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Code Smell 52 - Fragile Tests

Maxi Contieri
Learn something new every day. - I am a senior software engineer working in industry, teaching and writing on software design, SOLID principles, DDD and TDD.
Originally published at maximilianocontieri.com Updated on ・2 min read

Tests are our safety nets. If we don't trust on their integrity, we will be in great danger.

TL;DR: Don't write non-deterministic tests.

Problems

  • Determinism

  • Confidence loss

  • Wasted time

Solutions

  1. Test should be in full control. There should be no space for erratic behavior and degrees of freedom.

  2. Remove all tests coupling.

Examples

  • Fragile, Intermittent, Sporadic or Erratic tests are common in many organizations.

Nevertheless, they mine the developers trust.

We must avoid them.

Sample Code

Wrong

import static org.junit.Assert.assertEquals;

import org.junit.Test;

import components.set.Set;
import components.set.Set1L;

public abstract class SetTest {

    protected abstract Set<String> constructor();

    @Test
    public final void testAddEmpty() {
        Set<String> s = this.constructor();
        s.add("green");
        s.add("blue");
        assertEquals("{green. blue}", s.toString());
       //This is fragile since it dependes on set sort (which is not defined)
    }   
}
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Right

import static org.junit.Assert.assertEquals;

import org.junit.Test;

import components.set.Set;
import components.set.Set1L;

public abstract class SetTest {

    protected abstract Set<String> constructor();

    @Test
    public final void testAddEmpty() {
        Set<String> s = this.constructor();
        s.add("green");
        assertEquals("{green}", s.toString());
    }   

    @Test
    public final void testEntryAtSingleEntry() {
        Set<String> s = this.createFromArgs("red");
        Boolean x = s.contains("red");
        assertEquals(true, x);
    } 
}
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Detection

Detection can be done with test run statistics.

It is very hard to put some test in maintenance since we are removing a safety net.

More Info

%[https://softwareengineering.stackexchange.com/questions/109703/how-to-avoid-fragile-unit-tests]

Relations

Tags

  • Coupling

  • Determinism

Conclusion

Fragile tests show system coupling and not deterministic or erratic behavior.

Developers spend lots of time and effort fighting against this false positives.

Credits

Photo by Jilbert Ebrahimi on Unsplash


The amateur software engineer is always in search of magic.

Grady Booch


This article is part of the CodeSmell Series.

Last update: 2021/06/12

Discussion (1)

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yoursunny profile image
Junxiao Shi

Should tests verify the internals (private fields) of a class?
I've seen many test cases that assert every internal variable. I don't like that.

My reason is: Every time you refactor the class you have to rewrite the tests. However, tests are supposed to ensure the external behaviour of a class doesn't change unexpectedly. If the tests are rewritten, it's hard to confirm whether the external behaviour changed.