final Dialog dialog = new Dialog(ac, R.style.notitle);
                LayoutInflater inflater = LayoutInflater.from(ac);
                View view = inflater.inflate(R.layout.switch_account, null);
 TextView tv_sa_account = (TextView)  view.findViewById(R.id.tv_sa_account);
dialog.setCancelable(true);
                dialog.setCanceledOnTouchOutside(false);
                WindowManager wm = this.getWindowManager();
                int width = wm.getDefaultDisplay().getWidth();
                int height = wm.getDefaultDisplay().getHeight();
                 
                LinearLayout    lin_switch = (LinearLayout)  view.findViewById(R.id.lin_switch);
 
                dialog.setContentView(view, new LinearLayout.LayoutParams(
                        width,
                        height));


lin_switch.getBackground().setAlpha(50);//透明
             
                dialog.show();
                 Window window = dialog.getWindow();
                window.setGravity(Gravity.TOP);



<style name="notitle" >



 <!-- 背景颜色及透明程度 -->
        <item name="android:windowBackground">@android:color/transparent</item>
        <!-- 是否有标题 -->
        <item name="android:windowNoTitle">true</item>
        <!-- 是否浮现在activity之上true在中间 -->
        <item name="android:windowIsFloating">true</item>
     
        
    </style>