There are many times we have to switch from one application to another while in the middle of execution. In this case we have to switch to the second application and then perform some action and then again return back to the previous application.

DesiredCapabilities capabilities = new DesiredCapabilities();
capabilities.setCapability(CapabilityType.BROWSER_NAME, "");
capabilities.setCapability("device", "Android");
capabilities.setCapability("deviceName", "Galaxy Note3");
capabilities.setCapability("platformVersion", "5.0");
capabilities.setCapability("platformName","Android");

capabilities.setCapability("appPackage","com.android.contacts");
capabilities.setCapability("appActivity","com.android.contacts.activities.PeopleActivity");
driver = new AndroidDriver(new URL("http://127.0.0.1:4723/wd/hub"), capabilities);
driver.manage().timeouts().implicitlyWait(20L, TimeUnit.SECONDS);

//launch settings Application
driver.startActivity(settingsAppPackageName, settingsAppActivityName);

//Switch ON Bluetooth
driver.findElement(By.id("Bluetooth_id")).click();

//Re launch Caller Application
driver.startActivity(callerAppPackageName, callerAppActivityName);

Suppose we are on the mobile calling and we want to switch on Bluetooth then first the caller screen will be launched, then we will launch the settings screen and switch on the Bluetooth. Now again move back to the previous activity.