Why is my DataPoints method being called multiple times?

Question

This test class:

@RunWith(Theories.class)
public class TheoriesConfusion
{

    @DataPoints
    public static int[] ints()
    {
        System.out.println("Generator called");
        return new int[]{1, 2, 3, 4, 5};
    }

    @Theory
    public void twoArgTest(int x, int y)
    {
        assertTrue(x < y || x >= y);
    }
}

Prints the following output:

Generator called
Generator called
Generator called
Generator called
Generator called
Generator called
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.137 sec

This is quite counterintuitive, as I expect the data-generating function to be called only once. This has implications when creating random data, or any case where the data-generating method returns different results on each call, so I'd like to understand it.

Answer 1

After some experimentation, I've found that testing an array of length n against a Theory with c args, the generate function is called x times, where x = n^c + n^(c-1) + ... + n^0 .

The source is a little difficult to comprehend, but my assumption is that it works something like this (pseudocode):

for firstArg in generateArgs():
    for secondArg in generateArgs():
        for thirdArg in generateArgs():
            testTheory(firstArg, secondArg, thirdArg)

Which makes some sense, basically it's just not caching the results of the method, so if you want the method to be called just once, you have to annotate a static field, like:

@DataPoints
public static int[] ints = ints();

public static int[] ints()
{
    System.out.println("Generator called");
    return new int[]{1, 2, 3, 4, 5};
}