how to return the number of times Singleton accessed getInstance()?

Question

I want to use a private int type accessCount() to return the times of Singleton has been accessed via the getInstance() method, but always get error. Here's my two piece of code:

ExampleSingleton.java

public class ExampleSingleton {
  private int accessCount;
  private static ExampleSingleton instance = new ExampleSingleton();
  private ExampleSingleton() {
    System.out.println("I, the ExampleSingleton, am being created");
   }
  public static ExampleSingleton getInstance() {
    System.out.println("The sole instance of ExampleSingleton is being retrieved")
    
  }

}

ExampleTest.java

public class ExampleTest {
  public static void main(String[] args) {
    ExampleSingleton s = ExampleSingleton.getInstance();
    System.out.println("The ExampleSingleton has been "
    + "accessed via the getInstance() method "
    + s.accessCount()
    + " time(s)");
    s = ExampleSingleton.getInstance();
    System.out.println("The ExampleSingleton has been "
    + "accessed via the getInstance() method "
    + s.accessCount()
    + " time(s)");
  }
}

So I can get the following output output

Answer 1

You may use static AtomicInteger class type variable and increment it when you are retrivig the instance:

public class ExampleSingleton {
    private static AtomicInteger accessCounter = new AtomicInteger(0);

    public static int getAccessCount() {
        return accessCounter.get();
    }
    private static ExampleSingleton instance = new ExampleSingleton();
    private ExampleSingleton() {
        System.out.println("I, the ExampleSingleton, am being created");
    }
    public static ExampleSingleton getInstance() {
        accessCounter.getAndAdd(1);
        System.out.println("The sole instance of ExampleSingleton is being retrieved");
        return instance;
    }
}

public class ExampleTest {
    public static void main(String[] args) {
        ExampleSingleton s = ExampleSingleton.getInstance();
        System.out.println("The ExampleSingleton has been "
                + "accessed via the getInstance() method "
                + s.getAccessCount()
                + " time(s)");
        s = ExampleSingleton.getInstance();
        System.out.println("The ExampleSingleton has been "
                + "accessed via the getInstance() method "
                + s.getAccessCount()
                + " time(s)");
    }
}

Answer 2

This block of code should be placed inside the ExampleSingleton class, and incrementCounter() should then be called from getInstance() .

The AtomicInteger is a class that wraps an integer and supports concurrency, so its far safer than using a standard Integer or Long . The variable itself is private static , so that it's read-only and exists with the singleton class versus the singleton instance.

private static AtomicInteger counter = new AtomicInteger(0);


public Integer getCount() {
    return counter.get();
}
/**
 * @return the new counter
 */
private Integer incremenetCounter() {
    return counter.incrementAndGet();
}