Test.java

import org.opencv.core.*;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

import java.io.File;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;

public class Test {
   public static void main(String[] args) {

      System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
      String info = "Java " + System.getProperty("java.version") + " "
               + Core.NATIVE_LIBRARY_NAME + Core.VERSION_STATUS;
      System.out.println(info);

      var filename = "duke.png";
      var file = new File(filename);

      if (!file.exists()) {
         System.out.println(filename + "not found!");
         System.exit(-1);
      }

      var srcImage = Imgcodecs.imread(filename, Imgcodecs.IMREAD_UNCHANGED);

      List<Mat> channels = new ArrayList<>();
      Core.split(srcImage, channels);
      var chAlpha = channels.get(3); // 4th channel = Alpha

      Imgproc.cvtColor(srcImage, srcImage, Imgproc.COLOR_BGRA2GRAY);

      List<Mat> greyChannel = new ArrayList<>();
      Core.split(srcImage, greyChannel);
      var chGray = greyChannel.get(0);

      var grayDuke = new Mat();
      var listMat = Arrays.asList(chGray, chGray, chGray, chAlpha); // 3 channels + Alpha
      Core.merge(listMat, grayDuke);

      Imgcodecs.imwrite("duke_gray.png", grayDuke);

   }
}