How can I readfile txt and input information to Array Data?

You would need to read file for example like this:

try {
      File file = new File("PATH/TO/FILE/filename.txt");
      Scanner myReader = new Scanner(file);
      while (myReader.hasNextLine()) {
        String line = myReader.nextLine(); // this will be your single line in a file
        someLogicHere(line);
      }
      myReader.close();
    } catch (FileNotFoundException e) {
      // handle file not found
    }

After a line is parsed into program you would need some logic to determine if this is a doctor or a pet and create objects inside code.

This logic would look soomething like this :

String firstLine = "name joao";
String secondLine = "specialisation cat";
String name = firstLine.replace("name ", ""); // get rid of this name prefix
String specialisation = secondLine.replace("specialisation ", ""); // get rid of this specialisation prefix
Doctor doctor = new Doctor(name, specialisation);

I would strongly suggest to split doctors and pets into different files to make it easier for you. Addicionally what would make it even easier for you to have a doctor and pet writen one line per each not to have to parse to or more lines to create single object. Ex name joao specialisation cat