Hi everybody,
I recently needed to be able to sort a list of beans on many properties. So I
thought I would try and pick one of the collections comparators.
I had to to the sorting close to the sql way : be able to sort on n properties,
some ascending, some descending.
I haven't found what I'm looking for (:p), but I found some comparators I used
to do this : I used BeanComparator, NullComparator and ComparatorChain to
create a class : MultiPropertyBeanComparator.
Is there already something in one of the commons package that could be used
instead of it ?
If not, I'd be glad to contribute the small piece of code if wanted. It has
dependencies against commons-beanutils (BeanComparator, which is moving from
one package to another at the moment, no ?) and commons-lang
(StringUtils.isBlank()). I think some things might not satisfactory for
everybody, but hey, could still be improved without problems, that's not big
work :p.
Here it is :
=======================================
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import org.apache.commons.beanutils.BeanComparator;
import org.apache.commons.collections.ComparatorUtils;
import org.apache.commons.collections.comparators.NullComparator;
import org.apache.commons.collections.comparators.ReverseComparator;
import org.apache.commons.lang.StringUtils;
/**
* This comparator lets you sort a list using a list of properties. You can
specify ascending or
* descending order on these properties.
* <p>
* For example, if you want to sort with natural order a list of beans with
properties firstname,
* nickname and address, sorting the firstname descending, you can do it this
way:
* </p>
* <code>List l = ...</code>
* <code>...</code>
* <code>MultiPropertyBeanComparator multiPropBeanComp = new
MultiPropertyBeanComparator();</code>
* <code>multiPropBeanComp.append("firstname",
true).append("nickname").append("address");</code>
* <code>Collections.sort(l,multiPropBeanComp);</code>
*
* @author Baptiste MATHUS
*/
public class MultiPropertyBeanComparator implements Comparator, Serializable
{
private static final long serialVersionUID = -1431852774261001458L;
private List comparatorList = new ArrayList();
/**
* Use this method to add a comparator to the list.
*
* @param property
* the property on which to apply the given comparator.
* @param comparator
* the comparator to be added. If null, natural order will
be used.
* @param reverse
* <p>
* must be true if the given comparator must be used in
opposite order to sort. For
* example, if the comparator is designed to sort in
ascending order, put this
* parameter to <code>true</code> if you want descending
order.
* </p>
* <p>
* If the comparator is null, then the reversed natural
order is used.
* </p>
*/
public MultiPropertyBeanComparator append(String property, Comparator
comparator,
boolean reverse)
{
if (StringUtils.isBlank(property))
{
throw new IllegalArgumentException("The given property
is blank");
}
// If the comparator is null, then compare only on the given
property
// with a natural sort.
if (comparator == null)
{
comparator = new BeanComparator(property, new
NullComparator(false));
}
// Else : compare on the property, but with given comparator.
else
{
comparator = new BeanComparator(property, comparator);
}
// Here, the comparator cannot be null anymore, so reverse it if
// necessary.
if (reverse)
{
comparator = new ReverseComparator(comparator);
}
comparatorList.add(comparator);
return this;
}
public MultiPropertyBeanComparator append(String property, Comparator c)
{
return append(property, c, false);
}
public MultiPropertyBeanComparator append(String property)
{
return append(property, null, false);
}
public MultiPropertyBeanComparator append(String property, boolean
reverse)
{
return append(property, null, reverse);
}
/**
* Use this method to clear the
*/
public void clear()
{
comparatorList.clear();
}
/**
* Considered to be equal when all properties and comparators equal.
*
* @see java.lang.Object#equals(java.lang.Object)
* @overrides
*/
public boolean equals(Object obj)
{
MultiPropertyBeanComparator comp =
(MultiPropertyBeanComparator) obj;
if (this.comparatorList.size() != comp.comparatorList.size())
{
return false;
}
for (int i = 0; i < comparatorList.size(); ++i)
{
if
(!this.comparatorList.get(i).equals(comp.comparatorList.get(i)))
{
return false;
}
}
return true;
}
/**
* @see Comparator#compare(T, T)
* @overrides
*/
public int compare(Object arg0, Object arg1)
{
return getComparator().compare(arg0, arg1);
}
private Comparator getComparator()
{
return ComparatorUtils.chainedComparator(comparatorList);
}
}
=======================================
Thanks for the answers.
Baptiste
--
Baptiste MATHUS - [EMAIL PROTECTED]
Systèmes & Méthodes
MIPIH - Midi Pyrénées Informatique Hospitalière
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