For the blog ( https://no1fushi.hateblo.jp/entry/2019/12/21/130328 )

openFile.java

public String openFile() {
  String fn = "";
  String all = "";
  try {
    FileReader fr = new FileReader(fn);
    BufferedReader br = new BufferedReader(fr);
    String buf;
    while ((buf = br.readLine()) != null) {
      all += buf;
      all += System.getProperty("line.separator");
    }
    br.close();
    fr.close();
  } catch (Exception e) {
    System.out.println("Error reading file");
    System.exit(1);
  }
  return all;
}