Jakarta Commons Cookbook

5.8. Creating a Priority Queue

Problem

You need to prioritize objects: removing higher priority objects from a Collection before lower priority objects.

Solution

Use a PriorityBuffer to hold objects to be prioritized. Objects will be removed from this Buffer according to a priority generated with a Comparator. Whenever the remove( ) method is called on this Buffer implementation, a PriorityBuffer uses the Comparator to sort its contents, returning the element of greatest priority. Using a PriorityBuffer without a Comparator causes the buffer to prioritize objects by natural order, casting each object to Comparable and comparing objects with the compareTo( ) method; all objects must implement the Comparable interface if no Comparator is supplied. The following example demonstrates the use of a PriorityBuffer without supplying a Comparator :

import java.util.*;
import org.apache.commons.collections.Buffer;
import org.apache.commons.collections.buffers.PriorityBuffer;

// Create a PriorityBuffer with no Comparator
Buffer priority = new PriortyBuffer( );
priority.add( new Long( 2 ) );
priority.add( new Long( 1 ) );
priority.add( new Long( 20 ) );
priority.add( new Long( 7 ) );
priority.add( new Long( 18 ) );
priority.add( new Long( 1 ) );

// Print the results in priority order
Iterator priorityIterator = priority.iterator( );
while( priorityIterator.hasNext( ) ) {
    Long value = (Long) priority.next( );
    System.out.prinltn( "Value: " + value );
}

The previous example removes values from ...