Question

I’m trying to implement singleton concept in my play application. But before going into singleton concept can we find a class is instantiated, how many times it is instantiated

Answer 1

The simplest way would be to include a thread-safe static counter which is updated in your constructor:

public class CountedClass {
    private static final AtomicInteger counter = new AtomicInteger();

    public CountedClass() {
        counter.incrementAndGet();
    }

    public static int getInstanceCount() {
        return counter.get();
    }
}

Note that this will not show how many instances are currently alive, as it doesn’t decrement the counter on destruction. You could override finalize to do that, but I wouldn’t.

Also note that this is not the same thing as a singleton by a long chalk. The recommended ways of achieving a singleton are either using a single-value enum:

public enum Singleton
{
    INSTANCE;

    // Methods here
}

or like this:

public final class Singleton {
    // Thread-safe due to guarantees about initializers
    private static final Singleton instance = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return instance;
    }
}

You can use a nested type to achieve lazier initialization if you really need to, but I’ve rarely found that to be worth the complexity – the above patterns have always done me fine.