DateTimeFormatter with multiple pattern not working

Both of these patterns working fine when tested separately but they are not working together in a single ofPattern { "[]" + "[]"} statement.

Please guide what am I missing here

import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;

public class SampleProgram {
    
    private static final DateTimeFormatter dateTimeFormats = DateTimeFormatter.ofPattern(
            "[yyyy-MM-dd'T'HH:mm:ss.SSS]" + //: Year, month, day, 'T' separator, hour, minute, second, and milliseconds (e.g., 2023-05-21T12:34:56.789)
            "[yyyy-MM-dd'T'HH:mm:ss.SSSSSS]"  //: Year, month, day, 'T' separator, hour, minute, second, and microseconds (e.g., 2023-05-21T12:34:56.123456)
    );

    private static final String[] inputArray = new String[]{
        "2021-01-30T23:45:00.123",
        "2023-08-15T12:34:56.789123",
    };

    
    public static void main(String[] args) {
        
        for (var input : inputArray) {
            var ignore = LocalDateTime.parse(input, dateTimeFormats);
        }
        System.out.println("No exception occured!");
    }
}

Output:

Exception in thread "main" java.time.format.DateTimeParseException: Text '2023-08-15T12:34:56.789123' could not be parsed, unparsed text found at index 23
at java.base/java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:2049)
    at java.base/java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1948)at java.base/java.time.LocalDateTime.parse(LocalDateTime.java:492)
    at SampleProgram.main(SampleProgram.java:25)

You do not need any DateTimeFormatter because both of your date-time strings are in ISO 8601 format which is the default format used by java.time types.

public static void main(String[] args) {
    for (var input : inputArray) {
        var ignore = LocalDateTime.parse(input);
    }
    System.out.println("No exception occured!");
}

Output:

No exception occurred!

In case, you want to use your DateTimeFormatter, change it to make just the fraction-of-second part optional.

DateTimeFormatter dateTimeFormats = DateTimeFormatter.ofPattern(
                    "yyyy-MM-dd'T'HH:mm:ss[.[SSSSSS][SSS]]");

Online Demo

Learn more about the modern Date-Time API from Trail: Date Time.