"Thufir" wrote ...

...

Hear!  Hear!

<http://java.sun.com/javase/6/docs/api/java/util/Comparator.html>

Example: suppose you have an entity class (simplified):

public class Person
{
 private String name;
 private Double age;
 public void setName( String n ) { name = n; }
 public String getName() { return name; }
 public void setAge( Double a ) { age = a; }
 public Double getAge() { return age; }
}

Sometimes you want to sort by name, sometimes by age, sometimes by first one
then the other.

Here's a sample Comparator for age:

 Comparator <Person> personAgeComparator = new Comparator <Person> ()
  {
    public int compare( Person lef, Person rig )
    {
      if ( lef == null ) { return (rig == null? 0 : -1); }
      if ( rig == null ) { return 1; }
      if ( lef.getAge() == null ) { return (rig.getAge() == null? 0 : -1 }
      if ( rig.getAge() == null ) { return 1; }
      return lef.getAge().compareTo( rig.getAge() );
    }
  }

A more realistic example would use birthdates, which don't change as
continuously as age does.

A name Comparator would be similar, but use a different implementation of the
compare() method, one based on getName() instead of getAge()

One could easily write a Comparator that sorts by age first, then name (or
vice versa).

If a Comparator were to be widely re-used, you'd consider making it a
top-level class:

 class PersonAgeComparator implements Comparator <Person>
 {
    public int compare( Person lef, Person rig )
    {
     // etc.
    }
 }