Anonymous class definition based on interface... maybe?

Question

I saw this Java snippet in the book Spring in Action, but I'm not familiar with the language construct.

new RowMapper() {
  public Object mapRow() throws SQLException, DataAccessException {
    Motorist motorist = new Motorist();
    motorist.setId(rs.getInt(1));
    motorist.setEmail(rs.getString(2));
    motorist.setPassword(rs.getString(3));
    motorist.setFirstName(rs.getString(4));
    motorist.setLastName(rs.getString(5));
    return motorist;
  }
}

According the Spring documentation, RowMapper is an interface. It looks to me like an anonymous class definition based on the RowMapper interface. The new keyword is a little confusing, making me wonder if this also creates one instance of the anonymous class. I would guess yes, because if the class has no name, how will you ever create an instance after the line that defines it?

Can anyone confirm my guesses that:

• this is an anonymous class definition based on the RowMapper interface, and
• it creates a single instance of that class?

Answer 1

This is an anonymous class definition based on the RowMapper interface

That's precisely what it is.

It creates a single instance of that class?

Yep. That's correct.

Answer 2

That code is implementing the interface in an anonymous way.

The syntax would be similar to:

Runnable runnable = new Runnable() {
    public void run() {
    }
};

Note the semicolon at the end of the declaration. Here the runnable object, though holds the reference to the Runnable interface actually contains the implemented object. That's runtime polymorphism for you!