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<h1 class="page-header">AssertJ Core new website!</h1>
<h2 style="color:darkblue;font-style: italic;">AssertJ Core site has moved to <a href="https://assertj.github.io/doc/">https://assertj.github.io/doc/</a></h2>
<h1 class="page-header">AssertJ Core features highlight</h1>
<p>AssertJ has many great features that not everybody is aware of, here are some of them.</p>
<p>Basic tips :</p>
<ul>
<li>Java 8 assertions, see release notes : <a href="assertj-core-news.html#assertj-core-3.8.0">3.8.0</a> / <a href="assertj-core-news.html#assertj-core-3.7.0">3.7.0</a> / <a href="assertj-core-news.html#assertj-core-3.6.0">3.6.0</a> / <a href="assertj-core-news.html#assertj-core-3.5.0">3.5.0</a> / <a href="assertj-core-news.html#assertj-core-3.4.0">3.4.0</a> / <a href="assertj-core-news.html#assertj-core-3.3.0">3.3.0</a> /<a href="assertj-core-news.html#assertj-core-3.2.0">3.2.0</a> / <a href="assertj-core-news.html#assertj-core-3.1.0">3.1.0</a> / <a href="assertj-core-news.html#assertj-core-3.0.0">3.0.0</a> </li>
<li><a href="#ide-automatic-static-import">IDE configuration to directly get <span class="small-code">assertThat</span> in code completion</a></li>
<li><a href="#describe-assertion">Describe your assertion using <span class="small-code">as(String description, Object... args)</span></a></li>
<li><a href="#exception-assertion">Exception assertions guide</a></li>
<li><a href="#file-content-string-assertions">Using String assertions on the content of a file</a></li>
</ul>
<p>Iterable and arrays assertions :</p>
<ul>
<li><a href="#filters">Combining filtering and assertions on iterables or arrays</a></li>
<li><a href="#extracted-properties-assertion">Assertions on extracted properties/fields of iterable/array elements</a></li>
<li><a href="#flat-extracted-properties-assertion">Flat(map) extracting</a></li>
<li><a href="#extracted-method-result-assertion">Assertions on results of a method call on iterable/array elements</a></li>
</ul>
<p>Advanced tips :</p>
<ul>
<li><a href="#soft-assertions">Gather all errors message with soft assertions</a></li>
<li><a href="#custom-comparison-strategy">Using a custom comparison strategy in assertions</a></li>
<li><a href="#field-by-field-comparison">Field by field comparisons</a></li>
<li><a href="#custom-representation">Using a custom representation in assertions</a></li>
<li><a href="assertj-core-conditions.html">Extending assertions with conditions</a></li>
<li><a href="assertj-core-custom-assertions.html">Creating assertions for your domain</a></li>
</ul>
<h3 class="page-header"><span id="ide-automatic-static-import"></span><i class="fa fa-star"></i> IDE configuration to get assertThat in code completion</h3>
<p>We want to start typing <span class="small-code">asser</span> and let code completion suggest <span class="small-code">assertThat</span> from AssertJ (and not the one from Hamcrest !).</p>
<p class="list-section">Eclipse configuration</p>
<ol>
<li>Go to : Window > Preferences > Java > Editor > Content Assist > Favorites > New Type</li>
<li>Enter : <span class="small-code">org.assertj.core.api.Assertions</span> and click OK</li>
<li>You should see : <span class="small-code">org.assertj.core.api.Assertions.*</span> in the list of favorites.</li>
</ol>
<p class="list-section">IntelliJ Idea</p>
<ul>
<li>No special configuration, just start typing <span class="small-code">assertThat</span> and then invoke completion (Ctrl-Space) twice.</li>
</ul>
<h3 class="page-header"><span id="describe-assertion"></span><i class="fa fa-star"></i> Describe your assertion with as(String description, Object... args)</h3>
<p>It is often valuable to describe the assertion performed, especially for boolean assertion where the default error message just complains that it got <span class="small-code">false</span> instead of <span class="small-code">true</span> (or vice versa).</p>
<p>You can set such a description with <span class="small-code">as(String description, Object... args)</span> but remember to do it <b>before</b> calling the assertion. Otherwise it is simply ignored as a failing assertion breaks the following chained calls.</p>
<p>Example of a failing assertion with a description : </p>
<pre><code class="java"> TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
// failing assertion, remember to call as() before the assertion, not after !
assertThat(frodo.getAge()).as("check %s's age", frodo.getName()).isEqualTo(100);</code></pre>
<p> The error message starts with the given description in [] :</p>
<pre><code> [check Frodo's age] expected:<100> but was:<33></code></pre>
<h3 class="page-header"><span id="filters"></span><i class="fa fa-star"></i> Combining filtering and assertions on iterables or arrays</h3>
<p>
Filtering can be done on arrays or iterables. Filter criteria are expressed by :
<ul>
<li>a Java 8 <span class="small-code">Predicate</span></li>
<li>a condition (<span class="small-code">org.assertj.core.api.Condition</span>)</li>
<li>some operation on a property/field of array/iterable elements</li>
</ul>
</p>
<p>Let's see both options in some examples taken from <a href="https://github.com/joel-costigliola/assertj-examples/blob/master/assertions-examples/src/test/java/org/assertj/examples/FilterExamples.java">FilterExamples.java</a> of the <a href="https://github.com/joel-costigliola/assertj-examples/">assertj-examples</a> project.</p>
<p class="list-section">Filtering with a <span class="small-code">Predicate</span></p>
<p>You specify the filter condition using simple predicate, best expressed with a lambda.</p>
<p>Example :</p>
<pre><code class="java">assertThat(fellowshipOfTheRing).filteredOn( character -> character.getName().contains("o") )
.containsOnly(aragorn, frodo, legolas, boromir);</code></pre>
<p class="list-section">Filtering on a property or a field</p>
<p>First you specify the property/field name to filter on and then its expected value. The filter first tries to get the value from a property, then from a field. Reading private fields is supported by default, but can be disabled globally by calling <span class="small-code">Assertions.setAllowExtractingPrivateFields(false)</span>.</p>
<p>Filters support reading nested properties/fields. Note that if an intermediate value is null the whole nested property/field is considered to be null. Thus reading <span class="small-code">"address.street.name"</span> will return null if <span class="small-code">"address.street"</span> is null.</p>
<p>Filters support these basic operations : <span class="small-code">not</span>, <span class="small-code">in</span>, <span class="small-code">notIn</span></p>
<pre><code class="java">import static org.assertj.core.api.Assertions.in;
import static org.assertj.core.api.Assertions.not;
import static org.assertj.core.api.Assertions.notIn;
...
// filters use introspection to get property/field values
assertThat(fellowshipOfTheRing).filteredOn("race", HOBBIT)
.containsOnly(sam, frodo, pippin, merry);
// nested properties are supported
assertThat(fellowshipOfTheRing).filteredOn("race.name", "Man")
.containsOnly(aragorn, boromir);
// you can apply different comparison
assertThat(fellowshipOfTheRing).filteredOn("race", notIn(HOBBIT, MAN))
.containsOnly(gandalf, gimli, legolas);
assertThat(fellowshipOfTheRing).filteredOn("race", in(MAIA, MAN))
.containsOnly(gandalf, boromir, aragorn);
assertThat(fellowshipOfTheRing).filteredOn("race", not(HOBBIT))
.containsOnly(gandalf, boromir, aragorn, gimli, legolas);
// you can chain multiple filter criteria
assertThat(fellowshipOfTheRing).filteredOn("race", MAN)
.filteredOn("name", not("Boromir"))
.containsOnly(aragorn);
</code></pre>
<p class="list-section">Filtering with a <span class="small-code">Condition</span></p>
<p>Filter the iterable/array under test keeping only elements matching the given <a href="assertj-core-conditions.html"><span class="small-code">Condition</span></a>. </p>
<p>Two methods are available : <span class="small-code">being(Condition)</span> and <span class="small-code">having(Condition)</span>. They do the same job - pick the one that makes your code more readable !</p>
<pre><code class="java">import org.assertj.core.api.Condition;
Condition<Player> mvpStats= new Condition<Player>() {
@Override
public boolean matches(Player player) {
return player.pointsPerGame() > 20 && (player.assistsPerGame() >= 8 || player.reboundsPerGame() >= 8);
}
};
List<Player> players;
players.add(rose); // Derrick Rose : 25 ppg - 8 assists - 5 rebounds
players.add(lebron); // Lebron James : 27 ppg - 6 assists - 9 rebounds
players.add(noah); // Joachim Noah : 8 ppg - 5 assists - 11 rebounds
// noah does not have more than 20 ppg
assertThat(players).filteredOn(mvpStats)
.containsOnly(rose, lebron);
</code></pre>
<h3 class="page-header" ><span id="extracted-properties-assertion"></span><i class="fa fa-star"></i> Assertions on extracted properties/fields of iterable/array elements</h3>
<p>Let's say you have queried some service/dao and got a list (or an array) of TolkienCharacters. To check the results you have to build the expected TolkienCharacter(s), which can be a lot of work !</p>
<pre><code class="java">List<TolkienCharacter> fellowshipOfTheRing = tolkienDao.findHeroes(); // frodo, sam, aragorn ...
// requires creation of frodo and aragorn, the expected TolkienCharacters
assertThat(fellowshipOfTheRing).contains(frodo, aragorn);</code></pre>
<p>Instead, it is usually sufficient to only check some fields or properties of the TolkienCharacter results elements.
This easy but you still have to write code to extract the fields/properties before performing your assertions, something like :</p>
<pre><code class="java">// extract the names ...
List<String> names = new ArrayList<String>();
for (TolkienCharacter tolkienCharacter : fellowshipOfTheRing) {
names.add(tolkienCharacter.getName());
}
// ... and finally assert something
assertThat(names).contains("Boromir", "Gandalf", "Frodo", "Legolas");</code></pre>
<p>Hey ! It is too much work to extract fields and/or properties by hand. Let AssertJ help you taking care of that. Here is
how to check the names of the TolkienCharacter elements in the fellowshipOfTheRing list:</p>
<pre><code class="java">// "name" needs to be either a property or a field of the TolkienCharacter class
assertThat(fellowshipOfTheRing).extracting("name")
.contains("Boromir", "Gandalf", "Frodo", "Legolas")
.doesNotContain("Sauron", "Elrond");</code></pre>
<p>But there is more ! You can extract several fields/properties and check them using tuples. As an example, let's check the name, age and race's name of each TolkienCharacter element :</p>
<pre><code class="java">// when checking several properties/fields you have to use tuples :
import static org.assertj.core.api.Assertions.tuple;
// extracting name, age and and race.name nested property
assertThat(fellowshipOfTheRing).extracting("name", "age", "race.name")
.contains(tuple("Boromir", 37, "Man"),
tuple("Sam", 38, "Hobbit"),
tuple("Legolas", 1000, "Elf"));</code></pre>
<p>The extracted name, age and race's name values of the current element are grouped in a tuple, thus you need to use tuples for expected values.</p>
<p>More examples are available in the test method <span class="small-code">iterable_assertions_on_extracted_property_values_example</span> in <a href="https://github.com/joel-costigliola/assertj-examples/blob/master/assertions-examples/src/test/java/org/assertj/examples/IterableAssertionsExamples.java">IterableAssertionsExamples.java</a> of the <a href="https://github.com/joel-costigliola/assertj-examples/">assertj-examples</a> project.</p>
<p>Note that <span class="small-code">extracting</span> one property can be made strongly typed by giving the property type as the second argument.</p>
<pre><code class="java">// to have type safe extracting, use the second parameter to pass the expected property type:
assertThat(fellowshipOfTheRing).extracting("name", String.class)
.contains("Boromir", "Gandalf", "Frodo", "Legolas")
.doesNotContain("Sauron", "Elrond");</code></pre>
<h3 class="page-header" ><span id="flat-extracted-properties-assertion"></span><i class="fa fa-star"></i> Flat(map) extracting </h3>
<p>Flat extracting is hard to explain but easy to understand with an example, so let's see how it works (if you are familiar with functional programming, it's just a <span class="small-code">flatMap</span>).</p>
<p>You have a <span class="small-code">Player</span> with a <span class="small-code">teamMates</span> property returning a <span class="small-code">List<Player></span>, you want to assert that it returns the expected players:</p>
<pre><code class="java">Player jordan = ... // initialized with Pippen and Kukoc team mates
Player magic = ... // initialized with Jabbar and Worthy team mates
List<Player> reallyGoodPlayers = list(jordan, magic);
// check all team mates by specifying the teamMates property (Player has a getTeamMates() method):
assertThat(reallyGoodPlayers).flatExtracting("teamMates")
.contains(pippen, kukoc, jabbar, worthy);
// alternatively, you can implement an Extractor to extract the team mates:
assertThat(reallyGoodPlayers).flatExtracting(teamMates)
.contains(pippen, kukoc, jabbar, worthy);</code></pre>
<p>where <span class="small-code">teamMates</span> is an instance of <span class="small-code">PlayerTeammatesExtractor</span>:</p>
<pre><code class="java">public class PlayerTeammatesExtractor implements Extractor<Player, List<Player>> {
@Override
public List<Player> extract(Player input) {
return input.getTeamMates();
}
}</code></pre>
<h3 class="page-header" ><span id="extracted-method-result-assertion"></span><i class="fa fa-star"></i> Assertions on results of a method call on iterable/array elements</h3>
<p>Like <span class="small-code">extracting</span> but instead of extracting properties/fields, it extracts the result of a given method invocation on the elements of the Iterable/Array under test and puts the results into a new Iterable/Array which becomes the object under test.<br>
It allows you to test the results of a method call on elements instead of testing the elements themselves. It's especially useful for classes that do not conform to Java Bean's getter specification (e.g <span class="small-code">toString()</span> or <span class="small-code">String status()</span> instead of <span class="small-code">String getStatus()</span>).
</p>
<p>Let's take an example to make things clearer :</p>
<pre><code class="java">// WesterosHouse class has a method: public String sayTheWords()
List<WesterosHouse> greatHouses = new ArrayList<WesterosHouse>();
greatHouses.add(new WesterosHouse("Stark", "Winter is Coming"));
greatHouses.add(new WesterosHouse("Lannister", "Hear Me Roar!"));
greatHouses.add(new WesterosHouse("Greyjoy", "We Do Not Sow"));
greatHouses.add(new WesterosHouse("Baratheon", "Our is the Fury"));
greatHouses.add(new WesterosHouse("Martell", "Unbowed, Unbent, Unbroken"));
greatHouses.add(new WesterosHouse("Tyrell", "Growing Strong"));
// let's verify the words of the great houses of Westeros:
assertThat(greatHouses).extractingResultOf("sayTheWords")
.contains("Winter is Coming", "We Do Not Sow", "Hear Me Roar")
.doesNotContain("Lannisters always pay their debts");</code></pre>
<h3 class="page-header"><span id="soft-assertions"></span><i class="fa fa-star"></i> Collect all errors with soft assertions</h3>
<p>Using soft assertions, AssertJ collects all assertion errors instead of stopping at the first one.<br>
So, assuming something goes awry at your dinner party, when using "standard" assertions, this test ...</p>
<pre><code class="java">@Test
public void host_dinner_party_where_nobody_dies() {
Mansion mansion = new Mansion();
mansion.hostPotentiallyMurderousDinnerParty();
assertThat(mansion.guests()).as("Living Guests").isEqualTo(7);
assertThat(mansion.kitchen()).as("Kitchen").isEqualTo("clean");
assertThat(mansion.library()).as("Library").isEqualTo("clean");
assertThat(mansion.revolverAmmo()).as("Revolver Ammo").isEqualTo(6);
assertThat(mansion.candlestick()).as("Candlestick").isEqualTo("pristine");
assertThat(mansion.colonel()).as("Colonel").isEqualTo("well kempt");
assertThat(mansion.professor()).as("Professor").isEqualTo("well kempt");
} </code></pre>
<p>... will yield the less-than-ideal exception message :<br>
<pre><code class="diff">org.junit.ComparisonFailure: [Living Guests] expected:<[7]> but was<[6]></code></pre>
<p>Using soft assertions you can collect all failed assertions:</p>
<pre><code class="java">@Test
public void host_dinner_party_where_nobody_dies() {
Mansion mansion = new Mansion();
mansion.hostPotentiallyMurderousDinnerParty();
// use SoftAssertions instead of direct assertThat methods
SoftAssertions softly = new SoftAssertions();
softly.assertThat(mansion.guests()).as("Living Guests").isEqualTo(7);
softly.assertThat(mansion.kitchen()).as("Kitchen").isEqualTo("clean");
softly.assertThat(mansion.library()).as("Library").isEqualTo("clean");
softly.assertThat(mansion.revolverAmmo()).as("Revolver Ammo").isEqualTo(6);
softly.assertThat(mansion.candlestick()).as("Candlestick").isEqualTo("pristine");
softly.assertThat(mansion.colonel()).as("Colonel").isEqualTo("well kempt");
softly.assertThat(mansion.professor()).as("Professor").isEqualTo("well kempt");
// Don't forget to call SoftAssertions global verification !
softly.assertAll();
}</code></pre>
<p>When the collected assertions are all asserted together they yield a more descriptive error message:</p>
<pre><code class="diff">org.assertj.core.api.SoftAssertionError:
The following 4 assertions failed:
1) [Living Guests] expected:<[7]> but was:<[6]>
2) [Library] expected:<'[clean]'> but was:<'[messy]'>
3) [Candlestick] expected:<'[pristine]'> but was:<'[bent]'>
4) [Professor] expected:<'[well kempt]'> but was:<'[bloodied and dishevelled]'></code></pre>
<p>AssertJ also provides a few ways to avoid having to call <span class="small-code">softly.assertAll()</span> manually:</p>
<ul>
<li><a href="#JUnitSoftAssertions"><span class="small-code">JUnitSoftAssertions</span> with a JUnit rule</a></li>
<li><a href="#AutoCloseableSoftAssertions"><span class="small-code">AutoCloseableSoftAssertions</span> </a></li>
<li><a href="#assertSoftly">Using the static <span class="small-code">assertSoftly</span> method</a></li>
</ul>
<p><span class="small-code">JUnitSoftAssertions</span> example:</p>
<span id="JUnitSoftAssertions"></span><pre><code class="java">@Rule
public final JUnitSoftAssertions softly = new JUnitSoftAssertions();
@Test
public void host_dinner_party_where_nobody_dies() {
Mansion mansion = new Mansion();
mansion.hostPotentiallyMurderousDinnerParty();
// use SoftAssertions instead of direct assertThat methods
softly.assertThat(mansion.guests()).as("Living Guests").isEqualTo(7);
softly.assertThat(mansion.kitchen()).as("Kitchen").isEqualTo("clean");
softly.assertThat(mansion.library()).as("Library").isEqualTo("clean");
softly.assertThat(mansion.revolverAmmo()).as("Revolver Ammo").isEqualTo(6);
softly.assertThat(mansion.candlestick()).as("Candlestick").isEqualTo("pristine");
softly.assertThat(mansion.colonel()).as("Colonel").isEqualTo("well kempt");
softly.assertThat(mansion.professor()).as("Professor").isEqualTo("well kempt");
// No need to call softly.assertAll(), it is automatically done by JUnitSoftAssertions rule
}</code></pre>
<p><span class="small-code">AutoCloseableSoftAssertions</span> example:</p>
<span id="AutoCloseableSoftAssertions"></span><pre><code class="java">@Test
public void host_dinner_party_where_nobody_dies() {
Mansion mansion = new Mansion();
mansion.hostPotentiallyMurderousDinnerParty();
try (AutoCloseableSoftAssertions softly = new AutoCloseableSoftAssertions()) {
softly.assertThat(mansion.guests()).as("Living Guests").isEqualTo(7);
softly.assertThat(mansion.kitchen()).as("Kitchen").isEqualTo("clean");
softly.assertThat(mansion.library()).as("Library").isEqualTo("clean");
softly.assertThat(mansion.revolverAmmo()).as("Revolver Ammo").isEqualTo(6);
softly.assertThat(mansion.candlestick()).as("Candlestick").isEqualTo("pristine");
softly.assertThat(mansion.colonel()).as("Colonel").isEqualTo("well kempt");
softly.assertThat(mansion.professor()).as("Professor").isEqualTo("well kempt");
// no need to call assertAll, it is done when softly is closed.
}
}</code></pre>
<p><span class="small-code">assertSoftly</span> example:</p>
<span id="assertSoftly"></span><pre><code class="java">@Test
public void host_dinner_party_where_nobody_dies() {
Mansion mansion = new Mansion();
mansion.hostPotentiallyMurderousDinnerParty();
SoftAssertions.assertSoftly(softly -> {
softly.assertThat(mansion.guests()).as("Living Guests").isEqualTo(7);
softly.assertThat(mansion.kitchen()).as("Kitchen").isEqualTo("clean");
softly.assertThat(mansion.library()).as("Library").isEqualTo("clean");
softly.assertThat(mansion.revolverAmmo()).as("Revolver Ammo").isEqualTo(6);
softly.assertThat(mansion.candlestick()).as("Candlestick").isEqualTo("pristine");
softly.assertThat(mansion.colonel()).as("Colonel").isEqualTo("well kempt");
softly.assertThat(mansion.professor()).as("Professor").isEqualTo("well kempt");
// no need to call assertAll, it is done by assertSoftly.
});
}</code></pre>
<p>Note that in TestNG it is not possible to make the global verification in an <span class="small-code">@AfterMethod</span> method because if it throws an exception, TestNG skips all subsequent tests.</p>
<h3 class="page-header"><span id="file-content-string-assertions"></span><i class="fa fa-star"></i> Using String assertions on the content of a file</h3>
<p>File assertions are rather poor for checking the file content, so we had the idea to reuse AssertJ String assertions on the content of a file.<br>
See the example below :</p>
<pre><code class="java">File xFile = writeFile("xFile", "The Truth Is Out There");
// classic File assertions
assertThat(xFile).exists().isFile().isRelative();
// String assertions on the file content : contentOf() comes from Assertions.contentOf static import
assertThat(contentOf(xFile)).startsWith("The Truth").contains("Is Out").endsWith("There");</code></pre>
<p>Note that this is meant to be used with small files only, as the whole content is loaded into memory.</p>
<h3 class="page-header"><span id="exception-assertion"></span><i class="fa fa-star"></i> Exception assertions guide</h3>
<p>How to assert that an exception has been thrown and check that it is the expected one ?</p>
<p class="list-section">With Java 8 (AssertJ 3.x) :</p>
<p>Testing assertions in Java 8 is elegant, use <span class="small-code">assertThatThrownBy(ThrowingCallable)</span> to capture and then assert on a <span class="small-code">Throwable</span>, <span class="small-code">ThrowingCallable</span> being a functional interface it can be expressed by a lambda.</p>
<p>Example :</p>
<pre><code class="java">@Test
public void testException() {
assertThatThrownBy(() -> { throw new Exception("boom!"); }).isInstanceOf(Exception.class)
.hasMessageContaining("boom");
}</code></pre>
<p>An alternative syntax is provided as some people find it more natural :</p>
<pre><code class="java">@Test
public void testException() {
assertThatExceptionOfType(IOException.class).isThrownBy(() -> { throw new IOException("boom!"); })
.withMessage("%s!", "boom")
.withMessageContaining("boom")
.withNoCause();
}</code></pre>
<p>This later syntax has been enriched for common exceptions :</p>
<ul>
<li><span class="small-code">assertThatNullPointerException</span></li>
<li><span class="small-code">assertThatIllegalArgumentException</span></li>
<li><span class="small-code">assertThatIllegalStateException</span></li>
<li><span class="small-code">assertThatIOException</span></li>
</ul>
<p>The previous test can be rewritten as:</p>
<pre><code class="java">@Test
public void testException() {
assertThatIOException().isThrownBy(() -> { throw new IOException("boom!"); })
.withMessage("%s!", "boom")
.withMessageContaining("boom")
.withNoCause();
}</code></pre>
<p>You can test that a piece of code does not throw any exception:</p>
<pre><code class="java">assertThatCode(() -> {
// code that should throw an exception
...
}).doesNotThrowAnyException();
</code></pre>
<p>Finally, BDD aficionados can separate <i>when</i> and <i>then</i> steps by using <span class="small-code">catchThrowable(ThrowingCallable)</span> to capture the throwable and then perform assertions.</p>
<p>Example :</p>
<pre><code class="java">@Test
public void testException() {
// given some preconditions
// when
Throwable thrown = catchThrowable(() -> { throw new Exception("boom!"); });
// then
assertThat(thrown).isInstanceOf(Exception.class)
.hasMessageContaining("boom");
}</code></pre>
<p class="list-section">With Java 7 (AssertJ 2.x) :</p>
<p>Asserting on exceptions is not as nice compared to the Java 8 way, this is how you would do it in AssertJ 2.x :</p>
<ol>
<li>Put the code to should throw in a try-catch.</li>
<li>Call <span class="small-code">fail</span> method immediately after the code that should throw the exception, so that if it is not thrown, the test fails.</li>
<li>Make assertions on the caught exception</li>
</ol>
<p>Note that <span class="small-code">fail</span> method can be statically imported from <span class="small-code">Assertions</span> class</p>
<p>Example taken from <a href="https://github.com/joel-costigliola/assertj-examples/blob/master/assertions-examples/src/test/java/org/assertj/examples/FailUsageExamples.java">FailUsageExamples.java</a> in <a href="https://github.com/joel-costigliola/assertj-examples/">assertj-examples</a>.</p>
<pre><code class="java">import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.fail;
import static org.assertj.core.api.Assertions.failBecauseExceptionWasNotThrown;
// ... code omitted for brevety
assertThat(fellowshipOfTheRing).hasSize(9);
// here's the typical pattern to use Fail :
try {
fellowshipOfTheRing.get(9); // argggl !
// we should not arrive here => use fail to expresses that
// if IndexOutOfBoundsException was not thrown, test would fail the specified message
fail("IndexOutOfBoundsException expected because fellowshipOfTheRing has only 9 elements");
} catch (IndexOutOfBoundsException e) {
assertThat(e).hasMessage("Index: 9, Size: 9");
}
// Warning : don't catch Throwable as it would also catch the AssertionError thrown by fail method
// another way to do the same thing
try {
fellowshipOfTheRing.get(9); // argggl !
// if IndexOutOfBoundsException was not thrown, test would fail with message :
// "Expected IndexOutOfBoundsException to be thrown"
failBecauseExceptionWasNotThrown(IndexOutOfBoundsException.class);
} catch (IndexOutOfBoundsException e) {
assertThat(e).hasMessage("Index: 9, Size: 9");
}</code></pre>
<h3 class="page-header"><span id="custom-comparison-strategy"></span><i class="fa fa-star"></i> Using a custom comparison strategy in assertions</h3>
<p>Sometime you want to compare objects with a strategy not relying on the <span class="small-code">equals</span> method, this is possible in AssertJ thanks to two methods :
<ul>
<li><span class="small-code">usingComparator(Comparator)</span> : concerns object under assertion</li>
<li><span class="small-code">usingElementComparator(Comparator)</span> : concerns elements of iterable/array under assertion</li>
</ul>
</p>
<p><span class="small-code">usingComparator(Comparator)</span> example :</p>
<pre><code class="java">// frodo and sam are instances of TolkienCharacter with Hobbit race (obviously :)), they are not equal ...
assertThat(frodo).isNotEqualTo(sam);
// ... but if we compare race only, they are (raceComparator implements Comparator<TolkienCharacter>)
assertThat(frodo).usingComparator(raceComparator).isEqualTo(sam);</code></pre>
<p><span class="small-code">usingElementComparator(Comparator)</span> example :</p>
<pre><code class="java">// standard comparison : the fellowshipOfTheRing includes Gandalf but not Sauron (believe me) ...
assertThat(fellowshipOfTheRing).contains(gandalf).doesNotContain(sauron);
// ... but if we compare race only, Sauron is in fellowshipOfTheRing (he's a Maia like Gandalf)
assertThat(fellowshipOfTheRing).usingElementComparator(raceComparator).contains(sauron);</code></pre>
<h3 class="page-header"><span id="field-by-field-comparison"></span><i class="fa fa-star"></i> Field by field comparisons</h3>
<p>This is a special case of <a href="#custom-comparison-strategy">custom comparison</a> where objects are compared "by value", the comparison is performed on all fields/properties, several variants are provided :
<ul>
<li><a href="#field-by-field"><span class="small-code">isEqualToComparingFieldByField</span></a> : compares all fields/properties including inherited ones - not recursive</li>
<li><a href="#field-by-field-only"><span class="small-code">isEqualToComparingOnlyGivenFields</span></a> : compares only the specified fields/properties - not recursive</li>
<li><a href="#field-by-field-ignoring"><span class="small-code">isEqualToIgnoringGivenFields</span></a> : compares all fields/properties except specified ones - not recursive</li>
<li><a href="#field-by-field-null"><span class="small-code">isEqualToIgnoringNullFields</span></a> : compares non null fields/properties only - not recursive</li>
<li><a href="#field-by-field-recursive"><span class="small-code">isEqualToComparingFieldByFieldRecursively</span></a> : compares all fields/properties recursively</li>
</ul>
</p>
<p>Except <span class="small-code">isEqualToComparingFieldByFieldRecursively</span>, the comparison performed is not recursive, i.e. if one of the field is an <span class="small-code">Object</span>, it will be compared to the other field using its <span class="small-code">equals</span> method (whether <span class="small-code">equals</span> has been overriden or not). On the other hand <span class="small-code">isEqualToComparingFieldByFieldRecursively</span> compares <span class="small-code">Object</span> fields with a field by field strategy unless it has a custom <span class="small-code">equals</span> implementation (i.e. the overriden <span class="small-code">equals</span> method will be used instead of a field by field comparison).</p>
<p>You can specify a custom comparison strategy by <a href="assertj-core-news.html#assertj-core-2.4.0-usingComparatorForType">field type</a> or for a given <a href="assertj-core-news.html#assertj-core-2.4.0-usingComparatorForFields">group of fields</a>, this is useful when comparing doubles or floats as shown at the end of <span class="small-code">isEqualToComparingFieldByFieldRecursively</span> <a href="#field-by-field-recursive">example</a>.</p>
<p><u>Examples :</u></p>
<p>In the following examples, the <span class="small-code">TolkienCharacter</span> class has getters but its <span class="small-code">equals</span> method has not been overridden.</p>
<p><span id="field-by-field" class="small-code">isEqualToComparingFieldByField</span> :</p>
<pre><code class="java">TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
TolkienCharacter frodoClone = new TolkienCharacter("Frodo", 33, HOBBIT);
// Fail as equals compares object references
assertThat(frodo).isEqualsTo(frodoClone);
// frodo and frodoClone are equal when doing a field by field comparison.
assertThat(frodo).isEqualToComparingFieldByField(frodoClone);</code></pre>
<p><span id="field-by-field-only" class="small-code">isEqualToComparingOnlyGivenFields</span> :</p>
<pre><code class="java">TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
TolkienCharacter sam = new TolkienCharacter("Sam", 38, HOBBIT);
// frodo and sam both are hobbits, so they are equal when comparing only race
assertThat(frodo).isEqualToComparingOnlyGivenFields(sam, "race"); // OK
// they are also equals when comparing only race name (nested field).
assertThat(frodo).isEqualToComparingOnlyGivenFields(sam, "race.name"); // OK
// ... but not when comparing both name and race
assertThat(frodo).isEqualToComparingOnlyGivenFields(sam, "name", "race"); // FAIL</code></pre>
<p><span id="field-by-field-ignoring" class="small-code">isEqualToIgnoringGivenFields</span> :</p>
<pre><code class="java">TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
TolkienCharacter sam = new TolkienCharacter("Sam", 38, HOBBIT);
// frodo and sam are equal when ignoring name and age as the only remaining field is race
assertThat(frodo).isEqualToIgnoringGivenFields(sam, "name", "age"); // OK both are HOBBIT
// ... but they are not equals if only age is ignored as their names differ.
assertThat(frodo).isEqualToIgnoringGivenFields(sam, "age"); // FAIL</code></pre>
<p><span id="field-by-field-null" class="small-code">isEqualToIgnoringNullFields</span> :</p>
<pre><code class="java">TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
TolkienCharacter mysteriousHobbit = new TolkienCharacter(null, 33, HOBBIT);
// Null fields in expected object are ignored, mysteriousHobbit has a null name thus it's ignored
assertThat(frodo).isEqualToIgnoringNullFields(mysteriousHobbit); // OK
// ... but this is not reversible !
assertThat(mysteriousHobbit).isEqualToIgnoringNullFields(frodo); // FAIL</code></pre>
<p><span id="field-by-field-recursive" class="small-code">isEqualToComparingFieldByFieldRecursively</span> :</p>
<pre><code class="java">public class Person {
public String name;
public double height;
public Home home = new Home();
public Person bestFriend;
// constructor with name and height omitted for brevity's sake
}
public class Home {
public Address address = new Address();
}
public static class Address {
public int number = 1;
}
Person jack = new Person("Jack", 1.80);
jack.home.address.number = 123;
Person jackClone = new Person("Jack", 1.80);
jackClone.home.address.number = 123;
// cycle are handled in comparison
jack.bestFriend = jackClone;
jackClone.bestFriend = jack;
// will fail as equals compares object references
assertThat(jack).isEqualsTo(jackClone);
// jack and jackClone are equal when doing a recursive field by field comparison
assertThat(jack).isEqualToComparingFieldByFieldRecursively(jackClone);
// any type/field can be compared with a a specific comparator.
// let's change jack's height a little bit
jack.height = 1.81;
// assertion fails because of the height difference
// (the default precision comparison for double is 1.0E-15)
assertThat(jack).isEqualToComparingFieldByFieldRecursively(jackClone);
// use usingComparatorForType to specify how to compare the given type
// assertion succeeds because we allow a 0.5 tolerance on double
assertThat(jack).usingComparatorForType(new DoubleComparator(0.5), Double.class)
.isEqualToComparingFieldByFieldRecursively(jackClone);
// use usingComparatorForFields to specify how to compare some fields (nested fields are supported)
assertThat(jack).usingComparatorForFields(new DoubleComparator(0.5), "height")
.isEqualToComparingFieldByFieldRecursively(jackClone);</code></pre>
<h3 class="page-header"><span id="custom-representation"></span><i class="fa fa-star"></i> Using a custom
representation in assertions</h3>
<p>
Assertions error messages use a <span class="small-code">Representation</span> to format different types
involved.
There are multiple ways of registering a custom representation for assertions:
</p>
<ul>
<li>
global scope (default on startup): Since 2.9.0 / 3.9.0, one can register an implementation of <span class="small-code">Representation</span>
via a Service Provider Interface (SPI). This will cause AssertJ to use that assertion per default all the time.
There won't be a need to call <span class="small-code">Assertions.useRepresentation(myRepresentation)</span>.
And calling <span class="small-code">Assertions.useDefaultRepresentation()</span> will actually use the SPI registered <span class="small-code">Representation</span>
</li>
<li>
global scope (after calling): <span class="small-code">Assertions.useRepresentation(myRepresentation)</span>
- After this call AssertJ will use the implemented <span class="small-code">myRepresentation</span> globally
</li>
<li>
per assertion scope: <span class="small-code">assertThat(actual).withRepresentation(myRepresentation)</span>
- This will cause AssertJ to use to passed representation only during the scope of that particular assertion
</li>
</ul>
<p>
Examples:
</p>
<pre><code class="java">private class Example {} // dummy class
public class CustomRepresentation extends StandardRepresentation {
// override needed to hook non predefined type formatting
@Override
public String fallbackToStringOf(Object o) {
if (o instanceof Example) return "Example";
// fallback to default formatting.
return super.fallbackToStringOf(o);
}
// override a predefined type formatting : String
@Override
protected String toStringOf(String str) {
return "$" + str + "$";
}
}</code></pre>
<p>Global scope custom representation (default on startup) :</p>
<p>
You will need to register your <span class="small-code">Representation</span> as an SPI. You need to do the following for that:
</p>
<ul>
<li>
Create a file named <span class="small-code">org.assertj.core.presentation.Representation</span>
in the <span class="small-code">META-INF/services</span> directory
</li>
<li>
Put the fully qualified class name of your <span class="small-code">Representation</span> in it
</li>
<li>
Make sure <span class="small-code">META-INF/services/org.assertj.core.presentation.Representation</span>
is in the runtime classpath, usually putting it in <span class="small-code">src/test/resources</span> is enough
</li>
<li>
We recommend that you extend from the <span class="small-code">StandardRepresentation</span> and override the
<span class="small-code">fallbackToStringOf(Object)</span> method. By doing this all the defaults of AssertJ
would be applied and you can apply your own customization in there.
</li>
</ul>
<pre><code class="java">//You don't need to do anything, AssertJ will pick up the Representation automatically
// this assertion fails ...
assertThat(new Example()).isNull();
// ... with error :
expected:<[null]> but was:<[Example]>
// this one fails ...
assertThat("foo").startsWith("bar");
// ... with error :
Expecting:
<$foo$>
to start with:
<$bar$></code></pre>
<p>Global scope custom representation (after call) :</p>
<pre><code class="java">// register CustomRepresentation only once. However, all tests before calling this would use the default one
Assertions.useRepresentation(new CustomRepresentation());
// this assertion fails ...
assertThat(new Example()).isNull();
// ... with error :
expected:<[null]> but was:<[Example]>
// this one fails ...
assertThat("foo").startsWith("bar");
// ... with error :
Expecting:
<$foo$>
to start with:
<$bar$></code></pre>
<p>Per assertion scope custom representation :</p>
<pre><code class="java">// we need to register CustomRepresentation for each assertions
Representation customRepresentation = new CustomRepresentation();
// this assertion fails ...
assertThat(new Example()).withRepresentation(customRepresentation)
.isNull();
// ... with error :
expected:<[null]> but was:<[Example]>
// this one fails ...
assertThat("foo").withRepresentation(customRepresentation)
.startsWith("bar");
// ... with error :
Expecting:
<$foo$>
to start with:
<$bar$></code></pre>
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