Presents your 
JAVA E-NEWSLETTER for December 9, 2002

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IMPLEMENT A BEANCOMPARATOR WITH JAKARTA COMMONS

Reflection is a powerful tool that can save time and effort. One way
that it can be helpful is through the use of a BeanComparator.

BeanComparators are generic comparators that can act upon any JavaBean.
With a good underlying bean reflection system, they can handle a bean
with bean properties, arrays, collections, and maps.

If we have a java.util.List of Java Beans of type Person--where Person
has an age that's an Integer, a name that's a String, and a country that's
a Country bean (which itself has a String name)--then we can sort it
anyway we want with a BeanComparator.

Some examples:

List people = ...; // list of Person objects
BeanComparator comp = new BeanComparator("age");
Collections.sort(list, comp);

BeanComparator comp = new BeanComparator("name");
Collections.sort(list, comp);

BeanComparator comp = new BeanComparator("country.name");
Collections.sort(list, comp);

There is no need to write a PersonComparator with lots of options for
which property to sort by. Instead, a single BeanComparator class takes
care of it all.

All the magic is handled by the underlying reflection API. The 
following
is an example of a simple BeanComparator built on top of the Jakarta
Commons BeanUtils project:

import java.util.Comparator;
import org.apache.commons.beanutils.PropertyUtils;
import org.apache.commons.collections.comparators.ComparableComparator;

public class BeanComparator implements Comparator {

    private String attribute;
    private Comparator comp = new ComparableComparator();

    public BeanComparator(String attrib) {
        this.attribute = attrib;
    }

    public int compare(Object o1, Object o2) {
        if(o1 == null) {
            return 1;
        } else
        if(o2 == null) {
            return -1;
        }
        
        try {
            Object ret1 = PropertyUtils.getProperty(o1, 
this.attribute);
            Object ret2 = PropertyUtils.getProperty(o2, 
this.attribute);

            return this.comp.compare(ret1, ret2);
        } catch(Exception e) {
            return 0;
        }
    }
}

The exception handling has been simplified, and it's assumed that the
bean property to be compared implements Comparable, via the use of the
ComparableComparator in the Jakarta Commons Collections project.

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