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")

        assertNull(avPair3.getRaw(), "Raw data should be null if constructor argument was null.");
    }

    @Test
    void testConstants() {
        assertEquals(0x0, AvPair.MsvAvEOL, "MsvAvEOL constant should be 0x0.");
        assertEquals(0x6, AvPair.MsvAvFlags, "MsvAvFlags constant should be 0x6.");

            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

        assertEquals(SERVER, dcerpcBinding.getServer(), "Server should match the constructor argument.");
        assertNull(dcerpcBinding.getOptions(), "Options should be null initially.");
        assertNull(dcerpcBinding.getEndpoint(), "Endpoint should be null initially.");
        assertNull(dcerpcBinding.getUuid(), "UUID should be null initially.");

        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

     * It should always return 0.
     */
    @Test
    void testWriteSetupWireFormat() {
        byte[] dst = new byte[10];
        // The method should not write anything and return 0
        assertEquals(0, response.writeSetupWireFormat(dst, 0), "writeSetupWireFormat should return 0.");
    }

    /**
     * Test for the writeParametersWireFormat method.
     * It should always return 0.
     */
    @Test

        assertEquals("Remark 1", share1.remark.trim(), "Share 1 remark should be correct.");

        SmbShareInfo share2 = (SmbShareInfo) response.results[1];
        assertEquals("IPC$", share2.getName().trim(), "Share 2 name should be correct.");
        assertEquals(3, share2.type, "Share 2 raw type should be 3.");
        assertEquals("Inter-Process Communication", share2.remark.trim(), "Share 2 remark should be correct.");
    }

    @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() {