Expected Exception

Question 1

Why

Answer 1

When writing unit tests, it make sense to check whether certain methods throw the expected exceptions when we supply invalid inputs or pre-conditions to not satisfy.

Question 2

assertThrows()

Answer 2

The assertThrows() method verifies that a particular type of exception (or any of its subclasses) is thrown when a code block is executed.

The assertThrows() will FAIL:

If no exception is thrown from the executable block
If an exception of a different type is thrown

@Test
void testExpectedExceptionFail() {

NumberFormatException thrown = Assertions.assertThrows(NumberFormatException.class, () -> {
Integer.parseInt(“1”);
}, “NumberFormatException error was expected”);
}

For example, in below example “1” is a valid number so no exception will be thrown. This test will fail with the message in the console.

Question 3

Using the thrown exception in a subsequent assertion

Answer 3

@Test
void testExpectedException() {

NumberFormatException thrown = Assertions.assertThrows(NumberFormatException.class, () -> {
	Integer.parseInt("One");
}, "NumberFormatException was expected");

Assertions.assertEquals("For input string: \"One\"", thrown.getMessage()); }

Question 4

assertThrowsExactly()

Answer 4

The assertThrowsExactly() method is similar to assertThrows(), but it verifies that only the exact type of exception specified is thrown, and not any subclass

Question 5

assertDoesNotThrow()

Answer 5

The assertDoesNotThrow() method verifies that no exception is thrown by the block of code.