Is it possible to choose which overridden method to use in Java?

Question

I have a java.sql.Date instance and I want to use the java.util.Date.toInstant() method instead of java.sql.Date.toInstant(), for example, is it possible?

Answer 1

java.time and JDBC 4.2

I recommend that you stick to java.time, the modern Java date and time API to which Instant belongs and forget about the two Date classes mentioned in the question. They are both poorly designed and both long outdated. Since JDBC 4.2 we can retrieve java.time types from a ResultSet.

I am assuming that your SQL query is returning SQL datatype date.

    PreparedStatement stmt = yourDatabaseConnection
            .prepareStatement("select your_date_column from your_table;");
    ResultSet rs = stmt.executeQuery();
    while (rs.next()) {
        LocalDate date = rs.getObject("your_date_column", LocalDate.class);
        // do something with date
    }

You want to convert the date to an Instant? That conversion doesn’t readily make sense. An SQL date usually is a calendar date defined by year, month and day of month, without time zone. An Instant is a point in time, a completely different beast. If you can decide on a time of day and a time zone to use, a conversion is possible. One option is:

        Instant inst = date.atStartOfDay(ZoneId.systemDefault()).toInstant();

If you cannot avoid getting a java.sql.Date

Having a java.sql.Date, again we need a time of day and a time zone if we want to convert to Instant. There is a reason why java.sql.Date.toInstant() unconditionally throws UnsupportedOperationException (which I guess caused you to ask the question). I would convert the Date to a LocalDate and then proceed as before:

    java.sql.Date oldfashionedSqlDate = getFromSomewhere();
    Instant inst = oldfashionedSqlDate.toLocalDate()
            .atStartOfDay(ZoneId.systemDefault())
            .toInstant();

A shorter but more low-level and cryptic alternative is:

    Instant inst = Instant.ofEpochMilli(oldfashionedSqlDate.getTime());

The two ways may not always give the same result, which should be your first guidance for choosing. In case the Date contrary to the specification holds a time of day other than the start of day, the latter method will give you that time, whereas the former will give you the start of the day as the code says. BTW the latter is what java.util.Date.toInstant() does.

In general is it possible to circumvent the method implementation in the subclass and call the superclass method directly?

Is it possible to choose which overridden method to use in Java?

No, that is not possible in the language. Java doesn’t offer any syntax for such a trick. Holger’s comment under this answer seems to suggest that it is possible through reflection under some circumstances such as you being able to open the java.base module. See the last link at the bottom for more details.

Links

• Related question: Insert & fetch java.time.LocalDate objects to/from an SQL database such as H2
• Oracle tutorial: Date Time explaining how to use java.time.
• Related question: I want to print hi GrandFather;but it seems to print hi father

Answer 2

To answer your question as asked:

It is not possible for your code that uses a java.sql.Date to call an method in java.util.Date which java.sql.Date overrides.

That would break data abstraction, and the Java language doesn't allow it. You can't even do it reflectively.

(As other answers and comments indicate, there are alternative approaches that avoid this problem. Some more practical than others¹.)

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^{1 - The idea of creating a custom subtype of java.sql.Date is not practical. You don't want to be modifying JDBC drivers to try to solve this.}