only want check-boxes or radio buttons in recyclerview to check one box

I am currently using checkboxes but I have tried radio buttons as well. I cant seem to get only one item to be selected at a time. My desired outcome is one item to be checked at a time and for that item to remain checked even if the app is closed. here is where I am implementing the checkbox

        options1 = new FirebaseRecyclerOptions.Builder<contact>().setQuery(databaseReference1,contact.class).build();
        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());
                holder.chk.setTag(holder.getAdapterPosition());
                holder.chk.setChecked(position == lastCheckedPos);
                holder.chk.setOnCheckedChangeListener(null);
                if(holder.getAdapterPosition() == lastCheckedPos){
                    holder.chk.setChecked(true);
                }else{
                    holder.chk.setChecked(false);
                }
                holder.chk.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
                    @Override
                    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                        if(isChecked){
                            holder.chk.setChecked(true);
                        }else {
                            holder.chk.setChecked(false);
                        }
                    }
                });
                holder.chk.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        for(int i =0; i <adapter1.getItemCount(); i++){
                            if(adapter1.getItem(i).isSelected(true)){
                                number = ((TextView) contactsView.findViewHolderForAdapterPosition(position).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();
                            }
                        }
                    }
                });
            }

I am using a firebase recyclerAdapter to pull the data down from firebase. This is my model class

public class contact {
    String contactName;
    String contactPhone;
    private boolean isSelected;

    public contact() {
    }

    public contact(String contactName, String contactPhone) {
        this.contactName = contactName;
        this.contactPhone = contactPhone;
    }

    public String getContactName() {
        return contactName;
    }

    public void setContactName(String contactName) {
        this.contactName = contactName;
    }

    public String getContactPhone() {
        return contactPhone;
    }

    public void setContactPhone(String contactPhone) {
        this.contactPhone = contactPhone;
    }

    public boolean isSelected(boolean selected){
        return isSelected= selected;
    }
}

And this is my 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);
    }
}

I've tried a number of things but a lot of the examples that I've seen dont seem to help. Any suggestions.