fluent-assert

name

fluent-assert

description

Write and refactor Kotlin assertions with FluentAssert's AssertJ-backed `.assert()` extensions and `assertThrownBy`. Use when a project depends on `fluent-assert-core`, the user asks for FluentAssert, or the user asks to migrate direct AssertJ calls to FluentAssert; covers JUnit and Wow/SagaSpec tests, nullable values, supported JDK types, collections, time, futures, predicates, exceptions, recursive comparison, and custom `AssertProvider` types.

FluentAssert

Use FluentAssert for Kotlin assertions in this repository and in downstream Kotlin test examples. Prefer the Kotlin extension style over direct AssertJ calls.

Core Rule

import me.ahoo.test.asserts.assert

actual.assert().isEqualTo(expected)

Avoid assertThat(actual) in ordinary test code. FluentAssert keeps nullable receivers natural:

val name: String? = null
name.assert().isNull()

Common Patterns

Need	Pattern
Primitive/string/object	value.assert().isEqualTo(expected)
Collection/list	items.assert().hasSize(2).contains("a")
Map	map.assert().hasSize(2).containsEntry("a", 1)
Nullable value	value.assert().isNull() or value.assert().isNotNull()
Exception lambda	assertThrownBy<IllegalArgumentException> { call() }.hasMessage("bad")
Existing exception	throwable.assert().hasMessageContaining("bad")
LocalDate/YearMonth month	date.assert().hasMonth(Month.APRIL)
CompletableFuture success/failure	completableFuture.assert().isCompletedWithValue(value) / .isCompletedExceptionally()
Recursive comparison	actual.assert().usingRecursiveComparison().isEqualTo(expected)

For exception lambdas, also import:

import me.ahoo.test.asserts.assertThrownBy

For LocalDate and YearMonth month assertions, import java.time.Month; those assertions expect Month, not an integer.

Wow/Saga Tests

Use FluentAssert inside expectation blocks too:

expectCommandBody<UpdateDemo> {
    data.assert().isNotNull().isEqualTo("updated")
}

AssertProvider

For types implementing AssertJ's AssertProvider<A>, .assert() returns the provider's custom assertion type:

provider.assert().isPositive()

Only use direct assertThat when implementing the provider's assertThat() method or when a project explicitly requires raw AssertJ.

Avoid

• Do not chain .assert() after assertThrownBy; it already returns ThrowableAssert<T>.
• Do not assert contradictory states on one value, such as Optional.of("x").assert().isPresent().isEmpty().
• Do not use unavailable AssertJ shortcuts like OffsetDateTimeAssert.hasOffset(...).
• Do not use numeric hasMonth(4) for LocalDate or YearMonth; use hasMonth(Month.APRIL).
• Do not switch to assertThat(value).usingRecursiveComparison(); keep chaining from value.assert().
• Do not call .assert() on primitive arrays (IntArray, LongArray, ...) — they fall back to ObjectAssert with no collection APIs. Convert first: ints.toList().assert().hasSize(3).contains(42).
• Do not expect string assertions from non-String text receivers. A statically CharSequence-typed value resolves to ObjectAssert; StringBuilder (it implements Comparable<StringBuilder>) resolves to GenericComparableAssert. Use a String (.toString()) instead.
• There is no CompletionStageAssert; stage.assert() returns CompletableFutureAssert.

References

• Read references/FULL-API.md when you need installation, the exact type-to-assertion mapping, overload-resolution rules, or special API examples.
• Use evals/evals.json as the regression prompt set when changing this skill.