Creating subjects in tests with private constructors

_Class.kt

@JvmOverloads
fun <T> createForTests(
    type: Class<T>,
    whenever: ((index: Int, Constructor<T>) -> Boolean),
    vararg arguments: Any = emptyArray()
) = createForTests(type, *arguments, whenever)

@JvmOverloads
@Suppress("UNCHECKED_CAST")
fun <T> createForTests(
    type: Class<T>,
    vararg arguments: Any = emptyArray(),
    whenever: ((index: Int, Constructor<T>) -> Boolean) = { _, _ -> true }
): T =
    constructorFor(type, whenever)
        .newInstance(*arguments)
        ?: error("Could not create instance of ${type.simpleName}")

@Suppress("UNCHECKED_CAST")
fun <T> constructorFor(type: Class<T>, whenever: (index: Int, Constructor<T>) -> Boolean): Constructor<T> =
    type.constructors
        .let { it as? Array<Constructor<T>> }
        ?.filterIndexed(whenever)
        ?.first()
        ?.also { it.isAccessible = true }
        ?: error("Could not find constructor for ${type.simpleName}")


inline fun <reified T> createForTests(
    vararg arguments: Any = emptyArray(),
    noinline whenever: ((index: Int, Constructor<T>) -> Boolean) = { _, _ -> true }
): T = createForTests(T::class.java, arguments, whenever = whenever)

inline fun <reified T> constructorFor(
    noinline whenever: ((index: Int, Constructor<T>) -> Boolean) = { _, _ -> true }
): Constructor<T> = constructorFor(T::class.java, whenever = whenever)

Creating subjects in tests with private constructors

Left empty

JavaUsage.java

Subject subject = createForTests(Subject.class);
Subject subjectWithIndex = createForTests(Subject.class, new Object[0], (index, c) -> index == 0);
Subject subjectWithCondition = createForTests(Subject.class, new Object[0], (index, c) ->
        ArraysKt.contains(c.getParameterTypes(), String.class)
);

Subject subjectWithParams = createForTests(Subject.class, "name");
Subject subjectWithIndexAndParams = createForTests(Subject.class, (index, c) -> index == 0, "name");

KotlinUsage.kt

val subject = createForTests<Subject>()
val subjectWithIndex = createForTests<Subject> { index, _ -> index == 0 }
val subjectWithCondition = createForTests<Subject> { _, constructor ->
    (constructor.parameterTypes ?: emptyArray<Any>()).contentEquals(arrayOf(String::class.java))
}
val subjectWithParams = createForTests<Subject>("name")
val subjectWithIndexAndParams = createForTests<Subject>("name") { index, _ -> index == 0 }

val subjectConstructor = constructorFor<Subject>()
val subjectFromConstructor = subjectConstructor.newInstance("name")