webView关闭progrossBar以及出错提示

package com.maxpowersoft.example;
 
import android.app.Activity;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.util.Log;
import android.view.Window;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;
 
public class Main extends Activity {
    private WebView webview;
    private static final String TAG = "Main";
    private ProgressDialog progressBar;  
 
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        requestWindowFeature(Window.FEATURE_NO_TITLE);
 
        setContentView(R.layout.main);
 
        this.webview = (WebView)findViewById(R.string.webview);
 
        WebSettings settings = webview.getSettings();
        settings.setJavaScriptEnabled(true);
        webview.setScrollBarStyle(WebView.SCROLLBARS_OUTSIDE_OVERLAY);
 
        final AlertDialog alertDialog = new AlertDialog.Builder(this).create();
 
        progressBar = ProgressDialog.show(Main.this, "MaxPowerSoft Example", "Loading...");
 
        webview.setWebViewClient(new WebViewClient() {
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
                Log.i(TAG, "Processing webview url click...");
                view.loadUrl(url);
                return true;
            }
 
            public void onPageFinished(WebView view, String url) {
                Log.i(TAG, "Finished loading URL: " +url);
                if (progressBar.isShowing()) {
                    progressBar.dismiss();
                }
            }
 
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Log.e(TAG, "Error: " + description);
                Toast.makeText(activity, "Oh no! " + description, Toast.LENGTH_SHORT).show();
                alertDialog.setTitle("Error");
                alertDialog.setMessage(description);
                alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        return;
                    }
                });
                alertDialog.show();
            }
        });
        webview.loadUrl("http://www.google.com");
    }
}