函数式编程将代码转换为声明式样式

Question

I have a for loop like this.

for (int i = 0; i < filePaths.size(); i++) {
  String filePath = filePaths.get(i);
  Mat mat = Imgcodecs.imread(filePath);
  Mat gray = new Mat();
  cvtColor(mat, gray, 6);

  if (i != filePaths.size()-1) {
       threshold(gray, gray, 150, 255, THRESH_TRUNC);

   }
  Imgcodecs.imwrite(filePath, gray);
}

is it possible to convert it to declarative code.

Thanks.

Answer 1

I suppose you want to use some kind of functional style in your application, and remove this imperative code style to functional code style?

So, if I understood correctly, you should make some additional actions. First, you have to create DTO(Data transfer object) to pass through this part of your logic.

String filePath = filePaths.get(i);
Mat mat = Imgcodecs.imread(filePath);

You creating this DTO:

class FilePathDTO {
        private final String filepath;
        private final Mat mat;
        private final Mat grey = new Mat();
        private final boolean isNotLastFilepath;

        public FilePathDTO(String filepath, Mat mat, boolean isLast) {
            this.filepath = filepath;
            this.mat = mat;
            this.isNotLastFilepath = isLast;
        }

        public String getFilepath() {
            return filepath;
        }

        public Mat getMat() {
            return mat;
        }

        public Mat getGrey() {
            return grey;
        }

        public boolean isNotLastFilepath() {
            return isNotLastFilepath;
        }
    }

And then, the code will be looks like:

filePaths.stream()
                .map(filePath -> new FilePathDTO(filePath, Imgcodecs.imread(filePath), filePaths.indexOf(filePath) != filePaths.size() - 1))
                .forEach(dto -> {
                    cvtColor(dto.getMat(), dto.getGrey(), 6);

                    if(dto.isNotLastFilepath) {
                        threshold(dto.getGrey(), dto.getGrey(), 150, 255, THRESH_TRUNC);
                    }

                    Imgcodecs.imwrite(dto.getFilepath(), dto.getGrey());
                });

But, you have something side-effect, like

if(dto.isNotLastFilepath) {
    threshold(dto.getGrey(), dto.getGrey(), 150, 255, THRESH_TRUNC);
}

And you won't get rid of imperative code style at all. You can make you code more closer to functional.