Android – why does the screen always have the wrong inch size?

I want to calculate the device size in inches. I'm using this code and it will appear every time I search. But the problem is that when I put a 4.5-inch device in Android studio, my answer is 4. I tried a 5.2-inch device, and then got 4.3-inch and 10.1-inch - > the answer is 9.0. So how do I get an accurate answer?

 DisplayMetrics dm = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(dm);

    double wi=(double)dm.widthPixels/dm.xdpi;
    double hi=(double)dm.heightPixels/dm.ydpi;
    double x = Math.pow(wi,2);
    double y = Math.pow(hi,2);
    double screenInches = Math.sqrt(x+y);
    Log.e("hello"," "+screenInches);

resolvent:

The problem with your code is that it is not taken into account

action bar

Soft key

Screen orientation

Use the following answers

 WindowManager windowManager = getWindowManager();
Display display = windowManager.getDefaultDisplay();
DisplayMetrics displayMetrics = new DisplayMetrics();
display.getMetrics(displayMetrics);

// since SDK_INT = 1;
mWidthPixels = displayMetrics.widthPixels;
mHeightPixels = displayMetrics.heightPixels;

// includes window decorations (statusbar bar/menu bar)
if (Build.VERSION.SDK_INT >= 14 && Build.VERSION.SDK_INT < 17)
{
    try
    {
        mWidthPixels = (Integer) Display.class.@R_419_1075@("getRawWidth").invoke(display);
        mHeightPixels = (Integer) Display.class.@R_419_1075@("getRawHeight").invoke(display);
    }
    catch (Exception ignored)
    {
    }
}

// includes window decorations (statusbar bar/menu bar)
if (Build.VERSION.SDK_INT >= 17)
{
    try
    {
        Point realSize = new Point();
        Display.class.@R_419_1075@("getRealSize", Point.class).invoke(display, realSize);
        mWidthPixels = realSize.x;
        mHeightPixels = realSize.y;
    }
    catch (Exception ignored)
    {
    }


 DisplayMetrics dm = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(dm);
double x = Math.pow(mWidthPixels/dm.xdpi,2);
double y = Math.pow(mHeightPixels/dm.ydpi,2);
double screenInches = Math.sqrt(x+y);
Log.d("debug","Screen inches : " + screenInches);

Thanks to him