The function “getWindowManager()” does not exist.

Hi all! I am working on an android app and need to get screen size in cm to make it compatible with all devices. For that, I found this function in another discussion:

import android.view.WindowManager;
import android.util.DisplayMetrics;

import android.os.Bundle;

DisplayMetrics metrics;

//code like setup and draw etc.

//@Override
void onCreate(Bundle bundle)
{
      super.onCreate(bundle);

      metrics = new DisplayMetrics();
      getWindowManager().getDefaultDisplay().getMetrics(metrics);
}

float getXdpi()
{
      return metrics.xdpi;
}

float getYdpi()
{
      return metrics.ydpi;
}

However, when I paste it to my code, the getWindowManager.getDefaultDisplay.getMetrics(metrics) gets underlined, specifically the getWindowManager part and it says that such a function does not exist.

What could be the problem?