Monday, 5 December 2011

Paginated Lists

Extending Java List to create paginated lists for JSTL jsps

JSTL often need to display lists of Java Beans.

The Pageable interface can be used to display these lists with pagination:

package com.brewer.list.util;

import java.util.List;

/**
 * An extension of the {@link java.util.List} interface for use in JSPs,
 * to allow display of a paginated list.
 *
 * For example, the JSTL construct <c:forEach> can be populated with this type,
 * to allow simple instantiation of the necessary variables.
 *
 * @author gavin (gavcos@gmail.com).
 */
public interface Pageable<E> extends List<E> {

    /**
     * The number of pages in the list
     */
    public int getNumberOfPages();

    /**
     * Returns the currently selected page.
     *
     * @return int  The current page.
     */
    public int getCurrentPage();

    /**
     * Sets the current page.
     * On instantiation, this should be set to 1.
     *
     * @param   page    The page to set.
     */
    public void setCurrentPage(int page);

    /**
     * Sets the page size.
     *
     * @param   pageSize    The page size to set.
     */
    public void setPageSize(int pageSize);

    /**
     * Returns the current page size.
     *
     * @param int   The page size.
     */
    public int getPageSize();

    /**
     * Returns the index the first row of the current page.
     *
     * @return  int The index.
     */
    public int getFirstIndexOfCurrentPage();

    /**
     * Returns the index of the last row of the current page.
     *
     * @return  int The index.
     */
    public int getLastIndexOfCurrentPage();
}

The PagedList class implements the interface:

package com.ecomm.brewer.util.list;

import java.util.ArrayList;
import java.util.Collection;

/**
* @see {@link nsi.fonctionnel.partnens.technique.util.Pageable}.
* @author p127001
*/
public class PagedList<E> extends ArrayList<E> implements Pageable<E> {

    private static final long serialVersionUID = 3469596120762185396L;
    /** The page size */
    private int pageSize;
    /** Default page */
    private static final int DEFAULT_PAGE = 1;
    /** Default page size */
    private static final int DEFAULT_PAGE_SIZE = 20;
    /** The currently selected page, initially the first one. */
    private int currentPage = DEFAULT_PAGE;

    /** Default constructor, initialises page size to default. */
    public PagedList() {
        super();
        this.pageSize = DEFAULT_PAGE_SIZE;
    }

    /**
     * Calls super class and initialises page size to default.
     * @param initialCapacity   The initial size of the list.
     */
    public PagedList(int initialCapacity) {
        super(initialCapacity);
        this.pageSize = DEFAULT_PAGE_SIZE;
    }

    /**
     * Initialises the list with an initial capacity and a page size.
     * @param initialCapacity   The initial size of the list.
     * @param pageSize          The page size to set.
     */
    public PagedList(int initialCapacity, int pageSize) {
        super(initialCapacity);
        this.pageSize = pageSize;
    }

    /**
     * Calls super class and initialises page size to default.
     * @param c
     */
    public PagedList(Collection<E> c) {
        super(c);
        this.pageSize = DEFAULT_PAGE_SIZE;
    }

    /**
     * Returns the current page.
     *
     * @return int   The current page.
     */
    public int getCurrentPage() {
        return this.currentPage;
    }

    /**
     * Sets the current page.
     *
     * @param   int     The page to set as current.
     */
    public void setCurrentPage(int page) {
        this.currentPage = page;
    }

    /**
     * Sets the page size.
     *
     * @param   pageSize    The page size to set.
     */
    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    /**
     * Returns the current page size.
     *
     * @param int   The page size.
     */
    public int getPageSize() {
        return this.pageSize;
    }

    /**
     * Returns the number of pages in the list.
     * If the list is empty, returns zero.
     *
     * @return  int     The number of pages, based on the size of the list and the page size.
     */
    public int getNumberOfPages() {
        if (this.size() == 0 || this.pageSize == 0) {
            return 0;
        } else {
            int iDiv =  this.size() / this.pageSize;
            int iModulo = this.size() % this.pageSize;
            return iModulo == 0 ? iDiv : ++iDiv;
        }
    }

    /**
     * Returns the index of the first row of the current page.
     *
     * @return  int     The index of the current page's first element.
     */
    public int getFirstIndexOfCurrentPage() {
        if (getCurrentPage() < 2) {
            return 0;
        } else {
            return (getCurrentPage() - 1) * this.pageSize;
        }
    }

    /**
     * Returns the index of the last row of the current page.
     *
     * @return  int     The index of the current page's last element.
     */
    public int getLastIndexOfCurrentPage() {
        return getFirstIndexOfCurrentPage() + (this.pageSize - 1);
    }
} 

In the JSP, use a JSTL forEach tag to display the list:

<c:set var="noPages">
    <%= getList().getNumberOfPages() %> 
</c:set>
<c:set var="currentPage">
    <% getList().getCurrentPage() %> 
</c:set>
<c:set var="startIndex">
    <%= getList().getFirstIndexOfCurrentPage() %> 
</c:set>
<c:set var="endIndex">
    <% getList().getLastIndexOfCurrentPage() %> 
</c:set> 
<c:if test="${not empty people}">
    <c:if test="${noPages ne '1'}">
        <tr>
            <td>${currentPage}&nbsp;/&nbsp;${noPages}</td> 
        </tr>
        <c:forEach var="person" items="people" begin="startIndex" end="endIndex" varStatus="status">
            <tr>
                <td>${person.name}</td>
                <td>${person.firstName}</td>
            </tr> 
        </c:forEach>


        <c:if test="${pageNo ne '1'}">
            <tr>
                <td>
                <c:forEach var="i"  begin="0" end="noPages"> 
                    <c:choose> 
                    <c:when test="${currentPage eq i}">
                    </c:when>
                    <c:otherwise>
                        <a href="select-page-link">
                    </a>
                    </c:otherwise>
                    </c:choose>
                </c:forEach> 
                </td>
            </tr>
        </c:if> 
   </c:if>
</c:if> 

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