Render multi value hash map with single key and multiple values in JSP

Question

I have the following MultiHashMap in the controller.java

MultiHashMap multimap= new MultiHashMap();

List<SampleList> list1 = (List<SampleList>) 
                         request.getPortletSession().getAttribute("list1");  
Iterator<SampleList> sampleListIterator= list1.iterator();

while(sampleListIterator.hasNext()){
    SampleList sampleList = sampleListIterator.next();
    List <SubList> subList =  sampleList.getsubList();
    Iterator<SubList> subListIterator = subList.iterator();

    while(subListIterator.hasNext() ){
        SubList subList2 = subListIterator.next();
        multimap.put(subList2.getCategorySubcategory(),subList2.getCost());
    }
}

In the jsp I have a table which displays the above hashmap

<table>
    <tbody>
        <c:foreach var="item" items="${multimap}">
            <tr>
                <th> ${item.key}</th>
                <c:foreach var="valueList" items=${item.value}>
                    <td> ${valueList}</td>
                </c:foreach>
            </tr>
        </c:foreach>
    </tbody>
</table>

I'm getting an error when I try to render the value from the controller.

Answer 1

Apache Commons MultiHashMap is deprecated , you should use instead MultiValueMap .

Deprecated . Class now available as MultiValueMap in map subpackage. This version is due to be removed in collections v4.0.

Both the MultiHashMap and MultiValueMap classes have a getCollection(Object key) method, that will return a Collection of values for that specific key :

 java.util.Collection getCollection(java.lang.Object key) 

Gets the collection mapped to the specified key.

You can iterate it in JSP as any other Collection, by using JSTL, OGNL, etc...

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EDIT

Surprinsingly, there is not even one example of this Class usage in JSTL nor OGNL in the whole Internet... I guess your question will become popular, +1 ;)

Since MultiValueMap (and the deprecated MultiHashMap) is a Map with a Collection of values for every key of the Map, it should be iterated in JSTL like this (untested, but presumably right) ( now TESTED on JSTL 1.1.2 in a Servlet 2.4 / JSP 2.0 Container ):

<c:forEach var="entry" items="${multiMap}">
    <br/>-> Key: <c:out value="${entry.key}"/>
    <br/>-> Values for this key:
    <c:forEach var="currentValue" items="${entry.value}">
        <br/>|---> value: <c:out value="${currentValue}"/>
    </c:forEach>
</c:forEach>

Populating it like this

multiMap = new MultiValueMap();
multiMap.put("key1", "value1-1");
multiMap.put("key1", "value1-2");
multiMap.put("key1", "value1-3");
multiMap.put("key2", "value2-1");
multiMap.put("key2", "value2-2");
multiMap.put("key2", "value2-3");
multiMap.put("key3", "value3-1");
multiMap.put("key3", "value3-2");
multiMap.put("key3", "value3-3");

The output is this (copied and pasted from my JSP):

-> Key: key1
-> Values for this key:
|---> value: value1-1
|---> value: value1-2
|---> value: value1-3
-> Key: key3
-> Values for this key:
|---> value: value3-1
|---> value: value3-2
|---> value: value3-3
-> Key: key2
-> Values for this key:
|---> value: value2-1
|---> value: value2-2
|---> value: value2-3 

Then it works perfectly (apart from the curious fact that key3 is coming up before key2); if you still have problems, they're not code-related, but in your config.

Please check out the JSTL tag wiki on SO , this nice article or the numerous answers to JSTL problems from BalusC here on SO.

---

As a final though, looking at the code you are using to create the MultiValueMap, I strongly suggest you to start building some custom object , as Object Oriented Programming suggests, to avoid excessive nesting of Lists and Maps, that could result in unreadable, unmaintainable code.

Answer 2

I haven't used MultiHashMaps before, but this is how I usually display my HashMap contents in a jsp page (without generics example):

Iterator iter = map.entrySet().iterator();
while(iter.hasNext()){
   Map.Entry entry = iter.next();
   Object key = entry.getKey();
   Object value = entry.getValue();

Answer 3

Using JSTL Core taglibrary: <%

Map<String, String> countryList = new HashMap<String, String>();
countryList.put("United States", "Washington DC");
countryList.put("India", "Delhi");
countryList.put("Germany", "Berlin");
countryList.put("France", "Paris");
countryList.put("Italy", "Rome");

request.setAttribute("capitalList", countryList);
 %>



<c:forEach var="country" items="${capitalList}">
    Country: ${country.key}  - Capital: ${country.value}
</c:forEach>

If you are using struts, then you can struts logic tag library to iterate and display the map in JSP:

 <logic:iterate name="students" id="nameObj" scope="session">
    <bean:write name="nameObj" property="key"/>
    <bean:write name="nameObj" property="value"/>
 </logic:iterate>