应用退出时提示，监听OnkeyDown

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub
        
        if(keyCode==KeyEvent.KEYCODE_BACK){
            //弹出肯定退出对话框
            new AlertDialog.Builder(this)
            .setTitle("退出")
            .setMessage("肯定退出吗？")
            .setPositiveButton("肯定", new DialogInterface.OnClickListener() {
                
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    Intent exit = new Intent(Intent.ACTION_MAIN);
                    exit.addCategory(Intent.CATEGORY_HOME);
                    exit.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(exit);
                    System.exit(0);
                }
            })
            .setNegativeButton("取消", new DialogInterface.OnClickListener() {
                
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    // TODO Auto-generated method stub
                    dialog.cancel();
                }
            })
            .show();
            //这里不须要执行父类的点击事件，因此直接return
            return true;
        }
        //继续执行父类的其余点击事件
        return super.onKeyDown(keyCode, event);
    }