Object-Oriented Techniques - Java - page 318

The private field can be assigned from within a static initializer block or, more simply, using
an initializer. The getInstance() method (which must be public) then simply returns this
instance:

public public class class Singleton Singleton {

private private static static Singleton instance ;

/** A private Constructor prevents any other class from instantiating. */
private private Singleton () {

// nothing to do this time

}

/** The Static initializer constructs the instance at class loading time;
* this is to simulate a more involved construction process (it it
* were really simple, you'd just use an initializer)
*/
static

static {
instance = new new Singleton ();

}

/** Static 'instance' method */
public public static static Singleton getInstance () {

return

return instance ;

}

// other methods protected by singleton-ness would be here...

/** A simple demo method */
public

public String demoMethod () {
return return "demo" ;

}

}

Note that the method of using “lazy evaluation” in the getInstance() method (which is ad-
vocated in Design Patterns ), is not necessary in Java because Java already uses “lazy load-
ing.” Your Singleton class will probably not get loaded unless its getInstance() is called,
so there is no point in trying to defer the singleton construction until it's needed by having
getInstance() test the singleton variable for null and creating the singleton there.

Using this class is equally simple: simply get and retain the reference, and invoke methods
on it: