Migration to Java 8. API with default implementation.

[TikhomirovSergey]

Change list

Epic: #399

• each one interface extends the io.appium.java_client.ExecutesMethod
• both AppiumDriver and MobileElement implement io.appium.java_client.ExecutesMethod
• Current API has default implementation
• AppiumDriver, MobileElement and subclasses just declare interfaces are implemented.

Types of changes

What types of changes are you proposing/introducing to Java client?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Details

We have few targets:

• We are going to support Windows Mobile automation. Some interesting things were found. It supports some functions which were declared as Android-specific. As well it may support some functions which were declared as iOS-only as well. So we need for very flexible API which allows to implement it easily and without copy-paste.
• Some users need to use Android/iOS specific functions eventually. Now it should be possible in very pretty way:

import io.appium.java_client.android.StartsActivity;
...

private AppiumDriver<?> driver;

...
       StartsActivity startsActivity = new StartsActivity() {
            @Override
            public Response execute(String driverCommand, Map<String, ?> parameters) {
                return driver.execute(driverCommand, parameters);
            }

            @Override
            public Response execute(String driverCommand) {
                return driver.execute(driverCommand);
            }
        };
        startsActivity.startActivity("yourPackage", "yourActivity");

If somebody wants to use only Selenium and Appium interfaces when even this is possible:

import io.appium.java_client.ExecutesMethod;

public class ExtendedRemoteWebDriver extends RemoteWebDriver implements ExecutesMethod {
...
}

_______________________________________________________________________________________________

import io.appium.java_client.android.StartsActivity;
...

private ExtendedRemoteWebDriver driver;

...
       StartsActivity startsActivity = new StartsActivity() {
            @Override
            public Response execute(String driverCommand, Map<String, ?> parameters) {
                return driver.execute(driverCommand, parameters);
            }

            @Override
            public Response execute(String driverCommand) {
                return driver.execute(driverCommand);
            }
        };
        startsActivity.startActivity("yourPackage", "yourActivity");

[TikhomirovSergey]

It is just refactoring. API moving/restructuration is going after the merging.

Related issues/epics: #454, #471

[TikhomirovSergey]

@SrinivasanTarget could you review this PR
FYI @HeloiseRD @pfoster-youitv @philgreenyouilabs @simongranger

[SrinivasanTarget]

@TikhomirovSergey I like the changes. Its a great start 👍

[SrinivasanTarget]

@TikhomirovSergey I see checkstyle failures due to last PR's https://gist.github.com/SrinivasanTarget/60573e85fc7a85822f4ecdb1720026ef. Otherwise LGTM 👍

[TikhomirovSergey]

@SrinivasanTarget ok will fix ASAP but it is not related to this PR.