android 如何获得正确的分辨率[使用getRealSize

private void getScreenSizeOfDevice2() {  
    Point point = new Point();  
    getWindowManager().getDefaultDisplay().getRealSize(point);  
    DisplayMetrics dm = getResources().getDisplayMetrics();  
    double x = Math.pow(point.x/ dm.xdpi, 2);  
    double y = Math.pow(point.y / dm.ydpi, 2);  
    double screenInches = Math.sqrt(x + y);  
    Log.d(TAG, "Screen inches : " + screenInches);  
}  

小米5测试获取出来：Screen inches : 5.147105782031679。和官方说的5.15是对得上的