
It's written by Olav Maarsen and Stephen Stelting. Also the root of all patterns are the Gang of Four, you might want to use their book as a refference as well.
Design Pattern is not always good. Overuse of Design Pattern is bad.
Josh Bloch, however, is always right. Trust him and you are saved.
Forget Design Pattern. Josh Bloch's book gives you all the patterns you need.
What are the design patterns that every developer must know?
I'm interested in the context of Java web developers working with Spring & Hibernate. I have often heard that good knowledge in design patterns is essential for working with those frameworks. Can anyone list the specifics?
For example, I know that understanding abstract factory & factory pattern, singleton pattern etc is absolutely essential. I'm looking for a comprehensive list.
Inversion of Control
If you are ever going to design decoupled systems, you will need to know how to properly link dependencies between classes.
Command Pattern and Variants
In Java in particular, it is essential to learn how to pass a piece of functionality to another method as an object because of the lack of closures and function pointers in the language.
Factory Pattern
Factories are ubiquitous in Java frameworks and it is essential to learn why and when to use the factory pattern.
Singleton (pattern and anti-pattern)
Learning how to use the singleton pattern responsibly is very helpful for understanding the pitfalls in other people's code you may be reading.
Overall, learning the why with regards to patterns is much more important the the how. Knowing when not to apply a pattern is just as important as knowing when to.
Everybody should know about Singleton, but also when not to use it! It's currently the source of much pain for me on a project at work.
Singletons make the code hard to understand and follow, and make writing unit tests much more difficult. I like the blog post Singletons are Pathological Liars.
Most design patterns are pretty obvious--you already know and use them if you've been programming a few years.
The biggest advantage I've found to design patterns is sharing a common set of names. If someone says "Callback" that can mean quite a few things, but if someone says "Listener Pattern" that means a more specific set of calls and implies a higher level relationship between objects.
So in essence, read through a good design patterns book, get an idea of the name of each pattern, spend some time understanding any you don't know, and you're good to go.
I wouldn't completely ignore any of them--they are all good to have seen. You should be able to recognize a situation that might benefit from a specific pattern and know where to look to find out more about it.
Model–view–controller just has to be on the list, Spring has an MVC framework:
http://en.wikipedia.org/wiki/Model–view–controller
I recommend you to read Head First Design Patterns book. This is a well written book about all commons and useful patterns.
I would recommend you get and read the Design Patterns book, since it gives you the vocabulary.
But don't forget the fundamentals :)
Interviewing Java Developers With Tears in My Eyes
http://java.sys-con.com/node/1040135
Hibernate? Then the Unit Of Work is a must http://martinfowler.com/eaaCatalog/unitOfWork.html
Composite, it's present in the JUnit framework. (Test-TestCase-TestSuite)
The Adapter, Builder, Command, Template Method and Strategy patterns are easy and often usable in practice.
The State pattern also helped me to clean up mess in inherited source codes.
This would be a comment to Greg Hewgill's reference to "Singletons Are Pathological Liars", but I can't make comments yet.
That article makes a convincing case, but his ire is misdirected. As several commenters on his blog noted, his problem is really global state. His code fix could still be making use of singletons, and still gain the exact increase in clarity and testability.
Re-read the article. He's not bothered that OfflineQueue needs an initialized Database instance, nor that CreditCardProcessor needs an initialized OfflineQueue. He's bothered that those dependencies aren't visible, which causes issues with maintainability and testability.
His problem is with secret global state (does this make me sound like a conspiracy theorist?).
However, he's (imo) misinterpreting that secret global state as being the fault of singletons.
That doesn't mean I'm in favor of singletons where they're not necessary - certainly, they have drawbacks (including the obvious thread contention bottleneck possibility). But I prefer to be clear about what practices I'm eschewing.
Incidentally, I'd go further in my refactoring - based on the class names, I'd assert in a code review that CreditCardProcessor should, well, process the charges, so instead of his:
card.charge(cardProcessor, 100);
I'd have this, instead:
cardProcessor.chargeCard (card, 100);
(and yes, I replaced his variable names c and ccp with names I considered more readable)
Apart from Abstract factory , Factory Method and Singleton patterns, which you have quoted already, I think below patterns are useful.
Bridge Pattern : Abstraction and implementation can change independently
Decorator Pattern : Change the behaviour of object at run time
Mediator Pattern : Enable central communication medium between different objects
Chain of Responsibility : If you are adding filters to web service request, this is very useful.
Strategy Pattern : If you want to change the algorithm from a family of algorithms at run time by checking a parameter
Facade Pattern : If you have many services in your system and don't want to expose all the services to client, have one Facade class, which will interact with other services.
sourcemaking provides excellent details on each design-pattern : Intent, Strucutre, Checklist and Rules of thumb.
One more SE question would be definitely help you :
Design Patterns web based applications
Singleton - Singletons apparently can and should be used for everything