Item 12 - Consider implementing Comparable

From Effective Java 2/e by Joshua Bloch

Comparable interface

public interface Comparable<T> {
   int compareTo(T t);
}

By implementing Comparable, you allow your class to interoperate with all of the many generic algorithms and collection implementations that depend on this interface

public class WordList {
   public static void main(String[] args) {
      Set<String> s = new TreeSet<String>();
      Collections.addAll(s, args);
      System.out.println(s);
   }
}

General Contract

• Symmetric sgn(x.compareTo(y)) == -sgn(y.compare- To(x))
• Transitive (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0
• compareTo(y) == 0implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z))
• It is strongly recommended, but not strictly required, that (x.compareTo(y) == 0) == (x.equals(y))

compareTo method Example

public int compareTo(PhoneNumber pn) {
   // Compare area codes
   if (areaCode < pn.areaCode)
      return -1;
   if (areaCode > pn.areaCode)
      return 1;
   // Area codes are equal, compare prefixes
   if (prefix < pn.prefix)
      return -1;
   if (prefix > pn.prefix)
      return 1;
   // Area codes and prefixes are equal, compare line numbers
   if (lineNumber < pn.lineNumber)
      return -1;
   if (lineNumber > pn.lineNumber)
      return 1;
   return 0;  // All fields are equal
}

Posted by The Finest Artist