Does 100% code coverage mean that the code is correct?

Question

If you have 100% test coverage and all tests pass does that mean that the code is guaranteed to be correct and writing more tests is pointless?

Someone has voted to close this as opinion-based, which it clearly isn't. A simple counterexample can answer this question in the negative. — Mark Seemann, Oct 04 '22 at 13:59
[Code coverage is a useless target measure](https://blog.ploeh.dk/2015/11/16/code-coverage-is-a-useless-target-measure) — Mark Seemann, Oct 04 '22 at 14:00

score 6 · Accepted Answer · edited Oct 04 '22 at 13:59

It's only correct as far as the logic of your testing is correct.

I'll give the most stupid example possible...

If I for example have a class (Java):

public class Example {

    public int timesTwo(int x){
        return x*2;
    }

}

with the following test (which you can see it being illogical and useless)

public class ExampleTest {

    @Test
    public void timesTwo() {
        new Example().timesTwo(5);
        assertTrue(true);
    }
    
}

Most coverage tools will say that this class has been tested 100%! So no, coverage is a good indicator of what needs to be tested; but the test logic itself isn't assured.

anion · Answer 2 · 2023-02-20T22:04:52.163

If you have 100% test coverage and all tests pass does that mean that the code is guaranteed to be correct and writing more tests is pointless?

No. Counter-example: For a division-function a testcase with 0 as divisor can be missing (example taken from here: https://stackoverflow.com/a/555824/3905529 ). The tests verify that the function is working with the inputs used in the unit-tests. But there may be edge-cases where the code throws exceptions and there is maybe no unit-test which asserts exactly this, even with a coverage of 100%.

Another problem I have often seen is that for a condition the if-block will be handled and tested but else-block is missing and there is also no test that verifies that the else-case will be handled in the function. This is else-part may be missing completely. So the test is green, but the function has semantical flaws due to missing code. But the coverage can still be 100% because there are only tests for the existing code, but not for the missing parts of it.

Beside this there may be other issues, see: https://sqa.stackexchange.com/a/36940

Does 100% code coverage mean that the code is correct?

2 Answers2