Why not a class with static methods as replacement for Singleton object?

[Pushker Chaubey]

Hi Experts,

I have seen this question asked in an interview.

Why we cannot use a class with static methods (and possibly with private constructor) as replacement for Singleton?

Please share your views.

regards,
Pushker chaubey

[Max Rahder]

Good question. My 2 cents is that in some cases a class with static methods would work out fine. But if you use an interface to describe the behavior of the singleton, you need an object to provide the implementation.

[Pushker Chaubey]

Hi Max,

Thanks for a quick reply.


regards,
Pushker chaubey

[Rob Spoor]

If your singleton class implements an interface or extends another class, then without the singleton instance you will not be able to use your class as a parameter or return value. A good example is String.CASE_INSENSITIVE_ORDER - a singleton Comparator<String> so you can use it in Collections.sort for instance. Another example is Collections.EMPTY_SET and its brothers - singleton Set, List etc implementations you can use as a return value if you have nothing to return:

public Set<T> getItems()
{
    if (...) return Collections.emptySet(); // emptySet returns EMPTY_SET but with generics added
    Set<T> set = ...;
    // complex steps to fill the set
    return set;
}

This way, if you know that the complex steps are not necessary because you're going to return an empty Set anyway, you return the singleton which avoids the creation of a new object.

[Maneesh Godbole]

If the class has static methods, why do you even need to care if the constructor is public or private?

[R van Vliet]

Maneesh Godbole wrote:If the class has static methods, why do you even need to care if the constructor is public or private?

Code clarity, you'de be presenting a way to users of your code to instantiate a class that has no non-static methods. Private constructors remove that option and as a result make developers use the class the way it was intended. That said I have a preference for the Singleton pattern even if an all static method class would do as well.

[Pushker Chaubey]

Thanks guys!

[Muhammad Khojaye]

R van Vliet wrote:Code clarity, you'de be presenting a way to users of your code to instantiate a class that has no non-static methods. Private constructors remove that option and as a result make developers use the class the way it was intended

Note that this doesn't apply to abstract classes, since their subclasses may well include non-static methods.