getExternalStoragePublicDirectory deprecated in Android Q

Based on the docs, use DCIM/... for the RELATIVE_PATH, where ... is whatever your custom subdirectory would be. So, you would wind up with something like this:

      val resolver = context.contentResolver
      val contentValues = ContentValues().apply {
        put(MediaStore.MediaColumns.DISPLAY_NAME, "CuteKitten001")
        put(MediaStore.MediaColumns.MIME_TYPE, "image/jpeg")
        put(MediaStore.MediaColumns.RELATIVE_PATH, "DCIM/PerracoLabs")
      }

      val uri = resolver.insert(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, contentValues)

      resolver.openOutputStream(uri).use {
        // TODO something with the stream
      }

Note that since RELATIVE_PATH is new to API Level 29, you would need to use this approach on newer devices and use getExternalStoragePublicDirectory() on older ones.

@CommonsWare answer is amazing. But for those who want it in Java, you need to try this:

ContentResolver resolver = context.getContentResolver();
ContentValues contentValues = new ContentValues();
contentValues.put(MediaStore.MediaColumns.DISPLAY_NAME, name);
contentValues.put(MediaStore.MediaColumns.MIME_TYPE, mimeType);
contentValues.put(MediaStore.MediaColumns.RELATIVE_PATH, Environment.DIRECTORY_DOWNLOADS);

Uri uri = resolver.insert(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, contentValues);

As per the suggestion of @SamChen the code should look like this for text files:

Uri uri = resolver.insert(MediaStore.Files.getContentUri("external"), contentValues);

Because we wouldn't want txt files lingering in the Images folder.

So, the place where I have mimeType, you enter the mime type you want. For example if you wanted txt (@Panache) you should replace mimeType with this string: "text/plain". Here is a list of mime types: https://www.freeformatter.com/mime-types-list.html

Also, where I have the variable name, you replace it with the name of the file in your case.