EASILY REVERSING A COLLECTION SORT
If you're sorting a Collection and you want to provide an option to 
sort it in reverse order, try using a ReverseComparator.

When coding their first Comparators, Java beginners often add some form 
of flag to specify that the sort should be performed in reverse. In 
time, they discover Collections.reverse(List) and start to use this 
instead. However, a single line of implementation can help avoid both 
of these poor designs.

package com.generationjava.compare; 
 
import java.util.Comparator; 
 
public class ReverseComparator implements Comparator { 
 
    private Comparator comparator; 
 
    public ReverseComparator(Comparator comparator) { 
        this.comparator = comparator; 
    } 
 
    public int compare(Object o1, Object o2) { 
        // invert the wrapped comparator's result 
        return -1*comparator.compare(o1,o2); 
    } 
 
}

This is a simple but remarkably powerful implementation. For example, 
take a UrlComparator, which sorts java.net.URLs in a particular order, 
grouping ftp://www.generationjava.com/ with http://generationjava.com  
and not with all the other ftps. To sort using this, we would do:


    List listUrls = ....;  // contains java.net.URL 
    Collections.sort( listUrls, new UrlComparator() );

To reverse this list, we would do:

    List listUrls = ....;  // contains java.net.URL 
    Collections.sort( listUrls, new ReverseComparator( new 
UrlComparator() ) );

Using this snippet of code can help eradicate constructors like:

    public SomeComparator(boolean reverse) {

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