src/main/java/org/fest/assertions/internal/Collections.java
public void assertContainsNull(Description description, Collection<?> actual) {
    assertNotNull(description, actual);
    if (!actual.contains(null)) {
      throw failures.failure(description, ShouldContainNull.shouldContainNull(actual));
    }
  }
public void assertContainsNull(Description description, Collection<?> actual) {
    assertNotNull(description, actual);
    if (!actual.contains(null)) {
      throw failures.failure(description, ShouldContainNull.shouldContainNull(actual));
    }
  }
Asserts that the given {@code Collection} contains {@code null}.

@param description contains information about the assertion.
@param actual      the given {@code Collection}.
@throws AssertionError if the given {@code Collection} is {@code null}.
@throws AssertionError if the given {@code Collection} does not contain null.