Develop Reference

Static initialization vs Enum approach for Singleton pattern

I am learning singleton design pattern from the head first design pattern book.
They have given one approach which uses static initialization. Static initialization is done at the time of class loading and the problem with this approach is that it will create the instance even if we are not using it(at the time of class loading) which means it will do eager initialization.
public enum Singleton {
private static Singleton uniqueInstance = new Singleton();
// Other useful fields here (can be static or non-static)
private Singleton() {}
public static Singleton getInstance() {
return uniqueInstance;
}
// Other useful methods here (can be static or non-static)
}
They have given one more approach which uses an enum.
public enum Singleton {
UNIQUE_INSTANCE;
// other useful fields here (can be static or non-static)
public static Singleton getInstance() {
return UNIQUE_INSTANCE;
}
// other useful methods here (can be static or non-static)
}
I want to know what is the difference between the above approaches in terms of when the object is created(In both cases when we are using that object in the application and when we are not using the object throughout the application).
In other words, I want to know whether the enum approach will also do eager initialization?

When using an enum, the single constant is public. There's no need to add a getter for it.
As far as I'm concerned, only use an enum if you need any of its advantages. There aren't that many though, since comparison and switching is not relevant with only a single instance. There is one advantage I can think of (if you need it): enums are serializable. Getting serialization to work for non-enum singletons isn't trivial; you need to work with readResolve (and possibly writeReplace) to make sure that when the serialized singleton value is read from the steam, no new instance is returned. One is still created though, it will just be discarded. When using an enum you get serialization for free.

👉 Enum objects are instantiated when the enum class loads.
So yes, you could consider that to be "eager initialization".
Using an enum to define a Singleton in Java is the approach recommended by Dr. Joshua Bloch in his famous book, Effective Java.

Benefit of interface for getInstance call? [duplicate]

Is the Initialize-On-Demand idiom really necessary when implementing a thread safe singleton using static initialization, or would a simple static declaration of the instance suffice?
Simple declaration of instance as static field:
class Singleton
{
private static Singleton instance=new Singleton();
private Singleton () {..}
public static Singleton getInstance()
{
return instance;
}
}
vs
class Singleton {
static class SingletonHolder {
static final Singleton INSTANCE = new Singleton();
}
private Singleton () {..}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
I ask this because Brian Goetz recommends the 1st approach in this article:
http://www.ibm.com/developerworks/java/library/j-dcl/index.html
while he suggests the latter in this article
http://www.ibm.com/developerworks/library/j-jtp03304/
Does the latter approach provide any benefits that the former doesn't?

Well what i can say These articles are 7-9 years old.
Now we have > Java 1.5 where we have power of enumeration enum. According to 'Josh Block' The best way to write a singleton is to write a Single Element enum
public enum MySingleton{
Singleton;
// rest of the implementation.
// ....
}
But for your question I guess there is no issue in using either of the implementations. I personaly prefer the first option because its straightforward, simple to understand.
But watch out for the loop holes that we can be able to create more objects of these class in the same JVM at the same time by serializing and deserializing the object or by making the clone of the object.
Also make the class final, because we can violate the singleton by extending the class.

In first approach your singleton will get created once you load Singleton class. In the other, it will get created once you call getInstance() method. Singleton class may have many reasons to get loaded before you call getInstance. So you will most likely initialize it much earlier when you actually use it and that defeats the purpose of lazy initialization. Whether you need lazy initialization is a separate story.

The simple declaration pattern constructs the singleton when when the class Singleton is loaded. The initialize-on-demand idiom constructs the singleton when Singeton.getInstance() is called -- i.e., when class SingetonHolder is loaded.
So these are the same except for time; the second option allows you delay initialization. When to choose one or the other depends on (among other things) how much work you are doing in Singleton's constructor. If it's a lot, you may see improved application startup time with initialization-on-demand.
That said, my advice is to try not to do too much there so that the simplest pattern works for you.
-dg

Java - what's the difference between singleton and static-block-based classes initialization? [duplicate]

What real (i.e. practical) difference exists between a static class and a singleton pattern?
Both can be invoked without instantiation, both provide only one "Instance" and neither of them is thread-safe. Is there any other difference?

What makes you say that either a singleton or a static method isn't thread-safe? Usually both should be implemented to be thread-safe.
The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that's less common, in my experience), so you can pass around the singleton as if it were "just another" implementation.

The true answer is by Jon Skeet, on another forum here.
A singleton allows access to a single
created instance - that instance (or
rather, a reference to that instance)
can be passed as a parameter to other
methods, and treated as a normal
object.
A static class allows only static
methods.

Singleton objects are stored in Heap, but static objects are stored in stack.
We can clone (if the designer did not disallow it) the singleton object, but we can not clone the static class object
.
Singleton classes follow the OOP (object oriented principles), static classes do not.
We can implement an interface with a Singleton class, but a class's static methods (or e.g. a C# static class) cannot.

The Singleton pattern has several advantages over static classes. First, a singleton can extend classes and implement interfaces, while a static class cannot (it can extend classes, but it does not inherit their instance members). A singleton can be initialized lazily or asynchronously while a static class is generally initialized when it is first loaded, leading to potential class loader issues. However the most important advantage, though, is that singletons can be handled polymorphically without forcing their users to assume that there is only one instance.

static classes are not for anything that needs state. It is useful for putting a bunch of functions together i.e Math (or Utils in projects). So the class name just gives us a clue where we can find the functions and nothing more.
Singleton is my favorite pattern and I use it to manage something at a single point. It's more flexible than static classes and can maintain it's state. It can implement interfaces, inherit from other classes and allow inheritance.
My rule for choosing between static and singleton:
If there is a bunch of functions that should be kept together, then static is the choice.
Anything else which needs single access to some resources, could be implemented as a singleton.

Static Class:-
You cannot create the instance of static class.
Loaded automatically by the .NET Framework common language runtime (CLR) when the program or namespace containing the class is loaded.
We cannot pass the static class to method.
We cannot inherit Static class to another Static class in C#.
A class having all static methods.
Better performance (static methods are bonded on compile time)
Singleton:-
You can create one instance of the object and reuse it.
Singleton instance is created for the first time when the user requested.
You can create the object of singleton class and pass it to method.
Singleton class does not say any restriction of Inheritance.
We can dispose the objects of a singleton class but not of static class.
Methods can be overridden.
Can be lazy loaded when need (static classes are always loaded).
We can implement interface(static class can not implement interface).

A static class is one that has only static methods, for which a better word would be "functions". The design style embodied in a static class is purely procedural.
Singleton, on the other hand, is a pattern specific to OO design. It is an instance of an object (with all the possibilities inherent in that, such as polymorphism), with a creation procedure that ensures that there is only ever one instance of that particular role over its entire lifetime.

In singleton pattern you can create the singleton as an instance of a derived type, you can't do that with a static class.
Quick Example:
if( useD3D )
IRenderer::instance = new D3DRenderer
else
IRenderer::instance = new OpenGLRenderer

To expand on Jon Skeet's Answer
The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that's less common IME), so you can pass around the singleton as if it were "just another" implementation.
Singletons are easier to work with when unit testing a class. Wherever you pass singletons as a parameter (constructors, setters or methods) you can instead substitute a mocked or stubbed version of the singleton.

Here's a good article:
http://javarevisited.blogspot.com.au/2013/03/difference-between-singleton-pattern-vs-static-class-java.html
Static classes
a class having all static methods.
better performance (static methods are bonded on compile time)
can't override methods, but can use method hiding. (What is method hiding in Java? Even the JavaDoc explanation is confusing)
public class Animal {
public static void foo() {
System.out.println("Animal");
}
}
public class Cat extends Animal {
public static void foo() { // hides Animal.foo()
System.out.println("Cat");
}
}
Singleton
an object that can only be instantiated once.
methods can be overridden (Why doesn't Java allow overriding of static methods?)
easier to mock then static methods
better at maintaining state
In summary, I would only use static classes for holding util methods, and using Singleton for everything else.
Edits
static classes are lazy loaded as well. Thanks #jmoreno
(When does static class initialization happen?)
method hiding for static classes. Thanks #MaxPeng.

Another advantage of a singleton is that it can easily be serialized, which may be necessary if you need to save its state to disc, or send it somewhere remotely.

I'm not a great OO theorist, but from what I know, I think the only OO feature that static classes lack compared to Singletons is polymorphism.
But if you don't need it, with a static class you can of course have inheritance ( not sure about interface implementation ) and data and function encapsulation.
The comment of Morendil, "The design style embodied in a static class is purely procedural" I may be wrong, but I disagree.
In static methods you can access static members, which would be exactly the same as singleton methods accessing their single instance members.
edit:
I'm actually thinking now that another difference is that a Static class is instantiated at program start* and lives throughout the whole life span of the program, while a singleton is explicitly instantiated at some point and can be destroyed also.
* or it may be instantiated at first use, depending on the language, I think.

To illustrate Jon's point what's shown below cannot be done if Logger was a static class.The class SomeClass expects an instance of ILogger implementation to be passed into its constructor.
Singleton class is important for dependency injection to be possible.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication2
{
class Program
{
static void Main(string[] args)
{
var someClass = new SomeClass(Logger.GetLogger());
}
}
public class SomeClass
{
public SomeClass(ILogger MyLogger)
{
}
}
public class Logger : ILogger
{
private static Logger _logger;
private Logger() { }
public static Logger GetLogger()
{
if (_logger==null)
{
_logger = new Logger();
}
return _logger;
}
public void Log()
{
}
}
public interface ILogger
{
void Log();
}
}

Singleton's are instantiated. It's just that there's only one instance ever created, hence the single in Singleton.
A static class on the other hand can't be instantiated.

Well a singleton is just a normal class that IS instantiated but just once and indirectly from the client code. Static class is not instantiated.
As far as I know static methods (static class must have static methods) are faster than non-static.
Edit:
FxCop Performance rule description:
"Methods which do not access instance data or call instance methods can be marked as static (Shared in VB). After doing so, the compiler will emit non-virtual call sites to these members which will prevent a check at runtime for each call that insures the current object pointer is non-null. This can result in a measurable performance gain for performance-sensitive code. In some cases, the failure to access the current object instance represents a correctness issue."
I don't actually know if this applies also to static methods in static classes.

Main differences are:
Singleton has an instance/object while static class is a bunch of
static methods
Singleton can be extended e.g. through an interface while static
class can't be.
Singleton can be inherited which supports open/close principles in
SOLID principles on the other hand static class can't be inherited
and we need to make changes in itself.
Singleton object can be passed to methods while static class as it
does not have instance can't be passed as parameters

Distinction from static class
JDK has examples of both singleton and static, on the one hand java.lang.Math is a final class with static methods, on the other hand java.lang.Runtime is a singleton class.
Advantages of singleton
If your need to maintain state than singleton pattern is better choice than static class, because maintaining state in static class leads to bugs, especially in concurrent environment, that could lead to race conditions without adequate synchronization parallel modification by multiple threads.
Singleton class can be lazy loaded if its a heavy object, but static class doesn't have such advantages and always eagerly loaded.
With singleton, you can use inheritance and polymorphism to extend a base class, implement an interface and provide different implementations.
Since static methods in Java cannot be overridden, they lead to inflexibility. On the other hand, you can override methods defined in singleton class by extending it.
Disadvantages of static class
It is easier to write unit test for singleton than static class, because you can pass mock object whenever singleton is expected.
Advantages of static class
Static class provides better performance than singleton, because static methods are bonded on compile time.
There are several realization of singleton pattern each one with advantages and disadvantages.
Eager loading singleton
Double-checked locking singleton
Initialization-on-demand holder idiom
The enum based singleton
Detailed description each of them is too verbose so I just put a link to a good article - All you want to know about Singleton

Singleton is better approach from testing perspective.
Unlike static classes , singleton could implement interfaces and you can use mock instance and inject them.
In the example below I will illustrate this.
Suppose you have a method isGoodPrice() which uses a method getPrice() and you implement getPrice() as a method in a singleton.
singleton that’s provide getPrice functionality:
public class SupportedVersionSingelton {
private static ICalculator instance = null;
private SupportedVersionSingelton(){
}
public static ICalculator getInstance(){
if(instance == null){
instance = new SupportedVersionSingelton();
}
return instance;
}
#Override
public int getPrice() {
// calculate price logic here
return 0;
}
}
Use of getPrice:
public class Advisor {
public boolean isGoodDeal(){
boolean isGoodDeal = false;
ICalculator supportedVersion = SupportedVersionSingelton.getInstance();
int price = supportedVersion.getPrice();
// logic to determine if price is a good deal.
if(price < 5){
isGoodDeal = true;
}
return isGoodDeal;
}
}
In case you would like to test the method isGoodPrice , with mocking the getPrice() method you could do it by:
Make your singleton implement an interface and inject it.
public interface ICalculator {
int getPrice();
}
Final Singleton implementation:
public class SupportedVersionSingelton implements ICalculator {
private static ICalculator instance = null;
private SupportedVersionSingelton(){
}
public static ICalculator getInstance(){
if(instance == null){
instance = new SupportedVersionSingelton();
}
return instance;
}
#Override
public int getPrice() {
return 0;
}
// for testing purpose
public static void setInstance(ICalculator mockObject){
if(instance != null ){
instance = mockObject;
}
test class:
public class TestCalculation {
class SupportedVersionDouble implements ICalculator{
#Override
public int getPrice() {
return 1;
}
}
#Before
public void setUp() throws Exception {
ICalculator supportedVersionDouble = new SupportedVersionDouble();
SupportedVersionSingelton.setInstance(supportedVersionDouble);
}
#Test
public void test() {
Advisor advidor = new Advisor();
boolean isGoodDeal = advidor.isGoodDeal();
Assert.assertEquals(isGoodDeal, true);
}
}
In case we take the alternative of using static method for implementing getPrice() , it was difficult to the mock getPrice().
You could mock static with power mock, yet not all product could use it.

I'm agree with this definition:
The word "single" means single object across the application life
cycle, so the scope is at application level.
The static does not have
any Object pointer, so the scope is at App Domain level.
Moreover both should be implemented to be thread-safe.
You can find interesting other differences about: Singleton Pattern Versus Static Class

One notable difference is differed instantiation that comes with Singletons.
With static classes, it gets created by the CLR and we have not control on it.
with singletons, the object gets instantiated on the first instance it's tried to be accessed.

Below are some main differences between static class and singleton:
1.Singleton is a pattern, not a keyword like static. So for creating a static class static keyword is sufficient while in the case of singleton there is a need to write the logic for the singleton.
2.A singleton class must have a private default instance constructor, while a static class cannot contain any instance constructor.
3.A static class is neither instantiated nor extended, while a singleton class can be.
4.A static class is sealed implicitly, but the singleton class must be decorated as sealed explicitly.
5.It is possible for a singleton to implement the interface or inherit from another class, but the static class neither implements the interface nor extends from any other class.
6.We cannot implement the dependency injection with a static class, but DI is possible with the singleton class because it can be interface driven.
The scope of the static class is at the app domain level because it is managed by the CLR, while the scope of the singleton object is across the application lifecycle.
7.A static class cannot have any destructor but a singleton class can define a destructor.
8.The singleton class instance can be passed as a parameter to another method while a static class cannot be because it contains only static members.

Lazy Loading
Support of interfaces, so that separate implementation can be provided
Ability to return derived type (as a combination of lazyloading and interface implementation)

In many cases, these two have no practical difference, especially if the singleton instance never changes or changes very slowly e.g. holding configurations.
I'd say the biggest difference is a singleton is still a normal Java Bean as oppose to a specialized static-only Java class. And because of this, a singleton is accepted in many more situations; it is in fact the default Spring Framework's instantiation strategy. The consumer may or may not know it's a singleton being passed around, it just treat it like a normal Java bean. If requirement changes and a singleton needs to become a prototype instead, as we often see in Spring, it can be done totally seamlessly without a line of code change to the consumer.
Someone else has mentioned earlier that a static class should be purely procedural e.g. java.lang.Math. In my mind, such a class should never be passed around and they should never hold anything other than static final as attributes. For everything else, use a singleton since it's much more flexible and easier to maintain.

We have our DB framework that makes connections to Back end.To Avoid Dirty reads across Multiple users we have used singleton pattern to ensure we have single instance available at any point of time.
In c# a static class cannot implement an interface. When a single instance class needs to implement an interface for a business contracts or IoC purposes, this is where I use the Singleton pattern without a static class
Singleton provides a way to maintain state in stateless scenarios
Hope that helps you..

In an article I wrote I have described my point of view about why the singleton is much better than a static class:
Static class is not actually canonical class – it’s a namespace with functions and variables
Using static class is not a good practice because of breaking object-oriented programming principles
Static class cannot be passed as a parameter for other
Static class is not suitable for “lazy” initialization
Initialization and using of static class is always hard tracked
Implementing thread management is hard

Singleton class provides an object(only one instance) during the application lifeCycle such as java.lang.Runtime
While Static class only provide static methods such as java.lang.Math
Static methods in Java cannot be overridden, but methods defined in Singleton class can be overridden by extending it.
Singleton Class is capable of Inheritance and Polymorphism to extend a base class, implement an interface and capable of providing different implementations. whereas static not.
For eg: java.lang.Runtime,is a Singleton Class in Java, call to getRuntime() method returns the runtime object associated with the current Java application but ensures only one instance per JVM.

a. Serialization - Static members belong to the class and hence can't be serialized.
b. Though we have made the constructor private, static member variables still will be carried to subclass.
c. We can't do lazy initialization as everything will be loaded upon class loading only.

From a client perspective, static behavior is known to the client but Singleton behavior can be completed hidden from a client. Client may never know that there only one single instance he's playing around with again and again.

I read the following and think it makes sense too:
Taking Care of Business
Remember, one of the most important OO rules is that an object is responsible for itself. This means that issues regarding the life cycle of a class should be handled in the class, not delegated to language constructs like static, and so on.
from the book Objected-Oriented Thought Process 4th Ed.

We can create the object of singleton class and pass it to method.
Singleton class doesn't any restriction of inheritance.
We can't dispose the objects of a static class but can singleton class.

Singleton – the proper way

public enum YourSingleton {
INSTANCE;
public void doStuff(String stuff) {
System.out.println("Doing " + stuff);
}
}
YourSingleton.INSTANCE.doStuff("some stuff");
Here is the original link,
http://electrotek.wordpress.com/2008/08/06/singleton-in-java-the-proper-way/
I am asking why we can call the function doStuff this way in Java.

In Java, enum can do everything that class can [1]. YourSingleton.INSTANCE creates an instance of YourSingleton, so you can then invoke methods as if it were a regular class instance, which it basically is.
See the official Java docs for a more in-depth discussion on Enum Types: http://download.oracle.com/javase/tutorial/java/javaOO/enum.html
[1] enum does not have a practical implementation of inheritance. Since all enum types implicity inherit java.lang.Enum and Java does not support multiple inheritance, you cannot extend anything else.

The Traditional way to implementing singleton is fine, but to maintain its Status as true singleton, it needs to protect itself from sophisticated Serialization and Reflection Attacks. The general way of doing this, is by making the class Implement Serializable, make all instance fields Transient and also implement a readResolve method. (that return the same singleton instance).
The Enum Singleton pattern provides all these features out of the box. But the main reason, I like the Enum variant is its readability. According to me, it conveys what it does, in a much more concise fashion, than a traditional singleton.( You do not have to explain to a new developer, all the vagaries involved in serialization and how serialization might break the singleton guarantee and why you need readResolve method etc etc..)

I know this is not really what you asked for but this is what I do when I need a class to be a singleton, which may help. I create one static getInstance method that either creates and returns a new instance of the class if none exist or I return the existing reference of itself, and I make the constructor for this class private.
For example:
public class NameOfClass{
private static NameOfClass variableReferencingThisClass=new NameOfThisClass();
private NameOfClass(){}
public static NameOfClass getInstance(){
return variableReferencingThisClass;
}
}

You can also use the double-lock singleton creation. Assuming the class is MyObject, has a private constructor, and has a declared a static field instance as null. However, this is not a guarantee that 2 singletons will not end up getting created, but is a much closer attempt to thread safety than a single check.
public static MyObject getInstance()
{
if (instance == null)
{
synchronized(MyObject.class) {
if (instance == null)
instance = new MyObject();
}
}
return instance;
}

Is it possible to apply inheritance to a Singleton class?

Today I faced one question in interview. Is it possible to apply inheritance concept on Singleton Classes? I said since the constructor is private, we cannot extend that Singleton class.
Next thing he asked me is to apply inheritance on that Singleton class. So, I made the Singleton's constructor as protected thinking that child's constructor also has be protected. But I was wrong the child can have a modifier either equal to or higher than that.
So, I asked him to give a real world example on such a case. He was not able to give me one and said that I cant ask questions and wanted me to tell whether this scenario is possible or not.
I went kind of blank. My question here is,
Is this possible?
Even if it is possible, what is the use of it?
What real world scenario would demand such a use?

Citing the bible:
Use the Singleton pattern when [...]
the sole instance should be extensible
by subclassing, and clients should be
able to use an extended instance
without modifying their code.
The Singleton pattern has several
benefits: [...]
3. Permits refinement of operations and representation. The Singleton
class may be subclassed, and it's
easy to configure an application with
an instance of this extended class.
You can configure the application with
an instance of the class you need at
run-time.
As for how to implement this: the book suggests several way, the most sophisticated of which is a registry in which instances are looked up by name.

Yes, it is technically possible, as singleton is a design pattern and not a language construct that could have inheritance restrictions. I would just reimplement the public [Object] getInstance() method in the child classes (see below).
And, yes, singletons can also benefit from inheritance as they may share similar but not identifical behaviours with other singletons.
public class ParentSingleton {
private static ParentSingleton instance;
protected ParentSingleton() {
}
public static synchronized ParentSingleton getInstance() {
if (instance == null) {
instance = new ParentSingleton();
}
return instance;
}
public int a() {
// (..)
}
}
public class ChildSingleton extends ParentSingleton {
private static ChildSingleton instance;
public static synchronized ParentSingleton getInstance() {
if (instance == null) {
instance = new ChildSingleton();
}
return instance;
}
}
EDIT: as Eyal pointed out in his comments below, the constructor in the super class had to be made protected (instead of private), otherwise it would not be visible to the child classes and the code would not even compile.

You can create an abstract base class with a bunch of common attributes and methods, and then create a number of subclasses as singleton classes. That is "applying the inheritance concept" ... in a useful way.
But what you cannot do is create a subclass of a strictly implemented singleton class. If you declare the singleton classes constructor as private a subclass won't compile. If you declare it with some other access, the constructor could be used in another class to create multiple instances ... ergo it is not strictly a singleton. If you declare the singleton as abstract, it cannot be instantiated at all ... ergo it is not a singleton.

Its 'possible' to hack anything together really, but in this case it is not really advisable. There's no real reason to use the singleton pattern with inheritance, its just not meant for it.

A private constructor is visible to other inner classes of that singleton class. So yes, technically a singleton class with a private constructor can be extended by its inner class. But why would you do something like that is beyond me.

Below is one implementation of singleton pattern explained in GOF about creating singleton with inheritance. Here the parent class should be modified to add a new derived class. Environment variable can be used to instantiate appropriate derived class constructor.
Mainfile – 1
#include <iostream>
#include <string>
#include "Singleton.h"
using namespace std;
int main(){
Singleton::instant().print();
cin.get();
}
Singleton.h
#pragma once
#include <iostream>
using std::cout;
class Singleton{
public:
static Singleton & instant();
virtual void print(){cout<<"Singleton";}
protected:
Singleton(){};
private:
static Singleton * instance_;
Singleton(const Singleton & );
void operator=(const Singleton & );
};
Singleton.cpp
#include "Singleton.h"
#include "Dotted.h"
Singleton * Singleton::instance_ = 0;
Singleton & Singleton::instant(){
if (!instance_)
{
char * style = getenv("STYLE");
if (style){
if (strcmp(style,"dotted")==0)
{
instance_ = new Dotted();
return *instance_;
} else{
instance_ = new Singleton();
return *instance_;
}
}
else{
instance_ = new Singleton();
return *instance_;
}
}
return *instance_;
}
Dotted.h
#pragma once
class Dotted;
class Dotted:public Singleton{
public:
friend class Singleton;
void print(){cout<<"Dotted";}
private:
Dotted(){};
};

I'm guessing this isn't what he wasn't looking for, but if you want to get technical you could also mention that Singleton is a pattern, not an implementation. Messing with the class constructor isn't the only way to have a Singleton, you could have a factory enforcing the pattern, in which case you can use inheritance in exactly the same way as with other classes.

You could make the constructor package-private. This way, no classes outside the package could instantiate the Singleton class but derived classes could call the upper class constructor.
But, as others said, this is really not advisable.
Stupid interview question by the way.