Argument ordering conventions must be followed for assertions in their respectice libraries.

JUnit assertion methods such as assertEquals expect the first argument to be the expected value and second to be the actual value. On the other hand, AssertJ works in reverse. Methods such as assertThat() must be passed the actual value first, and the expected value is passed in the subsequent chained call to isEqualTo(), isGreaterThan() or a similar combinator method.

As far as test execution goes, the order of arguments does not matter; test results will not vary just because arguments are passed in the wrong order. But, changing the order can cause problems when reading test reports or test output (IntelliJ allows you to see a diff between expected and actual results, for example) leading to weird behavior. In general, it is a good practice to follow the conventions set by a particular test framework.

Bad Practice

Avoid passing arguments to test methods in arbitrary order.

class MyTest {
    @Test
    void test() {
        org.junit.Assert.assertEquals(value, 10);
        org.assertj.core.api.Assertions.assertThat(10).isEqualTo(value);
    }
}

Recommended

Instead, consider following the conventions set by the test framework that you are using.

class MyTest {
    @Test
    void test() {
        org.junit.Assert.assertEquals(10, value);
        org.assertj.core.api.Assertions.assertThat(value).isEqualTo(10);
    }
}

References

StackOverflow - JUnit assertEquals argument order
Vogella - Testing with AssertJ

ben-manes / caffeine

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