Why does java.sql.Timestamp extend java.util.Date

When I saw this I got baffled:

  public class Timestamp extends java.util.Date {
    //...
    public boolean equals(java.lang.Object ts) {
      if (ts instanceof Timestamp) {
        return this.equals((Timestamp)ts);
      } else {
        return false;
      }
    }
    public int hashCode() {
        return super.hashCode();
    }

It is indeed documented with a bold Note (see https://docs.oracle.com/javase/7/docs/api/java/sql/Timestamp.html)

What could be the cause to make such a, to me, very bad decision? Why not call super.equals(this) when compared to a java.util.Date object to make the equal comparison symmetrical?

Solution

Instead, use java.time

As others stated:

This class inheritance is poor design, a flawed hack.
Now moot, as this class is now legacy, supplanted by java.time.Instant.

Avoid all the old legacy date-time classes found outside the java.time package.

For databases, use a driver compliant with JDBC 4.2 or later to directly exchange java.time objects with your database. You can forget all about java.sql.Timestamp.

Storing:

Instant instant = Instant.now() ;
myPreparedStatement.setObject( … , instant ) ;

Retrieving:

Instant instant = myResultSet.getObject( … , Instant.class ) ;

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

Java SE 8, Java SE 9, and later
- Built-in.
- Part of the standard Java API with a bundled implementation.
- Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
- Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
- The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
- See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.