它是否读取了数组中的下一个文件？

Question

运行此代码时遇到此错误。看起来文件在数组中（我认为），但它仍然给出了错误。我怎么解决这个问题？

OpenCV Error: Assertion failed (m.dims >= 2) in cv::Mat::Mat, file ..\..\..\..\opencv\modules\core\src\matrix.cpp, line 269
CvException [org.opencv.core.CvException: cv::Exception: ..\..\..\..\opencv\modules\core\src\matrix.cpp:269: error: (-215) m.dims >= 2 in function cv::Mat::Mat]
at org.opencv.core.Mat.n_Mat(Native Method)
    at org.opencv.core.Mat.<init>(Mat.java:676)
    at last.Last1.main(Last1.java:39)

代码。

public class Last1 {
    public static void main (String[] args){
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);

        try{

            File folder = new File("C:\\Documents and   Settings\\James2\\Desktop\\JE\\testplatenum\\images");
            File[] listOfFiles = folder.listFiles();

            for (int a = 0; a < listOfFiles.length; a++){
                for (File file : listOfFiles) {
                    if (file.isFile()) {
                        System.out.println(file.getName());
                        String img[] = {file.getPath()};


                        Mat img_grayROI =  Highgui.imread(img[a], Highgui.CV_LOAD_IMAGE_GRAYSCALE);

                        Rect roi = new Rect(300,250,300,200);
                        Mat cropped = new Mat(img_grayROI, roi);

                        Imgproc.threshold(cropped, cropped, -1, 255, Imgproc.THRESH_BINARY+Imgproc.THRESH_OTSU);
                        Imgproc.dilate(cropped, cropped, Imgproc.getStructuringElement(Imgproc.MORPH_RECT, new Size(2, 2)));


                        Mat result = cropped;
                        Highgui.imwrite("1)Crop_"+a+".jpg",result);

                    }
                }
            }
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}