During the initialization of a class, the class makes an active use of a subclass. That subclass will not yet be initialized at the time of this use. For example, in the following code, foo will be null.

public class CircularClassInitialization {
	static class InnerClassSingleton extends CircularClassInitialization {
		static InnerClassSingleton singleton = new InnerClassSingleton();
	}
	
	static CircularClassInitialization foo = InnerClassSingleton.singleton;
}