How A Certain Field Is Being Incremented

I've come across a piece of code and I am totally baffled by how a certain field is being incremented.

public class Foo{
private static int counter;
private final int id = counter++;
public int id(){return id;}

[Code] ....

Now. The output for this is:
0
1
2
3
4

I understand all the code in it quite well. It's all basic. Containers, generics, looping, foreach loop. What I DON'T understand though is how on each iteration of the foreach loop, the call to the method id() returns an incremented number. I am assuming it has something to do with the field "counter" in class Foo being static because when I made it non-static it didn't increment. I must have missed something way back when I started learning Java because I just don't get it.

The id() method only returns the value of the id field in class Foo and that is the value of counter++. counter is not initialized, though I'm guessing it gets the default 0 when the Foo() constructor is run? Or do statics get initialized BEFORE the constructor is even run? Anyway. How does calling the id() method cause an incrementation? I'd understand if there was some code that tracked the number of Foo objects being created and incrementing for each one, but I cannot see how each time the id() method is called it returns an incremented value.