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Im trying to learn how to use OpenCV in java . Im studying this article . haarcascade_frontalface_alt.xml and lena.png files are in src folder.

package javaapplication5;

import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.MatOfRect;
import org.opencv.core.Point;
import org.opencv.core.Rect;
import org.opencv.core.Scalar;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
import org.opencv.objdetect.CascadeClassifier;

//
// Detects faces in an image, draws boxes around them, and writes the results
// to "faceDetection.png".
//
class DetectFaceDemo {
  public void run() {
    System.out.println("\nRunning DetectFaceDemo");

    // Create a face detector from the cascade file in the resources
    // directory.
    System.out.println("step 0");
    CascadeClassifier faceDetector = new CascadeClassifier(getClass().getResource("/haarcascade_frontalface_alt.xml").getPath());
    System.out.println("step 1");
    Mat image = Imgcodecs.imread(getClass().getResource("/lena.png").getPath());
    System.out.println("step 2");
    // Detect faces in the image.
    // MatOfRect is a special container class for Rect.
    MatOfRect faceDetections = new MatOfRect();
    System.out.println("step 3");
    faceDetector.detectMultiScale(image, faceDetections);
    System.out.println("step 4");
    System.out.println(String.format("Detected %s faces", faceDetections.toArray().length));

    // Draw a bounding box around each face.
    for (Rect rect : faceDetections.toArray()) {
        Imgproc.rectangle(image, new Point(rect.x, rect.y), new Point(rect.x + rect.width, rect.y + rect.height), new Scalar(0, 255, 0));
    }

    // Save the visualized detection.
    String filename = "faceDetection.png";
    System.out.println(String.format("Writing %s", filename));
    Imgcodecs.imwrite(filename, image);
  }
}

public class HelloOpenCV {
  public static void main(String[] args) {
    System.out.println("Hello, OpenCV");

    // Load the native library.
    System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
    new DetectFaceDemo().run();
  }
}

Output:

Hello, OpenCV

Running DetectFaceDemo
step 0
step 1
step 2
step 3
OpenCV Error: Assertion failed (!empty()) in cv::CascadeClassifier::detectMultiScale, file C:\builds\master_PackSlaveAddon-win64-vc12-static\opencv\modules\objdetect\src\cascadedetect.cpp, line 1639
Exception in thread "main" CvException [org.opencv.core.CvException: cv::Exception: C:\builds\master_PackSlaveAddon-win64-vc12-static\opencv\modules\objdetect\src\cascadedetect.cpp:1639: error: (-215) !empty() in function cv::CascadeClassifier::detectMultiScale
]