When You Need a Comparator

Posted 9 January 2015. 3 min read.

Introduction

Many of the XPages Managed Bean demonstrations point to your ability to populate an xp:comboBox with a custom defined List of SelectItems. One thing that seems to happen to me is that I wind up having to re-sort such lists to work off of their Label, as opposed to their value; so as to look sorted, at least to human eyes.

A Brief ComboBox Anatomy Lesson

An xp:comboBox lets us build out a list (preferably somewhat short) of values with their labels, which are selected from a "drop down" like interface. More specifically, from MDN,

The HTML select (<select>) element represents a control that presents a menu of options. The options within the menu are represented by <option> elements, which can be grouped by <optgroup> elements. Options can be pre-selected for the user.

But you're here for the code. Here's an incredibly simple select tag implemented with three options. If you switch to the HTML pane, you'll see that the value (which is what can be data bound for value in the xp:comboBox control) is 1, 2, or 3 while the labels are their English equivalent of One, Two, or Three. In classic Notes, we would achieve this by the usual list (line separated) by passing in sets of Label | Value, separated by the pipe character. You can still do this in XPages, but if you're defining the source for one in a bean, you'll want to build out your List<SelectItem>. My sample class below shows this, but the meat and potatoes here is the Comparator.

A Comparator

Enter java.util.Comparator. It's a member of the Collections Framework, making it ideal for sorting Collections (which a List is). So, to begin, we'll define a class (you can nest it in another class, as I have, a stand-alone class, or a member of another, utility class). This class contains a single, public compare method, which returns an int. It returns an int, as that's what's returned by the compareToIgnoreCase method of java.lang.String. All the compare method is doing is comparing whether the first string is before or after the second string.

Code

Here's my super simple sample bean, with the selectOptionsList being read-only (no setter method) as it's just the selectedOption being what the value to be stored is.

	package com.eric.test;

	import java.io.Serializable;
	import java.util.ArrayList;
	import java.util.Collections;
	import java.util.Comparator;

	import javax.faces.model.SelectItem;

	public class SampleComparatorUse implements Serializable {

	private static final long serialVersionUID = 1L;

	private String selectedOption;
	private List<SelectItem> selectOptionsList;

	public SampleComparatorUse() {}

	/**
	* Custom Comparator, for use with sorting (ascending) a List<SelectItem>.
	*/
	private static class LabelAscComparator implements Comparator<SelectItem> {
	//uses a one-off cmoparison which returns comparison boolean, as int
	public int compare(SelectItem s1, SelectItem s2) {
	//you can also do a case sensitive via s1.getLabel().compareTo(s2.getLabel())
	return s1.getLabel().compareToIgnoreCase(s2.getLabel());
	}
	}

	/**
	* Getter for Combo Box options, sorted alphabetically ascending
	* by the label. Read-only, as it's a computed value, so no setter.
	*/
	public List<SelectItem> getSelectOptionsList() {

	if( this.selectOptionsList == null ) {
	List<SelectItem> options = new ArrayList<SelectItem>();
	//normally I compute this by pulling in values from another source, iterated
	//these are statically added for demonstrative purposes
	options.add(new SelectItem( "value3", "label3" ));
	options.add(new SelectItem( "value1", "label1" ));
	options.add(new SelectItem( "value2", "label2" ));

	//auto-magic sorting! otherwise the order would be label3, label1, label2
	//results, based on the label, in label1, label2, label3
	Collections.sort( options, new LabelAscComparator() );

	selectOptionsList = options;
	}

	return selectOptionsList;
	}

	/**
	* @param selectedOption String value being set via the EL binding;
	* this is the data field for what has been selected, standard setter.
	*/
	public void setSelectedOption( String selectedOption ) {
	this.selectedOption = selectedOption;
	}

	/**
	* Standard getter for the selectedOption property of the bean.
	*/
	public String getSelectedOption() {
	return this.selectedOption;
	}

	}

view raw SampleComparatorUse.java hosted with ❤ by GitHub

The XPage control implementation is a standard xp:comboBox implemented with the value and select items (options) bound via EL. The value which the user selects is bound to the bean's property of selectedOption while the list of SelectItems (options list, with both value and labels populated) is the selectOptionsList property.

{% include gist.html id="34bc3a534c7e44ff5617" file="SampleSelectItemBean.xsp.xml" %}

Here's the full gist link, with class, XPage, and faces-config (in case anyone is looking for how my bean is registered).

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