Unit Testing Exceptions

  • Write JUnit tests to check whether exceptions are thrown when expected.
  • Differentiate when to throw exceptions and when to try/catch the exceptions.

Let's update IndexedListTest to include tests to check that the IndexOutOfBoundsException is thrown when expected. 

@Test
@DisplayName("get() throws exception if index is below the valid range.")
void testGetWithIndexBelowRangeThrowsException() {
  try {
    numbers.get(-1);
    fail("IndexOutOfBoundsException was not thrown for index < 0");
  } catch (IndexOutOfBoundsException ex) {
    return;
  }
}

@Test
@DisplayName("get() throws exception if index is above the valid range.")
void testGetWithIndexAboveRangeThrowsException() {
  try {
    numbers.get(size + 1);
    fail("IndexOutOfBoundsException was not thrown for index > length");
  } catch (IndexOutOfBoundsException ex) {
    return;
  }
}

Notice the approach to test that an exception is thrown when it is expected to be thrown:

try {
  // call a method with an invalid input
  obj.someMethodThatThrowsException(badInput);
  // we expect the execution stops 
  //   and goes to the catch block as a result of an exception
  // if we get passed this point, exception was not thrown
  //   we must deliberately signal the test has failed!
  fail("The expected exception was not thrown!");
} catch (ExpectedException ex) {
  // if we are here, the expected exception is thrown;
  //   there is nothing to do, we can "return" 
  //   to indicate the test has successfully passed
  return;
}

The fail method is part of JUnit's Assertions library

import static org.junit.jupiter.api.Assertions.fail;

Run the tests and see them fail!

Resources

If you need a refresher on Java's Exception, please visit Exceptions on Oracle's [online] Java Tutorial. Alternatively, you can read this article on Java Exceptions.

JUnit 5 includes assertThrows() method that provides a more elegant approach to check an exception is thrown. We will not use this alternative approach because it involves the use of Java's Lambda expression:

assertThrows(IndexOutOfBoundsException.class, () -> {
  numbers.get(-1);
});

This post provides a more detailed example on using assertThrows().