I use HashMap to store String followed by object Doctor, where the String is the copy of code in Doctor.

At first I thought it just some problem with checking condition and throw Exception but it not.
Secondly, I printed all the doctors that had been added but all the previous doctor was the same as the new doctor

public class Main {
    public static void main(String[] args) {
        DoctorController controler = new DoctorController();
        try {
            controler.addWorker();
            controler.addWorker();
            controler.displayAll();
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

This the part where the problem occurs:

public class DoctorController {
    private final DoctorInputer inputer;
    private final DoctorManagement management;

    public DoctorController() {
        management = new DoctorManagement();
        inputer = new DoctorInputer();
    }

    public void addWorker() throws Exception {
        Doctor doctor = inputer.getDoctorDetails();
        if (management.addDoctor(doctor)) {
            System.out.println("Add successful");
        } else {
            System.out.println("Can not add doctor!");
        }
    }

    public void displayAll() {
        management.displayAll();
    }
}

import java.util.HashMap;
public class DoctorManagement {
    
    private final HashMap<String, Doctor> doctors;

    public DoctorManagement() {
        doctors = new HashMap<>();
    }
    
    public boolean addDoctor(Doctor d) throws Exception {
        if (doctors.containsKey(d.getCode())) {
            throw new Exception("Doctor code " + d.getCode() + " is duplicate");
        }
        if (d == null) {
            throw new Exception("Data does not exist");
        }
        doctors.put(d.getCode(), d);
        return true;
    }

    //Method: Displat All the doctor
}

These parts are just INPUT but maybe you will need it

public class DoctorInputer {
    private final Doctor doctor;

    public DoctorInputer() {
        doctor = new Doctor();
    }

    public Doctor getDoctorDetails() {
        doctor.setCode(Validation.getString("Enter code: ",
                "Code should follow format 'DOC_number'",
                "^DOC\d+$"));
        doctor.setName(Validation.getString("Enter name: ",
                "Invalid",
                "[a-zA-Z]+"));
        doctor.setSpecialization(Validation.getString("Enter specialization: ",
                "Invalid",
                "[a-zA-Z]+"));
        doctor.setAvaiability(Validation.getInt("Enter avaiability: ",
                "Avaiability >= 0",
                "Invalid", 0, Integer.MAX_VALUE));
        return doctor;
    }
}

public class Doctor {
    private String code;
    private String name;
    private String specialization;
    private int avaiability;

// Getter and Setters
    
    @Override
    public String toString(){
        return String.format("%-6s %-10s %-12s %-2d", code, name, specialization, avaiability);
    }
}