Command Design Pattern

Could someone please explain the command design pattern.
Initially, I thought each action resulted in a the creation of a new command object that contained the data and methods to accomplish its task.
But it appears to me now that the command object representing a possible action would actually be created at initialization and it would be given to all the related objects that need to update the command object or respond to the command object .
Could someone provide some clarity
Stephen

I don't see that it makes a difference whether the command objects are reusable or not.

Similar Messages

What is command design pattern? what is the problem that it solves?

Hi i have been reading few articles on command design pattern but was unable to understand what it is and what problem does it solve. As far as my understanding goes if there is a pre specified method in an object that could execute and perform the necessary activities then it is called as command design pattern. Why is it named as command design pattern, it is because in a shell we use commands to retrieve output. For example when we type "dir" in dos command prompt it returns the list of files. so here we have to remember a preexisting exe file named "dir" which does something when used. So here like we call it command because when "dir" is passed to the computers it returns something that is expected. may be the computer obeys the command issued and return something back. So like wise if there is a method that is defined as as the command, for example in struts framework the action class must have a method "execute()" which is called by the framework to execute the necessary action mapped to it. So if there is a predefined method like "execute" in struts then the object is said to be followed command design pattern. So what actually does it solve?
What ever i have written is my understanding from the articles i have read about the design pattern. So kindly correct me if i am wrong. Help me understanding what actual problem does this pattern solves. Thanking you in advance.

This is really off-topic for these forums. You might do better on StackOverflow.com. I'll just say that although the first Design Patterns book came as a revelation to me, it was more for (a) the nomenclature (decorator, factory, facade, etc) which is still with us and (b) about five of the original patterns, which told me things I didn't already know. I've never used any of the others, including Command, and the whole design patterns thing quickly degenerated into farce with what seemed like a sustained attempt to reduce the whole of computer science to design patterns, which really isn't the point. As an example, there is one in another book called Type Object, which is nothing but 3rd normal form, which can be defined roughly in a couple of sentences. This pattern went on for dozens of pages with a very artificial example. I wrote to the mailing list about it: 'Type Object considered hilarious'. I got a rude reply from the original author, which I guess I expected, but also an incredible response from the list owner, saying I should recast my objections in the form of a design pattern. That lost it for me. True story. Moral: don't take this religion, or any IT religion, too seriously. They come around every 18 months or so with the regularity of the next bit of Moore's Law. I've seen dozens of them come and go since 1971, all with something to say, all 90% nonsense.
See here for a longer version of this incident.

JavaBean Models, Command Bean and J2EE Design Pattern

Hello,
I have read the available "How To", Using EJBs in Web Dynpro Applications where the author mentions that one should use a Command Bean (according to J2EE Design Patterns) so that one can invoke business methods in a Session Bean from our Web Dynpro Application. Well, although, I have read some available articles in the internet about J2EE design patterns, I still have some questions about command beans and its usage in Web Dynpro applications.
I have developed a WD App which uses EJBs to read data from the DB.
Let's suppose I only have two tables: BOOKS and AUTHORS.
Lets also suppose I have to Entity Beans (one for each table) and a Session Bean with two methods: Book[] getAllBooks() and Author[] getAllAuthors();
I also have a Command Bean which I imported in my WD App.
My questions are:
How should I design my Command Bean?
Can I have only one Command Bean with two methods each calling one of the methods of the Session Bean?
Or instead should I design two Command Beans, one for each call?
In the last case do the methods must be named <b>execute</b> or can I name them whatever I want?
Furthermore, how should I store the data in my command bean? In instance variables?
If so, can I use array of a class representing a record in the database table (for instance, classes Book and Author) or do I have to store the data in a generic collection (such as ArrayList for instance)?
I ask this last question because if I try to store the data in an array when I am importing the Command Bean as a JavaBean model I always get an error.
One last question: Can the Command Bean execute method directly return the data or should I always store the data in an instance variable an then get it using getter methods?
I know this questions are more about J2EE Design Patterns and JavaBeans specification than they are about Web Dynpro but I also have doubts about the rules that the command bean must obey in order to accomplish a successful JavaBean model import in WD.
Some guidance or tips would be highly appreciated.
King Regards

I have the same problem.
Does anyone know the solution?
Thanks
Davide

What is the best design pattern for this problem?

No code to go with the question. I am trying to settle on the best design pattern for the problem before I code. I want to use an Object Oriented approach.
I have included a basic UML diagram of what I was thinking so far.
Stated simply, I have three devices; Module, Wired Modem, and Wireless Modem.
In the Device Under Test parent class, I have put the attributes that are variable from device to device, but common to all of them.
In the child classes, I have put the attributes that are not variable to each copy of that device. The attributes are common across device types. I was planning to use controls in the class definition that have the data set to a default value, since it doesn't change for each serial number of that device. For example, a Module will always have a Device Type ID of 1. These values are used to query the database.
An example query would be [DHR].[GetDeviceActiveVersions] '39288', 1, '4/26/2012 12:18:52 PM'
The '1' is the device type ID, the 39288 is the serial number, and the return would be "A000" or "S002", for example.
So, I would be pulling the Serial Number and Device Type ID from the Device Under Test parent and child, and passing them to the Database using a SQL string stored in the control of the Active Versions child class of Database.
The overall idea is that the same data is used to send multiple queries to the database and receiving back various data that I then evaluate for pass of fail, and for date order.
What I can't settle on is the approach. Should it be a Strategy pattern, A Chain of Command pattern, a Decorator pattern or something else.
Ideas?

elrathia wrote:
Hi Ben,
I haven't much idea of how override works and when you would use it and why. I'm the newest of the new here.
Good. At least you will not be smaking with a OPPer dOOPer hammer if I make some gramatical mistake.
You may want to look at this thread in the BreakPoint where i trie to help Cory get a handle on Dynamic Dispatching with an example of two classes that inherit from a common parent and invoke Over-ride VIs to do the same thing but with wildly varying results.
The example uses a Class of "Numeric" and a sibling class "Text" and the both implement an Add method.
It is dirt simple and Cory did a decent job of explaining it.
It just be the motivation you are looking for.
have fun!
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction

Chain of Responsibility Design Pattern

Is there any java api which helps in implementing Chain of Responsibility Design pattern by parsing a XML file?. Can I use XML file to do this in Apache Commons Chain API?

A quick look at the javadocs for that project suggest that, yes, indeed they do provide a way to specify a chain of command using XML.
In fact I'm kind of confused why you even asked this question, since you already seem to know the answer.
I've never heard of such a thing in the standard API, if that's what you're asking, probably because it's a simple pattern to implement, and most of the pain of any implementation is likely to be due to details specific to the problem domain, not the pattern.
If you're really eager to use XML, you might want to check out Spring. You could configure a bunch of beans in a chain, if you like.

Choice of design pattern for data acquisition system

Hello all
I have a trouble about selecting the suitable design pattern / architecture for a data acquisition system.
Here is the details of the desired system:
There is data acquisition hardware and I need to use it by observing parameters on User interface.
the data acquisiton period, channel list to scan should be chosen on User interface. Besides, there are many user interface interactions. e.g. if user selects a channel to add scanlist, then I need to enable and make visible some other parts on user interface.
When user completes the channel selection, then he will press the button to start data acquisition. Then I also need to show the scanned values on a graph in real time and log them in txt file.
I know that I cannot use producer consumer pattern here. because the data acquisition loop should wait for parameters to scan channels. and it works in a given period by user. so the user interface loop performs higher rate then consumer loop (data acquisition loop). it means queue will be bigger bigger. if I use notifier it will loss some data come from user interface.
is there any idea about that ? is there any suitable design pattern for this case ?
Thanks in advance
best regards
Veli BAYAR
Embedded Systems Software and Hardware Engineer
"You live in a graphical world. Why not program in one?"
Solved!
Go to Solution.

johnsold wrote:
Veli,
I recommend the Producer/Consumer model with some modifications.
You might need three loops. I cannot tell for sure from your brief description.
The User Interface loop responds to the user inputs for configuration and start/stop of acquisition. The parameters and commands are passed to the Data Acquisition loop via a queue. In this loop is a state machine which has Idle, Configuration, Acquisition, and Shutdown states (and perhaps others). The data is sent to the Processing loop via a different queue. The Processing loop performs any data processing, displays the data to the user, and saves it to file. A notifier can be used to send the Stop or shutdown command from the User Interface loop to the other loops. If the amount of processing is minimal and the file write times are not too long, the Processing loop functions might be able to occur in the Timeout case of the UI loop Event structure. This simplifies things somewhat but is not as flexible when changes need to be made.
I am not sure that a Design Pattern for this exact setup exists but it is basically a combination of the Producer/Consumer (Events) and Producer/Consumer (Data) Design Patterns.
Lynn
Check out this thread: http://forums.ni.com/t5/LabVIEW/Multiple-poll-case-structures-to-event-help/td-p/2551309
There are discussions there about a 3-loop architecture that may help you.
Jeff
Jeffrey Zola

Design Pattern / Best Practice Question

Hi,
I have been using Flex for a while now, but there is a
scenario which I still have not found a solution I'm entirely happy
with. I'm wondering if anyone else out there might have suggestions
on a design pattern or best practice.
Suppose I have a view which depends on model data which
resides in some back end systems. That model data may or may not
have been loaded (e.g. via a web service or remote object call) at
the time the view is displayed.
I don't know if the user will ever visit this part of the
application so I would prefer to defer retrieval of the data until
the user actually navigates to this view. Or I want to retrieve the
data each time the view is displayed because the data is dynamic
and could change between one presentation of the view and the next.
Because the data comes from several systems, I cannot simply
make one service call and display the view when it completes and
all the data is available. I need to call several services which
could complete in any order but I only want to display my view
after I know all of them have completed and all of the model data
is available. Otherwise, I can present the user an incomplete view
(e.g. some combo boxes are empty until the corresponding service
call to get the data completes).
The solution I like best so far is to dispatch a single event
(I am using Cairngorm) handled by a single command which acts as
the caller and responder for all of the services. This command then
remembers which responses it has received and dispatches another
event to navigate to the view once all the results have returned.
If the services being called are used in different
combinations on different screens, this results in proliferation of
events and commands. An event and command for each service and
additional events and commands to bundle the services and the
handling of their responses in the right combinations for each of
the views.
Another approach is to have some helper class listen for all
of the model changes and only display the view when the model
enters some state that is acceptable. It is sometimes difficult to
determine just by looking at the model whether it is in the right
state (e.g. how can I tell that a collection is the new collection
that should just have been requested versus an old one lingering
from a previous call). The logic required can get kind of
convoluted and brittle.
Basically, all of the solutions I've come up with so far seem
less than ideal and a little hackish. I keep thinking there is some
elegant solution out there that I am just missing ... but so far,
no luck finding it. Thoughts?
Thanks.
Bill

i think a service class is right - to coordinate your calls.
i would have 1 event per call (so you could listen to individual
responses if you wanted to).
then i would use a flag. if you want to check for staleness,
you would probably want two objects to map your service flag to
lastRequested and lastCompleted. when you check, check if it's
completed, and if it's not stale and that your lastRequested is
less than lastCompleted (meaning that you're not currently waiting,
i.e. you've returned since making a request). then make the request
and update your lastRequested.
here's a snippet of what i mean.
./paul
public static const SVC1_LOADED:int = 1;
public static const SVC2_LOADED:int = 2;
public static const SVC3_LOADED:int = 4;
public static const SVCALL_LOADED:int = 7;
private var completedFlag:int = 0;
then each call would have it's own callback.
private function onSvc1Complete( evt:Event):void {
completedFlag |= SVC1_LOADED;
lastCompleted[ SVC1_LOADED ] = getTimer();
dispatchEvent( new Event("svc1complete") );
checkDone();
private function checkDone():void{
if( completedFlag == SVCALL_LOADED )
dispatchEvent(new Event( "allLoaded" ));

Is this a good design pattern?

Hi,
I am working with a team to create an application that can acquire data from up to 24 thermcouple channels. We plan to create a wizard to assist the user in setting up the test (specifiying stop conditions, sample rate, channels to sample, output file, and email/text notifications). After the setup, if the user does not cancel out, then the program continues. We have defined parallel loops and their roles, trying to fit into a producer/consumer design pattern. I have attached the main file which shows the structure of the program. We are trying to keep it clean and modular. Before diving into development, we are finalizing a detailed design specification so that different people can work on each of the loops' subvis. I was hoping that people could review the block diagram and offer suggestions for improvement or validate that we have chosen a good method of implementation.
Notes about the block diagram
The plan is for data driven stop conditions to be detected by Event Notifier, handled by the Main UI event, and then the Data Collector is instructed to stop (as well as the Main UI loop). When Data Collector stops, it releases queues, the queue reference becomes invalid, and then the loops with the Dequeue will be stopped.
I am not sure if an additonal event structure should be in the DataCollector to specifically handle the Stop button on the UI because if it is in the Main UI event structure, the DataCollector would not be stopped immediately. This is important to stop quickly because the sample rate could be VERY long. I understand about using an event structure instead of WAIT, but didn't know if there is an efficient way leaving it down in the Main UI event structure....????
Data Collector is the producer, it currently only collects data and passes it on. If the program is going to contain a start collection button, or pause/resume, I need to figure out how to handle this with the Main UI event structure.
Event Notifier consumes data from the Data Collector. It looks at all data and should set a flag if a stop condition is met. The flag will be handled by the Main UI event structure to tell Data Collector to stop. The event notifier also sends a notification to the Email/Text Caller when thresholds have been exceeded warranting a reason for the user to be notified via text or email.
Email/Text Caller waits for notification and then calls a subVI to send an email or text with certain data and channel information. The subVI call is lengthy, so we decided to put this in its own loop instead of a case structure inside the Event Notifier.
Report Generator logs all collected data to a file. A second queue was used, but uses the same data as the first. Is this the correct implementation since two consumers need the same data?
Main UI - handles changing of sample rate, possibly the stop button, possibly start/pause/resume of data collection. Also handles when Event Notifier sets flag for a data driven stop condition, which then should provide a mechanism to stop the Data Collector.
A big question that i have is.... Should the Main UI event structure just be in the Data Collector so that the loop can be stopped immediately? All of the UI events (stop button, start/pause/resume data collection) directly effect the Data Collector. Should the data Collector have a state machine instead?
I feel that the 3 consumer loops (Event notifier, Email/Text Caller, and Report Generator) seem like a good implementation, but i am not confident about the Data Collector and Main UI loops. Should they be combined?
Solved!
Go to Solution.
Attachments:
updatedMain.vi ‏13 KB

A couple of thoughts:
Overall it looks workable- remember to name your queus and notifiers- then you can depop the BD and launch each loop in its own sub-vi without wiring all those queues between loops- Just obtain ref to named queue.
"Data collector" needs a supervisor to control how often the data is collected- I'm thinking a QSM with "init- collect- stop" states. A simple timed loop could produce the "time to acq ticks" under control of the sample rate control. This puts both the DAQ init and cleanup functions within the Data Collector's perview so it aids coehesion and solves the exit without waiting for the loop to iterate.
I'm not a fan of duplicating data- I assume analisys is pretty much just some comparisons- so it is pretty quick- Why not append summary data to the same queue? And make it a single element queue- the report loop can then inspect the queue and look for a summary data "index" - if its greater than the last index written append the darn data to the report? Preview queue works great for these situations.
Do not combine the UI handeler with anything else! Do you see the snag?- hint: here thar be spaghetti. keep the functions seperated for solid scalable and maintainable code.
One last thought: Its getting less popular to stop loops on the error out of a dequeue. First it returns a default element that can cause your listeners some headaches. Second it doesn't let you run cleanup code. Use a stop command <enum> built into the element data as an executive to determine when to stop. We'll try to get the examples updated to show current best practices.
Jeff

Design patterns implemented in java API

Hi,
I have some questions on design patterns implemented in core java class or in general in java API.
1)Whether
java.util.Collections, the checkedXXX(), synchronizedXXX() and unmodifiableXXX() methods.
can be considered as a decorator pattern?
2) Whether
LinkedHashMap(LinkedHashSet<K>, List<V>) which returns an unmodifiable linked map which doesn't clone the items, but uses them
can be considered as a Bridge pattern?
3) Whether Facade pattern is implemented in java ? If so which API uses it?
4) Whether
•     All non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer.
•     All non-abstract methods of java.util.AbstractList, java.util.AbstractSet and java.util.AbstractMap
can be considered as a Template method pattern?
5) Whether
•     java.util.Comparator#compare(), executed by among others Collections#sort()
can be considered as a Stratergy pattern?
6)
Whether State pattern is implemented in java ? If so which API uses it?
7)
All implementations of java.lang.Runnable are considered as a Command pattern.
8)
Whether
•     java.io.InputStreamReader(InputStream) (returns a Reader)
•     java.io.OutputStreamWriter(OutputStream) (returns a Writer)
can be considered as an Adapter pattern?
Please clarify.
Thanks.

What do you think, and why?

CairnGorm Design Patterns

Can any one let me know the design patterns
implemented in Cairngorm?

Are you talking about Cairngorm 2 or 3 ?
Cairngorm 2:
Model View Controller
Command
Responder
Delegate
Singleton
Cairngorm 3 (on top of Parsley):
Dependency Injection
Presentation Model

What kind of design pattern is this?

Hi ,
I am just learning the designpattern. So when I went through different kinds of design pattern, I got some doubts as it looks same some design patterns. could anybody please tell what kind of design pattern is the following one?
public interface IExporter{
void export();
public class ExcelExporter implements IExporter{
public void export(){
//TODO implementation
public class CVSExporter implements IExporter{
public void export(){
//TODO implementation
}thanks in advance..

Well - in structure, it is nearest to the Command pattern as far as I can tellOr Strategy. Or Builder. Several design pattenrs may share the same class structure, the difference is in the context, forces, and the parts one want to be flexible. And if someone wants to make it explicit which pattern is applied, they should name things accordingly.
As Saish said this example shows little more that inheritance at work. Not a pattern in itself, but a core element of the language. Trying to "identify" a pattern that is not obvious from the context is either:
- a sign that the original coder completely missed the point of using a pattern (which is, as Saish said too, assentially about communication)
- a sign that the maintenance coder is looking for patterns where there aren't
- an exercise question
but it differs in so much as its motivation is to polymorphically allow the use of different exporters rather than provide a set of action commands.That shouldn't count as a difference, merely as an "application" (it's perfectly valid to have a hierarchy of commands all devoted to exporting, although, as stated, if the original coder applied the Command pattern, he should have made it explicit, by renaming the interface, e.g. to ExportCommand).

Design Patterns that are used in standard j2se/j2ee classes/interfaces

Hi All,
I am understanding following design patterns (used within standard j2se/j2ee):
Adapter
Facade
Composite
Bridge
Singleton
Observer
Mediator
Proxy
Chain of Responsibility
Flyweight
Builder
Factory Method
Abstract Factory
Prototype
Memento
Template Method
State
Strategy
Command
Interpreter
Decorator
Iterator
Visitor
I want to see if/where these design patterns are used in j2se/j2ee classes/interfaces. i am looking for few examples of standard java classes/interfaces/cases where these design patterns are used by jdk developers.
for e.g.
WindowAdapter class is an example of Adapter DP.
JOptionPane is an exmple of Facade DP.
MouseListener is an example of Observer DP.
Similarly, where can find examples of jdk classes/interfaces of the remaining DPs.
I searched a lot of books, but they explain the DPs by creating their own classes/interfaces.
I would like to see where these DPs are already utilised in std j2se/j2ee
thanks,
Madhu_1980

877316 wrote:
I searched a lot of books, but they explain the DPs by creating their own classes/interfaces.
I would like to see where these DPs are already utilised in std j2se/j2eeWell, you can go through the javadocs first, they sometimes mention the pattern used.
Then you can get the sources for the jdk and go through the classes yourself, identifying the patterns.

Suggest what design pattern is applicable for my case

I have an object called GrantAward , these awards I have to show to the user for selection
1. I call a web service to get list of GrantAwards.
2.for each GrantAward check if this is present in my database
a. if not there insert
3.for each GrantAward check some conditions
a if satisfied add to the list to show to user
b if not satisfied do not add to list .
these are the steps involved before I can show the GrantAwards to user for selection , please suggest me what design pattern can I use ?

I'm not sure you want to hunt for patterns when you already know what you are implementing. A lot of people have pattern fever. Patterns are useful constructs for communicating design ideas and enforcing best practices. But I rarely go out and hunt for a specific pattern (or name my objects after patterns). If you are doing a heavyweight methodology with RUP or something similar, I could see stereotyping your UML with patterns.
That having been said, you already have a Facade with your service. You are undoubtedly using DataAccessObject to fetch your records. Your conditions could be implemented with a ChainOfResponsibility and/or Strategy pattern. What you return could be a Composite. Potentially the request and response are a Command. And your overall architecture could be MVC.
But IMO that is overkill.
- Saish

Complex architecture - design pattern?

I am trying to determine which architecture, or combination woudl best suite my application:
The core aspect is a client receiving data from a network at variable speeds - I have this working using a producer consumer loop (data) design pattern, that starts and stops by placing it inside a case structure with start button. I'd now like to combine this with an event driven user interface, which also uses the serial port to talk to external hardware every now and then. I have that working seperately with a producer consumer (event) design pattern. I'd like to combine them and include regular events (handshaking with the external hardware) to take place whilst the producer-consumer (data) reads from the network.
How can I mix the two design patterns?! I'm struggling to get my head 'round this.
Thanks

Here is an example of two Producer-Consumers in one vi. Notice the two queues. One is for commands to the event loops. The other is for the serial data loops. When certain data is received, you can send commands to the event loop (actually to Consumer loop, 2nd from top) to cause some action. So the user can initiate some action via event structure, and received data can also initiate some action.
This is complex and it might not do all you want. But its a start.
- tbob
Inventor of the WORM Global
Attachments:
DoubleProducerConsumer.vi ‏29 KB

Design Pattern for low cohesion & coupling

Hi there!
I�m about to make a applictation in swing and don�t have so much experince in this API. Normally I program for the web using J2EE tech like Servlets, JSP, custom tag libs and EJB�s. Therefore my biggest challenge for the moment is to find (/ find out) a good framework for coding in swing and minimize coupling between the participants.
I have good knowledge about the MVC paradigm (in web programming) but finds it difficult to apply my knowledge in the Swing area. Do you have links or can recomend some litterature that handles my little but very frustrating problem?
In advance thank�s
Best regards
Niclas Rothman

Check out Java Design Patterns A Tutorial by James W. Cooper.
I am sure there are many other publications out there, but I am not quite sure what your frustration is related to swing.
Design patters are applicable in many aspects and are mostly language independent. In fact swing uses some of the design patters such at the command pattern in using Actions.
Hope this helps
Neelesh

Command Design Pattern

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