In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks { #an-app-with-callbacks }

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

    @DisplayName("MonotonicClock singleton instance should always return the same instance")
    void testSingletonInstance() {
        MonotonicClock clock1 = MonotonicClock.get();
        MonotonicClock clock2 = MonotonicClock.get();

        assertSame(clock1, clock2, "Multiple calls to get() should return the same instance");
    }

    @Test
    @DisplayName("MonotonicClock should always use UTC timezone")
    void testClockTimezone() {

    /**
     * Indicates whether Maven should never fail the build, regardless of project result.
     *
     * @return an {@link Optional} containing true if the build should never fail, false if it should fail normally, or empty if not specified
     */
    @Nonnull
    Optional<Boolean> failNever();

    /**
     * Indicates whether Maven should resume from the last failed project in a previous build.
     *

        // Project A should have the highest weight as it's at the root
        assertTrue(weightA > weightB, "Project A should have weight > Project B");
        assertTrue(weightA > weightC, "Project A should have weight > Project C");
        assertTrue(weightB > weightX, "Project B should have weight > Project X");
        assertTrue(weightC > weightX, "Project C should have weight > Project X");
    }

    @Test

        "overridden configs should be preserved"
    )
    assert '"deepLinking": true' not in response.text, (
        "overridden configs should not include the old value"
    )
    assert '"syntaxHighlight": false' not in response.text, (
        "not used parameters should not be included"
    )
    assert '"dom_id": "#swagger-ui"' in response.text, (
        "default configs should be preserved"
    )

        Plugin retrievedPlugin = project.getPlugin("org.apache.maven.plugins:maven-compiler-plugin");
        assertNotNull(retrievedPlugin, "Plugin should be found");
        assertEquals("3.8.1", retrievedPlugin.getVersion(), "Initial version should match");

        // Modify the plugin version
        retrievedPlugin.setVersion("3.11.0");

        // Verify the change persists when getting the plugin again

        ExclusionArtifactFilter filter = new ExclusionArtifactFilter(Collections.singletonList(exclusion));

        assertFalse(filter.include(artifact), "Maven artifact should be excluded by glob pattern");
        assertTrue(filter.include(artifact2), "JUnit artifact should not be excluded by maven-* glob pattern");
    }

    @Test
    void testExcludeWithGlobStar() {
        Exclusion exclusion = new Exclusion();

* `allow_origins` - A list of origins that should be permitted to make cross-origin requests. E.g. `['https://example.org', 'https://www.example.org']`. You can use `['*']` to allow any origin.
* `allow_origin_regex` - A regex string to match against origins that should be permitted to make cross-origin requests. e.g. `'https://.*\.example\.org'`.

            assertTrue(result.success(), "Result should be successful when no errors");
            assertEquals(2, result.processedCount(), "Should have 2 processed POMs");
            assertEquals(1, result.modifiedCount(), "Should have 1 modified POM");
            assertEquals(1, result.unmodifiedCount(), "Should have 1 unmodified POM");
            assertEquals(0, result.errorCount(), "Should have no errors");
        }

        @Test

 *             but should have been.
 *
 */
@Deprecated
public interface ProjectDependenciesResolver {

    /**
     * Resolves the transitive dependencies of the specified project.
     *
     * @param project         The project whose dependencies should be resolved, must not be {@code null}.
     * @param scopesToResolve The dependency scopes that should be resolved, may be {@code null}.