Here is an example of a simple abstract base class, explained in sections:

/**
 * Sample Base Class Page Object for Browser App
 *
 * @author Name
 *
 */
public abstract class BrowserBasePO <M extends WebElement> {
    public int elementWait = Global_VARS.TIMEOUT_ELEMENT;
    public String pageTitle = "";
    WebDriver driver = CreateDriver.getInstance().getDriver();

    // constructor
    public BrowserBasePO() throws Exception {
        PageFactory.initElements(driver, this);
    }
}

/**
 * Sample Base Class Page Object for Mobile App
 *
 * @author Name
 *
 */
public abstract class Mobile</span>BasePO <M extends MobileElement> {
    public int elementWait = Global_VARS.TIMEOUT_ELEMENT;
    public String pageTitle = "";
    AppiumDriver<MobileElement> driver =
                                CreateDriver.getInstance().getDriver(true);

    // constructor
    public MobileBasePO () throws Exception {
        PageFactory.initElements(new AppiumFieldDecorator(driver), this);
    }
}

Notice that in the class signature, there is a generic user that passes in WebElement; this is included now to allow future modification of the default behavior of the WebElement class. Again, the PageFactory.initElements method is called in the constructor that will automatically initialize all the subclass page objects when it is instantiated.