How to handle Maps on Visualforce page

Can anyone please help me to understand the below code. I am not able to understand how we are extracting value from map on visualforce page.

Actually i am trying to get the size of value list which is associated with respective key in map.

            <apex:outputPanel >
                <apex:repeat value="{!attributeWrapper.mapAllAttributesLabel}" var="M">
                    <div>
                        <div>{!attributeWrapper.mapLabels[M].Name} </div>
                        <div>
                            <ul>
                                <apex:repeat value="{!attributeWrapper.mapAllAttributesLabel[M]}" var="attribute">
                                    <li>
                                    <p>
                                        <apex:outputLink onclick="selectAttribute('{!attribute.attributeLabel.Label__c}','{!attribute.attributeLabel.Attribute__c}');return false;">
                                           <apex:outputLabel value="{!attribute.attributeLabel.Attribute__r.Name}"/>
                                        </apex:outputLink>
                                    </p>
                                    </li>
                                </apex:repeat>
                            </ul>
                        </div>
                    </div>
                </apex:repeat>
            </apex:outputPanel>



public with sharing class AttributeWrapper {
     public Map<String, List<AttributesLabelWrapper>> mapAllAttributesLabel{
      // some code to populate this map
    }
}

If i do:

{!attributeWrapper.mapAllAttributesLabel[M].size}

I get an error so how do i get the size of value list on VF page?

Answer

I tried to replicate your problem, and ended up trying different things, no luck doing it your way (confirmed that, got a visualforce error), no luck with <apex:param> but what I ended up doing was I created a second map, with the same keys, which basically just holds the size of the list in an Integer, and get those values.

Here is my code:

VisualForce Page

<apex:page controller="TestController">
    <apex:repeat value="{!testMap}" var="key">
        <apex:outputText value="{!listSizeMap[key]}" />
    </apex:repeat>
</apex:page>

Apex Controller

public with sharing class TestController 
{
    public Map<String, List<String>> testMap {get;set;}
    public Map<String, Integer> listSizeMap {get;set;}

    public TestController()
    {
        listSizeMap = new Map<String, Integer>();
        testMap = new Map<String, List<String>>();
        testMap.put('test1', new List<String>{'1', '2'});
        testMap.put('test2', new List<String>());

        for(String key : testMap.keySet())
        {
            listSizeMap.put(key, testMap.get(key).size());
        }
    }
}

Map Explanation (old answer, but perhaps useful for other people)

You can’t do a .size on a value of the map.

You try to get a single value from the map by adding the square brackets with the variable M {!attributeWrapper.mapAllAttributesLabel[M].size} which is basically equal to mapAllAttributesLabel.get(M); in apex.

if you want to get the size of your map just do {!attributeWrapper.mapAllAttributesLabel.size} which is equal to mapAllAttributesLabel.size(); in apex.

with the apex repeat, you loop over the keys in your map, and the var set by var=”M” defines the current key in your loop. Each time you do mapAllAttributesLabel[M] you get the value of that key in your map.

Attribution
Source : Link , Question Author : Pramod Kumar , Answer Author : pjcarly

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