blackboard.base
Class GenericFieldComparator

java.lang.Object
  blackboard.base.BaseComparator
      blackboard.base.GenericFieldComparator

All Implemented Interfaces:

java.util.Comparator

public class GenericFieldComparator
extends BaseComparator
implements java.util.Comparator

Comparator implementation that allows objects in a list to be compared based on the value of an attribute that they have in common. For example, a list of announcements could be ordered by their creation dates, returned by announcement.getCreationDate().

Note that this class is implemented using reflection.

 Example Use:

     The following code separates an announcement list into two sections.  First 
     the permanent announcements, and then the non-permanent announcements.  
     Each of these sections is sorted in order of date created.  Finally, those
     announcements with the same isPermanent value and same created date are 
     sorted in alphabetical order by their title.

     AnnouncementList list = loadAnnouncementList();
     GenericFieldComparator comparator = new GenericFieldComparator( BaseComparator.DESCENDING, "getIsPermanent", Announcement.class );
     comparator.appendSecondaryComparator( new GenericFieldComparator( BaseComparator.ASCENDING, "getCreatedDate", Announcement.class ) );
     comparator.appendSecondaryComparator( new GenericFieldComparator( BaseComparator.ASCENDING, "getTitle", Announcement.class ) );
     Collections.sort( list, comparator );
 

Since:

Bb 6.0

Version:

$Revision: #1 $ $Date: 2005/06/27 $

Field Summary

Fields inherited from class blackboard.base.BaseComparator

ASCENDING, DESCENDING

Constructor Summary

GenericFieldComparator(boolean ascending, java.lang.String fieldAccessorMethod, java.lang.Class targetClass)
Constructor.

GenericFieldComparator(java.lang.String fieldAccessorMethod, java.lang.Class targetClass)
Constructor.

Method Summary

protected int	compareFields(java.lang.Object field1, java.lang.Object field2) Compare method allowing explicit support for comparing objects that don't implement Comparable.
protected int	doCompare(java.lang.Object o1, java.lang.Object o2) Implementation of BaseComparator.doCompare().

Methods inherited from class blackboard.base.BaseComparator

appendSecondaryComparator, compare, convertToBaseComparator, isAscending, setAscending

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Comparator

compare, equals

Constructor Detail

GenericFieldComparator

public GenericFieldComparator(java.lang.String fieldAccessorMethod,
                              java.lang.Class targetClass)
                       throws java.lang.IllegalArgumentException,
                              java.lang.SecurityException

Constructor. Default to ASCENDING order.

Parameters:

fieldAccessMethod - the case-sensitive name of the accessor method defined in the target class that will be used to get the attribute used to compare objects.

targetClass - the class defining the requested accessor method. This can be a base class of the objects in a list of an interface that they all implement.

Throws:

java.lang.IllegalArgumentException - if the target class does not contain a method called fieldAccessorMethod.

java.lang.SecurityException - if the caller does not have access to the specified method in the specified class.

GenericFieldComparator

public GenericFieldComparator(boolean ascending,
                              java.lang.String fieldAccessorMethod,
                              java.lang.Class targetClass)
                       throws java.lang.IllegalArgumentException,
                              java.lang.SecurityException

Constructor.

Parameters:

ascending - the ascending/descending mode for this comparator as defined in BaseComparator.

fieldAccessMethod - the case-sensitive name of the accessor method defined in the target class that will be used to get the attribute used to compare objects.

targetClass - the class defining the requested accessor method. This can be a base class of the objects in a list of an interface that they all implement.

Throws:

java.lang.IllegalArgumentException - if the target class does not contain a method called fieldAccessorMethod.

java.lang.SecurityException - if the caller does not have access to the specified method in the specified class.

Method Detail

doCompare

protected int doCompare(java.lang.Object o1,
                        java.lang.Object o2)
                 throws java.lang.ClassCastException,
                        java.lang.IllegalArgumentException

Implementation of BaseComparator.doCompare().

Specified by:

doCompare in class BaseComparator

Parameters:

o1 - First object to compare

o2 - Second object to compare

Returns:

Comparison result, as defined by Comparable.compareTo()

Throws:

java.lang.ClassCastException - if the fields are not of the same type

java.lang.IllegalArgumentException - if the objects' fields are not a supported type, do not implement comparable, or the objects themselves do not match types

compareFields

protected int compareFields(java.lang.Object field1,
                            java.lang.Object field2)
                     throws java.lang.IllegalArgumentException

Compare method allowing explicit support for comparing objects that don't implement Comparable. Base implementation provides support for String, Comparable, Boolean, Calendar. Note that blackboard.persist.Id and blackboard.base.BbEnum already implement Comparable. Subclasses can override this method to add support for additional non- comparable types, but be sure to call super.compareFields() if the objects are not of one of the target types being supported.
Note that ascending order for booleans is defined as false before true. Calendars are compared by the Dates returned by their getTime() methods. BbEnum elements are sorted by their external string representation in alphabetical order.

Parameters:

field1 - First field to compare

field2 - Second field to compare

Returns:

Comparison result, as defined by Comparable.compareTo()

Throws:

java.lang.IllegalArgumentException - if the fields is not a supported type, does not implement comparable, or the fields do not match types