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`.

            }

            for (int state : connectedStates) {
                transport.setState(state);
                assertFalse(transport.isDisconnected(), "State " + state + " should not be disconnected");
            }
        }

        @Test
        @DisplayName("should correctly identify failed states")

        assertEquals(smbComNtTransaction.parameterOffset, SmbComTransaction.readInt4(dst, 23),
                "parameterOffset should be written correctly");
        assertEquals(smbComNtTransaction.dataCount, SmbComTransaction.readInt4(dst, 27), "dataCount should be written correctly");
        assertEquals(smbComNtTransaction.dataOffset, SmbComTransaction.readInt4(dst, 31), "dataOffset should be written correctly");

        assertNotNull(password, "Password char array should not be null");
        assertArrayEquals(testPassword.toCharArray(), password, "Password should match");

        // Verify it returns a clone, not the original
        password[0] = 'X';
        char[] password2 = authenticator.getPasswordAsCharArray();
        assertNotEquals(password[0], password2[0], "Should return a clone, not the original");
    }

    @Test

    @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() {

        // Assert
        assertNotNull(blob.get(), "get() should never return null for default instance");
        assertEquals(0, blob.get().length, "Internal array should be empty");
        assertEquals(0, blob.length(), "length() should be 0 for empty");
        assertEquals("", blob.toString(), "toString() of empty should be empty string");
        assertTrue(blob.equals(blob), "Object should be equal to itself");

        assertNull(ex.getMessage(), "Default ctor should not set a message");
        assertNull(ex.getCause(), "Default ctor should not set a cause");
        assertTrue(ex instanceof SmbException, "Should be an SmbException subtype");
        // SmbException default ctor leaves status 0, which equals NT_STATUS_SUCCESS
        assertEquals(NtStatus.NT_STATUS_SUCCESS, ex.getNtStatus(), "Default status should be success (0)");
    }

    /**
     * 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.
     *

        assertTrue(infoString.contains("alloc=1000"), "toString should contain alloc");
        assertTrue(infoString.contains("free=500"), "toString should contain free");
        assertTrue(infoString.contains("sectPerAlloc=100"), "toString should contain sectPerAlloc");
        assertTrue(infoString.contains("bytesPerSect=512"), "toString should contain bytesPerSect");
    }

        assertEquals((int) offset, getFieldValue(write, "offset"), "Offset should be updated");
        assertEquals(remaining, getFieldValue(write, "remaining"), "Remaining should be updated");
        assertEquals(buffer, getFieldValue(write, "b"), "Buffer should be updated");
        assertEquals(off, getFieldValue(write, "off"), "Off should be updated");