Google Sign in sample with webview

GoogleSignInActivity.java

package com.omerucel.sample;

import android.content.Intent;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.MenuItem;
import android.view.View;
import android.webkit.WebResourceError;
import android.webkit.WebResourceRequest;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.TextView;

import com.omerucel.sample.CustomApplication;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public class GoogleSignInActivity extends AppCompatActivity {
    final public static String CLIENT_ID = "";
    protected CustomApplication customApplication;
    protected OkHttpClient okHttpClient;
    protected Boolean firstPageInitialize = false;

    protected WebView webView;
    protected TextView loadingMessage;
    protected TextView successMessage;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_google_signin);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        customApplication = (CustomApplication) getApplication();
        webView = (WebView) findViewById(R.id.webview);
        loadingMessage = (TextView) findViewById(R.id.loading_message);
        successMessage = (TextView) findViewById(R.id.success_message);
        okHttpClient = new OkHttpClient();
        String url = "https://accounts.google.com/o/oauth2/v2/auth?"
                + "scope=https://www.googleapis.com/auth/drive.appdata"
                + "&redirect_uri=http://localhost"
                + "&response_type=code"
                + "&client_id=" + CLIENT_ID;
        webView.loadUrl(url);
        webView.getSettings().setJavaScriptEnabled(true);
        webView.setWebViewClient(new WebViewClient() {
            @Override
            public void onPageStarted(WebView view, String url, Bitmap favicon) {
                super.onPageStarted(view, url, favicon);
                Log.d("test", "onPageStarted:" + url);
                Log.d("test", "onPageStarted:" + view.getTitle());
                Uri uri = Uri.parse(url);
                if (uri.getHost().equals("localhost")) {
                    webView.setVisibility(View.INVISIBLE);
                    loadingMessage.setVisibility(View.INVISIBLE);
                    successMessage.setVisibility(View.VISIBLE);
                }
                if (uri.getQueryParameter("code") != null && uri.getQueryParameter("code").length() > 0) {
                    handleAccessCode(uri.getQueryParameter("code"));
                }
                if (uri.getQueryParameter("error") != null) {
                    Log.d("test", "onPageStarted:" + uri.getQueryParameter("error"));
                    loadingMessage.setText(getString(R.string.an_error_occurred));
                }
            }

            @Override
            public void onPageFinished(WebView view, String url) {
                super.onPageFinished(view, url);
                Log.d("test", "onPageFinished:" + url);
                Log.d("test", "onPageFinished:" + view.getTitle());
                if (!firstPageInitialize) {
                    firstPageInitialize = true;
                    loadingMessage.setVisibility(View.INVISIBLE);
                    webView.setVisibility(View.VISIBLE);
                }
            }

            @Override
            public void onReceivedError(WebView view, WebResourceRequest request, WebResourceError error) {
                super.onReceivedError(view, request, error);
                Log.d("test", "onReceiveError");
                loadingMessage.setText(getString(R.string.an_error_occurred));
            }
        });
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                finish();
                break;
            default:
                return super.onOptionsItemSelected(item);
        }
        return true;
    }

    public void handleAccessCode(String accessCode) {
        String content = "code=" + accessCode
                + "&client_id=" + CLIENT_ID
                + "&redirect_uri=http://localhost"
                + "&grant_type=authorization_code";
        RequestBody requestBody = RequestBody.create(MediaType.parse("application/x-www-form-urlencoded; charset=utf-8"), content);
        Request request = new Request.Builder()
                .url("https://www.googleapis.com/oauth2/v4/token")
                .post(requestBody)
                .build();
        okHttpClient.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.d("test", "onFailure:" + e.getMessage(), e);
                loadingMessage.setText(getString(R.string.an_error_occurred));
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                try {
                    JSONObject jsonObject = new JSONObject(response.body().string());
                    Log.d("test", "Response: " + response.body().string());
                    customApplication.preferences.saveAccessToken(jsonObject.getString("access_token"));
                    finish();
                } catch (JSONException e) {
                    Log.d("test", "onResponse:" + e.getMessage(), e);
                    loadingMessage.setText(getString(R.string.an_error_occurred));
                }
            }
        });
    }
}