Sample oAuth2 code flow, based on gist service

MainActivity.java

/**
 * Sample oauth2 based on code flow
 */
public class MainActivity extends AppCompatActivity {

    private static final String REDIRECT_URL = "http://redirect.com";
    private static final String CLIENT_ID = "your_client_id(oauth_key)";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        WebView browser = (WebView) findViewById(R.id.browser);
        browser.setWebViewClient(new WebViewClient() {
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) {

                if (url.startsWith(REDIRECT_URL)) {
                    HashMap<String, String> params = ParseUrlParams(url);

                    if(params != null) {
                        //get access token and stuff
                        return true;
                    }
                }

                return super.shouldOverrideUrlLoading(view, url);
            }
        });
        String params = "?client_id=" + CLIENT_ID + "&response_type=code&redirect_uri=" + REDIRECT_URL;
        browser.loadUrl("https://gitter.im/login/oauth/authorize" + params);
    }

    public static @Nullable
    HashMap<String, String> ParseUrlParams(String url) {
        String params = null;

        try {
            params = new URL(url).getQuery();
        } catch (MalformedURLException e) {
            e.printStackTrace();
            return null;
        }

        String[] exploded = params.split("&");
        HashMap<String, String> uniqueParams = new HashMap<String, String>();

        for (String keyValue : exploded) {
            String[] split = keyValue.split("=");
            uniqueParams.put(split[0], split[1]);
        }

        return uniqueParams;
    }
}