How to write a Consumer that counts how many times it was called?

Question

I need (mostly for testing purposes) to write a consumer whose purpose is to remember how many times it was called. However, I can't do

    int i = 0;
    Consumer<Object> handler = o -> i++;

because i must be final, and I can't increment final variables. I suppose I need something like MutableInteger class. What is the right way to count then? Writing my own new class or a new method just for that wouldn't count for a right way.

Answer 1

Use an AtomicInteger which is implemented using CAS.

AtomicInteger has an incrementAndGet() method you can use for this purpose.

It is also useful to know that there are more Atomic* variants in the JDK, so there is also AtomicLong if Integer is not enough for you.

Answer 2

You never could do that. Even with anonymous classes that would not work. You need some object wrapper that will allow you to hold the state, sth like

class NumberOfInvocations {
    private int i = 0;

    public void increment(){
       i +=1;
    }
}

but you would have to make sure that everything is thread safe, etc, or you could just use AtomicInteger as suggested by Adam.

Answer 3

For testing purposes where you know that access is single-threaded, a quick and dirty workaround is:

int[] i = new int[]{0};
Consumer<Object> handler = o -> i[0]++;