Static object

[Dev Cunningham]

What's the best way to create an instance of a Java class that 'globally' available throughout my application? For instance, with C++ I might create an object when my application starts and store a pointer to that object in a global variable.
In this case, I am creating a hash table, which is useful to many parts of my application. Obviously I don't want to create the hash table each time I need to use it.
Thanks!!

[Greg Brouelette]

Do a quick search for "Singleton" on this board.
A "Singleton" object is an object designed in such a way that you can't actually make a new instance of it. You use a static getInstance() method instead. If this is the first time that the getInstance method was called then the object is constructed. On the 2nd and subsequent time that it's called then the method returns the already constructed object.
Here's an example:

public class Foo {
    private static Foo theOnlyOne = null;
    
    public static Foo getInstance(){
        if (theOnlyOne == null){
            theOnlyOne = new Foo();
        }   
        
        return theOnlyOne;
    }
    
    // the constructor is private so people can't make
    // an instance of it.  Only the getInstance method
    // has access to it.
    private Foo() {
        // construtor code goes here
    }
    
// other code goes here
}

[ May 15, 2002: Message edited by: Greg Brouelette ]

[Dev Cunningham]

Thanks Greg!!
That looks perfect.