MainActivityTestSection.java

@Test
    public void testTeachersAreDisplayed() throws IOException {
        /**
         * Setting up mockWebServer at localhost:9900.
         */
        MockWebServer server = new MockWebServer();
        // start the server at port 9900
        server.start(9900);

        /**
         * Get the target application context during testing and cast onto the TestDemoApplication
         */
        TestDemoApplication testApp = (TestDemoApplication) InstrumentationRegistry.getTargetContext().getApplicationContext();
        // Set the base url of the test app using the url of the mocked local server
        testApp.setBaseUrl(server.url("/").toString());

        // Enqueue the response you want the server to respond with when querried in the course to this test.
        server.enqueue(new MockResponse().setBody(SUCCESS_RESPONSE));


        // LAUNCH ACTIVITY
        mainActivityTestRule.launchActivity(null);
        // UI testing 
        onView(withText("John Doe")).check(matches(isDisplayed()));
        onView(withText("William Smith")).check(matches(isDisplayed()));
      
        // Shut down the server when done with testing
        server.shutdown();
    }