java DTO中将日期类型转换为字符串格式，无需迭代

Question

I am trying to convert a date format to string format in dto class by having date type variable and string type variable which will hold string formatted fromdate variable, ie without using iteration for parsing and send it to ui as response

private LocalDateTime date_created;
private LocalDateTime date_updated;

private String dateCreated = Utility.getStringDate(date_created);    
private String dateUpdated = Utility.getStringDate(date_updated);

While trying like above im getting Null pointer exception , it may be like data is not set in date type variable and trying to convert to string

Answer 1

Indeed the field dateCreated were initialized once, when the date_created field still was null.

Make methods from them.

private String dateCreated() { return Utility.getStringDate(date_created); }
private String dateUpdated() { return Utility.getStringDate(date_updated); }

Answer 2

I don't know what would be wrong with formatting the date-time every time you need it formatted, need the string. But when you don't want to do that, cache the string. The setter for your date-time is a good place to do that, for example (not tested):

    public void setDateCreated(LocalDateTime dateCreated) {
        this.date_created = dateCreated;
        dateCreated = Utility.getStringDate(date_created);
    }

If the value may be null, you need to take that into account, of course.

Slightly more advanced, only format the string the first time you need to:

    public String getDateCreated() {
        if (dateCreated == null && date_created != null) {
            dateCreated = Utility.getStringDate(date_created);
        }
        return dateCreated;
    }

If the date-time can be changed, you may set the string to null in the setter so that a new string will be formatted next time one is asked for.

Answer 3

I did this in mongodb aggregation using $datetostring and straightway assign to string dateCreated variable "from":{ $dateToString: { format: "%Y-%m-%dT%H:%M:%S:%L"+"Z", date: "$from" } },

where from is dto variable name