How can I save an exception so it can be passed as an object to a function?

Question

I am trying to unit test a function whose parameter is an exception. However, it seems that because the function only throws the exception and doesn't return in, I can't just feed a function and parameter that would throw an exception.

How can I deal with this?

Hi, would you mind providing the code you're talking about, this will help us help you. — Yassin Hajaj, Mar 08 '19 at 21:40
JUnit4: `@Test(throws = ExpectedException.class)` and JUnit5: `assertThrows(...)` both seem to invalidate your "i can't do this" premise. — Roddy of the Frozen Peas, Mar 08 '19 at 21:41
@Makoto - For completeness' sake can you link to a TestNG duplicate as well? That's the other major testing framework. This one should be an acceptable target: https://stackoverflow.com/questions/3677271/testng-how-to-test-for-mandatory-exceptions — Roddy of the Frozen Peas, Mar 10 '19 at 18:35

score -1 · Answer 1 · answered Mar 08 '19 at 22:13

-1

You can add a try-catch clause with the Exception you expect to be thrown, with a pass in the catch clause and a fail() after the catch clause.

answered Mar 08 '19 at 22:13

Pim van Leeuwen

293
1
9

score -1 · Answer 2 · answered Mar 08 '19 at 22:25

The major unit testing frameworks support verifying an exception.

Consider this method:

public boolean throwIfBlank(String input) {
    if(input == null || input.trim().isEmpty()) {
        throw new IllegalArgumentException("Input should not be blank.");
    }
    return true;
}

For JUnit 4, you could use the expected attribute of the @Test annotation to verify that an exception is thrown. If the exception is not thrown, then the unit test will be failed.

// JUnit 4
@Test( expected = IllegalArgumentException.class )
public void testBlankStringThrowsException() {
  classUnderTest.throwIfBlank("");
  // test will automatically fail if exception is NOT thrown
}

Similarly, in JUnit 5 you can explicitly assert that something is thrown using assertThrows:

@Test
void testBlankStringThrowsException() {
  assertThrows(IllegalArgumentException.class, () -> classUnderTest.throwIfBlank(""));
}

And finally, TestNG's @Test annotation supports a variety of attributes to verify that a particular exception is thrown:

@Test(
  expectedExceptions = IllegalArgumentException.class,
  expectedExceptionsMessageRegExp = "Input should not be blank."
)
public void testBlankStringThrowsException {
  classUnderTest.throwIfBlank("");
}

Of course, you can always wrap your test method in a try-catch block and explicitly call fail() if you do not throw the exception; generally this old-school method is not required but for completeness:

@Test
public void testBlankStringThrowsException() {
  try {
    classUnderTest.throwIfBlank("");
    fail("Blank string should have thrown an IllegalArgumentException!");
  } catch (IllegalArgumentException e) {
    // expected
  } catch (Exception e) {
    fail("Expected an IllegalArgumentException but threw: " + e);
  }
}

How can I save an exception so it can be passed as an object to a function?

2 Answers2