Filed under: Java, TDD, Teaching, — Tags: JUnit, assertThat, assertTrue — Thomas Sundberg — 2015-11-06
When should you use which assert? It is a question developers new to unit tests may ask themselves.
It is easy as a seasoned developer to think that this is obvious. It isn't when you are learning.
I had a conversation today where the person I talked to used assertTrue and had a failing test. The
reason why the test failed was not clear. I realized after our conversation that it might have been a better idea to
use another assert. But why would that be a good idea? Let me give you an example that you can run and figure out
the answer to that question yourself.
I compare assertTrue and assertThat in the examples below. One of them will give me a
better error report when something goes wrong.
My recommendation is that you run this test a few times and changes what complicatedFunction() returns
to get a feeling why one of assertTrue and assertThat is better in many situations.
Run this code:
package se.thinkcode;
import org.junit.Test;
import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue;
public class CompareAsserts {
@Test
public void assertTrueExample() {
assertTrue(complicatedFunction().equals("bar"));
String message = "Expected bar, got " + complicatedFunction();
assertTrue(message, complicatedFunction().equals("bar"));
}
@Test
public void assertThatExample() {
assertThat(complicatedFunction(), is("bar"));
}
private String complicatedFunction() {
return "bar";
}
}
What happened? Did you get a feeling about the differences?
My I prefer assertThat in almost every situation. My motivation is that I get better feedback. You may
have other opinions. I would be interested in hearing them.
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