Extending a singleton class

[Ankur Sharma]

Hi,

I have a class(e.g. SuperClass), which is singleton. Now I want to create a sub singleton class(e.g. SubClass) from SuperClass with some specific new methods, which may use some SuperClass.methods() too.

I am giving below details about my class, and what I have done so far. but it's not working(I mean it's giving compilation error)

SUPER_CLASS

public class UnrecognizedDataDAO {
private static UnrecognizedDataDAO daoInstance;
 
public static UnrecognizedDataDAO getInstance() {
synchronized (UnrecognizedDataDAO.class) {
if (daoInstance == null) {
daoInstance = new UnrecognizedDataDAO();
}
}
return daoInstance;
}
 
protected UnrecognizedDataDAO() {
initialize();
}
 
private void initialize() {
//DOING SOMETHING OVER HERE
}
// AND SOME MORE METHODS IN THIS CLASS, WHICH I MAY NEED TO USE 
// IN SUB CLASS
}

Here is SUB_CLASS, I am trying to create

SUB_CLASS

public class AnaylsisTestDAO extends UnrecognizedDataDAO {
 
private static AnaylsisTestDAO daoInstance;
 
public static AnaylsisTestDAO getInstance() {
synchronized (AnaylsisTestDAO.class) {
if (daoInstance == null) {
daoInstance = new AnaylsisTestDAO();
}
}
return daoInstance;
}
 
private AnaylsisTestDAO() {
 
}
}

But this code is not compiling properly, as it's giving following compilation error @ AnaylsisTestDAO getInstance() method

The return type is incompatible with UnrecognizedDataDAO.getInstance()

I hope, I illustrated my problem very well, but still if anything disturbs you, just let me know. I will try again to voice my concern.

Thanks

[Alex Wong]

The return type for getInstance() within AnalysisTestDAO should be UnrecognizedDataDAO, not AnalysisTestDAO. Otherwise, you're trying to have a method with the same name and parameters but with different return types (since there is a already a getInstance method in the superclass). The rest of your code should be okay as-is, since AnalysisTestDAO is a UnrecognizedDataDAO; just remember to cast the returned object to an AnalysisTestDAO when you call AnalysisTestDAO.getInstance().

[Campbell Ritchie]

Extend a Singleton? Surely that is a contradiction in terms. Your Singleton is supposed to have one instance and one only; an instance of a subclass would represent a second instance of the original Singleton too.

And wouldn't a Singleton have a private constructor? In which case you cant extend it.

I also think this isn't a real beginner's topic, so I shall move it.

[Bill Shirley]

Some languages don't inherit static methods, they are basically just scoped. They would allow what you want.

But Java DOES inherit static methods (you can call SubClass.staticMethodFromParentClass() � ) and changing the return type of an overriden method is a no-can-do.

If what you're saying is really what you want to do, then you likely want to rename the static methods to getFooInstance() and getBarInstance().

But as mentioned, your Singleton implementation is a bit fragile. Google around for some examples.