What problem are you trying to solve?

• When the expression inside assertThat(...) is an instanceof check, the AssertJ best-practices recipe leaves it as a literal boolean port instead of using the dedicated isInstanceOf assertion — so the failure message is just "expected true but was false" instead of describing the actual type. This is the direct sibling of AssertJ: Refactor assertThat with < or > in the expression to assertThat(actual).isGreaterThan(expected) or assertThat(actual).isLessThan(expected) #814 (</> comparisons), which now is handled; the common root cause is that the argument of assertThat(...) is a non-J.MethodInvocation boolean expression (here J.InstanceOf), which the existing Simplify* recipes skip.

What precondition(s) should be checked before applying this recipe?

AssertJ available.

Describe the situation before applying the recipe

class A {
    void foo(Object error) {
        assertThat(error instanceof RuntimeException).isTrue();
        assertThat(error instanceof RuntimeException).isFalse();
    }
}

Describe the situation after applying the recipe

class A {
    void foo(Object error) {
        assertThat(error).isInstanceOf(RuntimeException.class);
        assertThat(error).isNotInstanceOf(RuntimeException.class);
    }
}

(Should also handle the parenthesized form assertThat((error instanceof RuntimeException)) and the negated assertThat(!(x instanceof T)).isTrue().)

Any additional context

• Found while migrating the TestNG suite to AssertJ (testng-team/testng#3278) via org.openrewrite.java.testing.testng.TestNgToAssertj. I ran org.openrewrite.java.testing.assertj.Assertj (rewrite-testing-frameworks 3.39.0) as a rewriteDryRun over the test sources to confirm coverage:

• comparisons are now correctly handled (AssertJ: Refactor assertThat with < or > in the expression to assertThat(actual).isGreaterThan(expected) or assertThat(actual).isLessThan(expected) #814) — long/int operands convert to isLessThan/isGreaterThan/isPositive/… ;

• method-call predicates convert (.size()→hasSize, .isEmpty()→isEmpty/isNotEmpty, etc.) ;

• but instanceof predicates produced 0 conversions (~20 occurrences), e.g. assertThat(error instanceof RuntimeException).isTrue().

• Related: AssertJ: Refactor assertThat with < or > in the expression to assertThat(actual).isGreaterThan(expected) or assertThat(actual).isLessThan(expected) #814 (comparisons, now handled), JUnit5's assertInstanceOf is not converted to AssertJ equivalent #607 (assertInstanceOf from JUnit 5 — a different source pattern).

Are you interested in contributing this recipe?

Can help with samples/testing; happy to review a PR.