Authenticate OAuth2 ArcGIS Services in Android

WebViewDialogFragment.java

public class WebViewDialogFragment extends DialogFragment {

    // The redirect_uri can be either a special value of urn:ietf:wg:oauth:2.0:oob 
	// or an application-specific custom URI that is handled on the device.
	private static final String REDIRECT_URI = "";
	// OAuth2 client_id = APPID
	private static final String CLIENT_ID = "";
	String username;
	String access_token;
	String refresh_token;
	private CountDownLatch latch;	
	
	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
		Bundle savedInstanceState) {
		getDialog().setTitle("Sign in to ArcGIS Online");	
	
		final View root = inflater.inflate(R.layout.activity_main, null);
		root.findViewById(R.id.progressbar).setVisibility(View.VISIBLE);
		final WebView webView = (WebView) root.findViewById(R.id.webview);
		webView.getSettings().setJavaScriptEnabled(true);
		webView.getSettings().setBuiltInZoomControls(true);
		webView.setWebViewClient(new WebViewClient() {
			@Override
			public boolean shouldOverrideUrlLoading(WebView view, String url) {
				final String code = url.substring(url.indexOf("?code=") + 6,
								url.length());
							
				AsyncTask.execute(new Runnable() {
					@Override
					public void run() {
						try {
							requestToken("https://www.arcgis.com/sharing/rest/oauth2/token?client_id="
											+ CLIENT_ID
											+ "&redirect_uri="
											+ REDIRECT_URI
											+ "&grant_type=authorization_code&code="
											+ code);
						} catch (Exception e) {
							Log.e(getClass().getSimpleName(), "", e);
						}
						dismiss();
					}
				});
				return true;
			}

			@Override
			public void onPageFinished(WebView view, String url) {
				webView.setVisibility(View.VISIBLE);
				root.findViewById(R.id.progressbar).setVisibility(View.GONE);
			}
		});
		webView.loadUrl("https://www.arcgis.com/sharing/oauth2/authorize?client_id="
							+ CLIENT_ID
							+ "&response_type=code&redirect_uri="
							+ REDIRECT_URI);
		return root;
	}
	
	@Override
	public void onDismiss(DialogInterface dialog) {
		if (latch != null)
		latch.countDown();
		super.onDismiss(dialog);
	}

	public void setLatch(CountDownLatch latch) {
		this.latch = latch;
	}

	private void requestToken(String url) throws JsonParseException,
			MalformedURLException, IOException {
		JsonFactory f = new JsonFactory();
		JsonParser json = f.createJsonParser(new URL(url));
		json.nextToken();
		while (json.nextToken() != JsonToken.END_OBJECT) {
			String name = json.getCurrentName();
			json.nextToken();
			if ("username".equals(name))
				username = json.getText();
			else if ("access_token".equals(name))
				access_token = json.getText();
			else if ("refresh_token".equals(name))
				refresh_token = json.getText();
			Log.i(getClass().getSimpleName(), name + " = " + json.getText());
		}
	}
}