Не правильно, но принимает. 
private Singleton readResolve();
Результат: Instance reference check : com.javarush.task.task20.task2008.Solution$Singleton@2ff4f00f Instance reference check : com.javarush.task.task20.task2008.Solution$Singleton@2ff4f00f ========================================================= Object reference check : com.javarush.task.task20.task2008.Solution$Singleton@6df97b55 Object reference check : com.javarush.task.task20.task2008.Solution$Singleton@3cbbc1e0 Правильно 
.... Object.....
Результат: Instance reference check : com.javarush.task.task20.task2008.Solution$Singleton@c818063 Instance reference check : com.javarush.task.task20.task2008.Solution$Singleton@c818063 ========================================================= Object reference check : com.javarush.task.task20.task2008.Solution$Singleton@c818063 Object reference check : com.javarush.task.task20.task2008.Solution$Singleton@c818063