JButton btnSaveReceipt = new JButton("Save receipt");
        btnSaveReceipt.addActionListener(e -> saveReceipt());
        btnSaveReceipt.setAlignmentX(Component.CENTER_ALIGNMENT);
        add(btnSaveReceipt);

private void saveReceipt(){
        BufferedWriter bw = null;
        try {
            bw = new BufferedWriter(new FileWriter(new SimpleDateFormat("yyyy-MM-dd-HHmmss").format(receipt.getDate())+ ".txt"));
            for (ReceiptItem item: receipt.getItems()){
                bw.write(item.getProduct().getName());
                bw.write("   ");
                bw.write(String.valueOf(item.getQuantity()));
                bw.write("   ");
                bw.write(String.valueOf(item.getProduct().getUnitPrice()));
                bw.write(item.getProduct().getUnit());
                bw.write("   ");
                bw.write(String.valueOf(item.getProduct().getUnitPrice()*item.getQuantity()+ "Kc"));
                bw.write("\n");
            }
            bw.write(String.valueOf(receipt.getTotalPrice())+"Kc");
        }catch (IOException e){
            e.printStackTrace();
        }finally {
            if (bw != null){
                try {
                    bw.close();
                }catch (IOException e){
                    e.printStackTrace();
                }
            }
        }
    }