What would be the best way to display a localDateTime in the correct format?

I've been building a small banking app, and have run into an issue where the localDateTime of a transaction is displayed as the full format "2020-10-06T11:54:00.517734".

This is obviously pretty not great to look at so I've tried a few different methods of formatting it however most end up in null pointer exceptions.

Here the data gets added to models from the database:

for (Transaction transaction : allTransactions) {
    TransactionInfo transactionInfo = new TransactionInfo();

    BankAccount bankAccount;

    if (transaction.getDebitAccount() == selectedBankAccount) {
        bankAccount = transaction.getCreditAccount();
        transactionInfo.setAmount(transaction.getAmount().negate());
    } else {
        bankAccount = transaction.getDebitAccount();
        transactionInfo.setAmount(transaction.getAmount());
    }
    
    transactionInfo.setDateTime(transaction.getDateTime());
    transactionInfo.setName(bankAccount.getAccountName());
    transactionInfo.setIban(bankAccount.getIban());
    transactionInfo.setDescription(transaction.getDescription());
    transactionInfo.setTransactionId(transaction.getId());

    transactions.add(transactionInfo);
}
modelAndView.addObject("transactions", transactions);
... 

So I've tried using .format( DateTimeFormatter.ofPattern( "HH:mm:ss" ) ) at transactionInfo.setDateTime(transaction.getDateTime()).

However this requires a localDateTime data type. when I try to change this in the object class I keep getting null pointer exceptions and I don't like the idea of representing the dateTime as a String.

this is the HMTL page:

<table class="transaction-table">
                    <tr>
                        <th>Afzender</th>
                        <th>Tegenrekening</th>
                        <th>Bedrag</th>
                        <th>Datum</th>
                        <th>Beschrijving</th>
                    </tr>

                    <tr th:each="transaction : ${transactions}">
                        <td th:text="${transaction.name}"></td>
                        <td th:text="${transaction.iban}"></td>
                        <td>€<span th:text="${transaction.amount}"></span></td>
                        <td th:text="${transaction.dateTime}"></td>
                        <td th:text="${transaction.description}"></td>
                    </tr>
                </table>

Should I try to make these formats in the HTML file instead? or is there a better method to do it in Java?

It should work. If you are getting NPE, you probably call some method on a reference with no actual object behind it (e.g. some getSomething() returns null and you try to do smth. on it).

Here are a few examples:

LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE); // 2020-10-06
LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME); 
LocalDateTime.now().format(DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss")); // 2020/10/06 15:20:03

There are some other useful methods, you can consider:

LocalDateTime.now().toLocalDate(); // get date only
LocalDateTime.now().toLocalTime(); // get time only
LocalDateTime.now().withNano(0); // prints something like 2020-10-06T15:26:58 (no nanos which usually we don't need :) )

Try this:

   SimpleDateFormat format = new SimpleDateFormat( "yyyy-MM-dd'T'HH:mm:ss.SSS",Locale.US);
       format.setTimeZone(TimeZone.getTimeZone("UTC"));
       try{
       Date date = format.parse("2020-10-06T11:54:00.517734");
       System.out.println(date);
       }catch(Exception ex){
           
       }