feat: add values methode in AbstractMapAssert to make assertions on v…

…alues of the map (#3291)

assertj-core/src/main/java/org/assertj/core/api/AbstractMapAssert.java

@@ -15,6 +15,7 @@
 import static java.util.Collections.singleton;
 import static java.util.Objects.requireNonNull;
 import static java.util.stream.Collectors.toList;
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.entry;
 import static org.assertj.core.description.Description.mostRelevantDescription;
 import static org.assertj.core.error.ShouldBeUnmodifiable.shouldBeUnmodifiable;
@@ -27,6 +28,7 @@
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Collection;
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -2219,4 +2221,24 @@ private static List<Object> flatten(Iterable<Object> collectionToFlatten) {
     }
     return result;
   }
+
+  /**
+   * <p>Returns an {@link AbstractCollectionAssert} to make assertions on the values of the map</p>
+   *
+   * <p><strong>Example</strong></p>
+   * <pre><code class='java'> TolkienCharacter pippin = new TolkienCharacter("Pippin", 28, HOBBIT);
+   * TolkienCharacter frodo = new TolkienCharacter("Frodo", 33, HOBBIT);
+   * TolkienCharacter merry = new TolkienCharacter("Merry", 36, HOBBIT);
+   *
+   * Map&lt;String, TolkienCharacter&gt; characters = mapOf(entry("Pippin", pippin),
+   *                                                  entry("Frodo", frodo),
+   *                                                  entry("Merry", merry));
+   * assertThat(characters).values()
+   *                       .contains(frodo, pippin, merry); </code></pre>
+   * @return An {@link AbstractCollectionAssert} to make collections assertion only on map values.
+   */
+  public AbstractCollectionAssert<?, Collection<? extends V>, V, ObjectAssert<V>> values() {
+    requireNonNull(actual, "Can not extract values from a null map.");
+    return assertThat(actual.values());
+  }
 }

assertj-core/src/test/java/org/assertj/core/api/map/MapAssert_values_Test.java

@@ -0,0 +1,38 @@
+package org.assertj.core.api.map;
+
+import org.assertj.core.api.AbstractCollectionAssert;
+import org.assertj.core.api.ObjectAssert;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collection;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatNullPointerException;
+import static org.assertj.core.api.Assertions.from;
+
+class MapAssert_values_Test {
+  @Test
+  void should_return_collection_assertion_on_values_for_map_values() {
+    Map<String, Integer> map = Map.of("first", 1, "second", 2);
+    AbstractCollectionAssert<?, Collection<? extends Integer>, Integer, ObjectAssert<Integer>> assertion =
+      assertThat(map).values();
+    assertion.contains(1, 2);
+  }
+
+  @Test
+  void should_return_collection_assertion_with_right_generic_type() {
+    assertThat(Map.of("first", "one"))
+      .values()
+      .singleElement()
+      .returns('o', from(s -> s.charAt(0)));
+  }
+
+
+  @Test
+  void should_have_an_helpful_error_message_when_size_is_used_on_a_null_map() {
+    Map<String, String> nullMap = null;
+    assertThatNullPointerException().isThrownBy(() -> assertThat(nullMap).values().contains("nothing"))
+      .withMessage("Can not extract values from a null map.");
+  }
+}