Is it bad practice to have more than one assertion in a unit test?

assertunit-testing

Is it bad practice to have more than one assertion in a unit test? Does it matter?

Best Solution

Sometimes I have exactly one assert per test case, but I think more often I have several assert statements.

I've seen the case that @Arkain eludes to, where a very large piece of code has a single unit test suite with just a few test cases, and they are all labeled testCase1, testCase2, etc, and each test case has hundreds of asserts. And even better, each condition usually depends upon the side-effects of previous execution. Whenever the build fails, invariably in such a unit test, it takes quite some time to determine where the problem was.

But the other extreme is what your question suggests: a separate test case for each possible condition. Depending on what you're testing, this might make sense, but often I have several asserts per test case.

For instance, if you wrote java.lang.Integer, you might have some cases that look like:

public void testValueOf() {
    assertEquals(1, Integer.valueOf("1").intValue());
    assertEquals(0, Integer.valueOf("0").intValue());
    assertEquals(-1, Integer.valueOf("-1").intValue());
    assertEquals(Integer.MAX_VALUE, Integer.valueOf("2147483647").intValue());
    assertEquals(Integer.MIN_VALUE, Integer.valueOf("-2147483648").intValue());
    ....
}

public void testValueOfRange() {
    assertNumberFormatException("2147483648");
    assertNumberFormatException("-2147483649");
    ...
}

public void testValueOfNotNumbers() {
    assertNumberFormatException("");
    assertNumberFormatException("notanumber");
    ...
}
private void assertNumberFormatException(String numstr) {
    try {
        int number = Integer.valueOf(numstr).intValue();
        fail("Expected NumberFormatException for string \"" + numstr +
             "\" but instead got the number " + number);
    } catch(NumberFormatException e) {
        // expected exception
    }
}

Some simple rules that I can think of off hand for how many assert's to put in a test case:

  • Don't have more than one assert that depends on the side-effects of previous execution.
  • Group asserts together that test the same function/feature or facet thereof--no need for the overhead of multiple unit test cases when it's not necessary.
  • Any of the above rules should be overridden by practicality and common sense. You probably don't want a thousand unit test cases with a single assert in each (or even several asserts) and you don't want a single test case with hundreds of assert statements.