Android实现退出界面弹出提示对话框

<?xml version="1.0" encoding="utf-8"?>  
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  
  android:orientation="vertical"  
  android:layout_width="fill_parent"  
  android:layout_height="fill_parent"  
  android:id="@+id/layout1" 
  android:gravity="center_horizontal" 
  >  
  <Button 
    android:id="@+id/button1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content"  
    android:text="退出"/> 
</LinearLayout> 

package com.example.test;  
  
import android.app.Activity; 
import android.app.AlertDialog; 
import android.content.DialogInterface; 
import android.os.Bundle; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
  
public class MainActivity extends Activity {  
 
 
  @Override  
  public void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.main);  
     
    Button button=(Button)findViewById(R.id.button1); 
    //为"退出"按钮添加单击事件监听器 
    button.setOnClickListener(new OnClickListener() { 
       
      @Override 
      public void onClick(View view) { 
        AlertDialog alert=new AlertDialog.Builder(MainActivity.this).create(); 
        alert.setIcon(R.drawable.stop); 
        alert.setTitle("退出？"); 
        alert.setMessage("真的要退出本软件吗？"); 
        //添加取消按钮 
        alert.setButton(DialogInterface.BUTTON_NEGATIVE,"不",new DialogInterface.OnClickListener() { 
           
          @Override 
          public void onClick(DialogInterface dialog, int which) { 
            // TODO Auto-generated method stub 
             
          } 
        }); 
        //添加"确定"按钮 
        alert.setButton(DialogInterface.BUTTON_POSITIVE,"是的", new DialogInterface.OnClickListener() { 
           
          @Override 
          public void onClick(DialogInterface arg0, int arg1) { 
             finish(); 
          } 
        }); 
        alert.show(); 
      } 
    }); 
  }  
}