public class Singleton {
// Private constructor prevents instantiation from other classes
private Singleton() {}
/**
* SingletonHolder is loaded on the first execution of Singleton.getInstance()
* or the first access to SingletonHolder.INSTANCE, not before.
*/
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
Refactorings
No refactoring yet !
There is a very important guaranteed behavior at work here explained by the comment above SingletonHolder. Java classes are loaded and initialized on first access - instantiating the class or accessing one of its static fields or methods for the first time. This is relevant to us because classes are verified by the VM when they are loaded, not before.
source: http://android-developers.blogspot.com/2010/07/how-to-have-your-cupcake-and-eat-it-too.html