Question

Is there any method to check whether any instance of a class is running?

I have a GUI and I don’t want open another instance of it, if a one is already running.
I know about including a counter in constructor. Any method except that?

Thanks in advance!!

Answer 1

Is there any method to check whether any instance of a class is running?

The simple answer is No.

The normal way to deal with this problem is to use the Singleton design pattern. The idea of the pattern is that you write the code to remove the possibility of creating multiple instance of the class.

In Java you typically do this by declaring the class constructor to be private, and providing access to the singleton instance via a static method. Depending on your precise requirements, the singleton can be created eagerly or lazily.

Bohemian’s answer provides a good example of this approach.

(In Java 5 or later, it is also possible to use enum types to implement singletons.)