Trying to save the state of checkbox and the data within that checkbox even when app is closed

I have created a checkbox that can only select one checkbox at a time. I am then storing the data of the checkbox in a string which i then use in a different method. the checkbox is used in a recyclerview and it is displayed using a firebase recycler adapter. my app requires the checkbox to remain checked at all times even when app is closed as I am launching the app with a foreground service and it uses the data that is checked in the checkbox for an intent in a different class. What i want to know is how to get the checkbox and the data that is stored using this checkbox to save even if my app is closed . I have done some digging and seen that you can use a shared preference but I cant seem to work out how to implement it in my application. here is the code I am currently using to implement the checkbox: firebase recycler adapter

 adapter1 = new FirebaseRecyclerAdapter<contact, contactAdapter>(options1) {
            @Override
            protected void onBindViewHolder(@NonNull final contactAdapter holder, final int position, @NonNull contact model) {
                holder.contactName.setText(model.getContactName());
                holder.contactPhone.setText(model.getContactPhone());
                if(lastCheckedPos == position){
                    holder.chk.setChecked(true);
                }
                else{
                    holder.chk.setChecked(false);
                }
                holder.chk.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
                    @Override
                    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                        if (position == lastCheckedPos) {
                            holder.chk.setChecked(false);
                            lastCheckedPos = -1;
                        }else {
                            lastCheckedPos = position;
                            for (int i = 0; i < adapter1.getItemCount(); i++) {
                                if (adapter1.getItem(i).isSelected(true)) {
                                    number = ((TextView) holder.itemView.findViewById(R.id.contactPhone)).getText().toString();
                                    Toast.makeText(getActivity(), "Data Inserted....." + number, Toast.LENGTH_SHORT).show();
                                } else {
                                    Toast.makeText(getActivity(), "Error", Toast.LENGTH_LONG).show();
                                }
                            }
                            contactsView.post(new Runnable()
                            {
                                @Override
                                public void run() {
                                    adapter1.notifyDataSetChanged();
                                }
                            });

                        }
                    }
                });
            }

            @NonNull
            @Override
            public contactAdapter onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                return new contactAdapter(LayoutInflater.from(getActivity()).inflate(R.layout.item_contacts, parent, false));

            }
        };

Adapter class

public class contactAdapter extends RecyclerView.ViewHolder{

    public TextView contactName, contactPhone;
    CheckBox chk;
    AdapterView.OnItemClickListener itemClickListener;




    public contactAdapter(@NonNull View itemView) {
        super(itemView);
        contactName = itemView.findViewById(R.id.contactName);
        contactPhone = itemView.findViewById(R.id.contactPhone);
        chk = itemView.findViewById(R.id.myCheckBox);
        this.setIsRecyclable(false);

    }
}

Any suggestions?