The Comparable Interface Type

• Collections  has static sort method:
  

  ArrayList a = . . .
  Collections.sort(a);

• Objects in list must implement the Comparable interface type
  

  public interface Comparable
  {
     int compareTo(Object other);
  }
  

• object1.compareTo(object2) returns
• Negative number if object1 less than object2
• 0 if objects identical
• Positive number if object1 greater than object2
• sort method compares and rearranges elements
  if (object1.compareTo(object2) > 0) . . .
• String class implements Comparable interface type: lexicographic (dictionary) order
• Country class: compare countries by area
  Country.java
  CountrySortTest.java
  

The Comparator interface type

• How can we sort countries by name?
• Can't implement Comparable twice!
• Comparator interface type gives added flexibility
  

  public interface Comparator
  {
     int compare(Object object1, Object object2);
  }

• Pass comparator object to sort:
  

  Collections.sort(list, comp);
  

• CountryComparatorByName.java
  ComparatorTest.java
• Comparator object is a function object
• This particular comparator object has no state
• State can be useful, e.g. flag to sort in ascending or descending order

Anonymous Classes

• No need to name objects that are used only once
  

  Collections.sort(countries,
     new CountryComparatorByName());

• No need to name classes that are used only once
  

  Comparator comp = new
     Comparator()
     {
        public int compare(Object obj1, Object obj2)
        {
           Country country1 = (Country)obj1;
           Country country2 = (Country)obj2;
           return country1.getName().compareTo(country2.getName());
        }
     };
  

Anonymous Classes

• anonymous new expression:
• defines anonymous class that implements Comparator
• defines compare method of that class
• constructs one object of that class
• Cryptic syntax for very useful feature

Anonymous Classes

• Commonly used in factory methods:
  public static Comparator comparatorByName()
  {
     return new Comparator()
     {
        public int compare(Object o1, Object o2) { . . . }
     };
  }
• Collections.sort(a, Country.comparatorByName());
• Neat arrangement if multiple comparators make sense
  (by name, by area, ...)
  

See: ComparatorTest2.java

Maintained by John Loomis, updated Thu Feb 22 22:30:04 2007