Test expected exceptions in Kotlin

Question

In Java, the programmer can specify expected exceptions for JUnit test cases like this:

@Test(expected = ArithmeticException.class)
public void omg()
{
    int bl         


        

Answer 1

Nobody mentioned that assertFailsWith() returns the value and you can check exception attributes:

@Test
fun `my test`() {
        val exception = assertFailsWith<MyException> {method()}
        assertThat(exception.message, equalTo("oops!"))
    }
}

Answer 2

JUnit5 has kotlin support built in.

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.assertThrows

class MyTests {
    @Test
    fun `division by zero -- should throw ArithmeticException`() {
        assertThrows<ArithmeticException> {  1 / 0 }
    }
}

Answer 3

The Kotlin translation of the Java example for JUnit 4.12 is:

@Test(expected = ArithmeticException::class)
fun omg() {
    val blackHole = 1 / 0
}

However, JUnit 4.13 introduced two assertThrows methods for finer-granular exception scopes:

@Test
fun omg() {
    // ...
    assertThrows(ArithmeticException::class.java) {
        val blackHole = 1 / 0
    }
    // ...
}

Both assertThrows methods return the expected exception for additional assertions:

@Test
fun omg() {
    // ...
    val exception = assertThrows(ArithmeticException::class.java) {
        val blackHole = 1 / 0
    }
    assertEquals("/ by zero", exception.message)
    // ...
}

Answer 4

Assert extension that verifies the exception class and also if the error message match.

inline fun <reified T : Exception> assertThrows(runnable: () -> Any?, message: String?) {
try {
    runnable.invoke()
} catch (e: Throwable) {
    if (e is T) {
        message?.let {
            Assert.assertEquals(it, "${e.message}")
        }
        return
    }
    Assert.fail("expected ${T::class.qualifiedName} but caught " +
            "${e::class.qualifiedName} instead")
}
Assert.fail("expected ${T::class.qualifiedName}")

}

for example:

assertThrows<IllegalStateException>({
        throw IllegalStateException("fake error message")
    }, "fake error message")

Answer 5

Kotlin has its own test helper package that can help to do this kind of unittest.

Your test can be very expressive by use assertFailWith:

@Test
fun test_arithmethic() {
    assertFailsWith<ArithmeticException> {
        omg()
    }
}

Answer 6

You can use KotlinTest for this.

In your test, you can wrap arbitary code with a shouldThrow block:

shouldThrow<ArithmeticException> {
  // code in here that you expect to throw a ArithmeticException
}