• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Paul Clapham
  • Ron McLeod
  • Jeanne Boyarsky
  • Tim Cooke
Sheriffs:
  • Liutauras Vilda
  • paul wheaton
  • Henry Wong
Saloon Keepers:
  • Tim Moores
  • Tim Holloway
  • Stephan van Hulst
  • Carey Brown
  • Frits Walraven
Bartenders:
  • Piet Souris
  • Himai Minh

right way to implement compareTo() method?

 
Ranch Hand
Posts: 342
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
the method below is sorting (descending) the student objects by their id (integer type).

I am not sure if I implemented it correctly (good design? etc).

can we return any value as long as they are positive, negative and zero? how the Collections.sort() implementation's codes use/understand those values?

 
Sheriff
Posts: 14691
16
Eclipse IDE VI Editor Ubuntu
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Any number will do. The calling method does not look at the value, but at the sign, positive, negative or zero.
 
Ranch Hand
Posts: 809
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
  • -1 means left less than right
  • 0 means both equal
  • +1 means left greater than right



  • Your code should be like this..



    From javadoc...

    compareTo

    public int compareTo(Object o)

  • Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

  • In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return one of -1, 0, or 1 according to whether the value of expression is negative, zero or positive. The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception iff y.compareTo(x) throws an exception.)

    The implementor must also ensure that the relation is transitive: (x.compareTo(y)>0 && y.compareTo(z)>0) implies x.compareTo(z)>0.

    Finally, the implementer must ensure that x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for all z.

    It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."


    Parameters:
    o - the Object to be compared.
    Returns:
    a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

    Naseem
    [ July 23, 2006: Message edited by: Naseem Khan ]
     
    What does a metric clock look like? I bet it is nothing like this tiny ad:
    Free, earth friendly heat - from the CodeRanch trailboss
    https://www.kickstarter.com/projects/paulwheaton/free-heat
    reply
      Bookmark Topic Watch Topic
    • New Topic