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Thread: Singletons - I'm having a hard time understand how to implement them.

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    Default Singletons - I'm having a hard time understand how to implement them.

    Heya. I'm pretty new at programming - currently following the BlueJ learning book.
    Our teacher is trying to beat some Singleton sense into our heads but I'm having a hard time going from theory to practice in one of my applications.

    I can change the "main" "runtime" class into a singleton using examples on the internet just fine - the problem arises when it comes to creating singletons of other classes from it.

    The main class is a console interface - a TUI if you will. Upon user request it will call a method to create an instance of another class (A specific interface) - Thing is those are supposed to be singletons as well. How do I go about ensuring they can't be made more than once? The main class runs the following method every time the user wants to change to the class (They are also console interfaces).

    Will changing the code in those classes to a singleton pattern fix it? The "new LibraryUI" bit is worrying me a bit.

    private void startLibraryUI()
        {
            libraryUI = new LibraryUI();
            libraryUI.start();
        }


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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    Post the full code for a sample class where you are trying to define it as a singleton.

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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    Make the constructor private and do something like this:

    public class Singleton
    {
     
    private Singleton()
    {
     
     
    }
     
     
    private static class SingletonHandler
    {
     
    static final Singleton instance = new Singleton();
     
    }
     
    public static Singleton getInstance()
    {
    return SingletonHandler.instance;
    }
     
     
     
     
    }

    Now the only way for stuff outside the class to get to it is by calling the getInstance method. Since the object is final, it can't be instantiated twice. However, if the object finally goes out of scope, then it can be instantiated again. (It will go out of scope if the window is closed or some other reason that would cause the object to "die" for lack of a better word.)

    However, and this goes for GUIs especially, you (probably, hey, there might perhaps be some cases where you DO want to do this) shouldn't do something like this (create a Singleton like that above)

    method.addActionListener(new ActionListener() {

    public void actionPerformed(ActionEvent e)
    {

    s = Singleton.getInstance();


    }
    });

    Rather, do it before the ActionListener, if the GUI is a Singleton, and make it invisible initially and have the ActionListener make it visible. Doing it the former way makes a new, and separate somehow, instance of Singleton. How that happens, I'm not quite sure. But it does.

    I guess a way to think of it would be if you had a FontChooser for a tabbed Notepad. You might want to make sure that each Notepad is allowed only 1 FontChooser at a time. However, there should be nothing wrong for EACH Notepad in the tabs having (at most) one FontChooser, and it wouldn't violate the instance thing somehow.

    I don't know the exact mechanisms of how a Singleton works, though I DO know that scoping is part of that mechanism.

    I know that the Calendar (Java Platform SE 7 ) Calendar class in Java is a Singleton.

    In fact, it seems you can have multiple getInstance() methods.

    static Calendar getInstance()
    static Calendar getInstance(Locale aLocale)
    static Calendar getInstance(TimeZone zone)
    static Calendar getInstance(TimeZone zone, Locale aLocale)
    Last edited by javapenguin; February 16th, 2012 at 12:24 AM.

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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    Quote Originally Posted by javapenguin View Post
    M
    I know that the Calendar (Java Platform SE 7 ) Calendar class in Java is a Singleton.

    In fact, it seems you can have multiple getInstance() methods.

    static Calendar getInstance()
    static Calendar getInstance(Locale aLocale)
    static Calendar getInstance(TimeZone zone)
    static Calendar getInstance(TimeZone zone, Locale aLocale)
    Calendar is NOT an example of a Singleton. Once again feeding incorrect information. The Runtime class is a much better example of why you would want only a single instance of a class in an application, and is implemented as such.

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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    Calendar isn't a Singleton?! Then why does it have a getInstance() method?

    What is the Runtime class for? It does indeed appear to be a Singleton.

    Calendar's constructor is protected, so maybe it isn't a Singleton, but it appears to be nearly a Singleton, as you can't access the constructor directly (unless you subclass Calendar.).
    Last edited by javapenguin; February 16th, 2012 at 05:36 PM.

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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    With a singleton, you expect there to be only one instance.
    Call getInstance() twice and compare the references that are returned to see if they are the same.

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    Default Re: Singletons - I'm having a hard time understand how to implement them.

    Quote Originally Posted by javapenguin View Post
    Calendar isn't a Singleton?! Then why does it have a getInstance() method?

    What is the Runtime class for? It does indeed appear to be a Singleton.

    Calendar's constructor is protected, so maybe it isn't a Singleton, but it appears to be nearly a Singleton, as you can't access the constructor directly (unless you subclass Calendar.).
    Having a getInstance() method and/or a private/protected constructor does not define a Singleton - it is defined just as Norm has defined it: there exists one instance in an application.

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