Override toString() for Freemarker Java List<Map> variable

Question

I would like to create a Java Freemarker variable that works in a template both as

${myListOfMapsVar}     <#-- calls toString() on myListOfMapsVar -->

and

<#list myListOfMapsVar as item>
  ${item.mapKeyName}
</#list>

It seems like this should be possible using something like

List<Map> myListOfMaps = ...
var myListOfMapsVar = new MyWrappedObject();
myListOfMapsVar.wrap( myListOfMaps );

class MyWrappedObject extends DefaultObjectWrapper {
  MyWrappedObject() { super(); }

  public String toString() {
    return "a string representation of my list";
  }
}

However, when I try this, the ${myListOfMapsVar} works as expected, but the list enumeration produces an error:

The value you try to list is an extended_hash+string (.... wrapped into f.e.b.StringModel), thus you must specify two loop variables after the "as"; one for the key, and another for the value, like <#... as k, v>).

What am I doing wrong?

btw, I cannot change any application-wide Freemarker configuration.

Answer 1

For those that come later, here's my solution

class MyListOfMap<K,V> implements TemplateSequenceModel, TemplateScalarModel {

    DefaultObjectWrapper wrapper = new DefaultObjectWrapperBuilder(Configuration.VERSION_2_3_27).build();
    List<Map<K,V>> listOfMaps;
    Function<Map<K,V>,String> mapper;

    MyListOfMap( List list, Function<Map<K,V>,String> mapper ) {
      listOfMaps = list;
      this.mapper = mapper;
    }

    @Override
    public String getAsString() throws TemplateModelException {
      var strings = listOfMaps.stream().map( mapper ).collect(Collectors.toList());
      return String.join( ", ", strings );
    }

    @Override
    public TemplateModel get(int index) throws TemplateModelException {
      return new MapModel( listOfMaps.get(index), wrapper );
    }

    @Override
    public int size() throws TemplateModelException {
      return listOfMaps.size();
    }
  }

var mylist = new ArrayList<Map<String,String>>();
mylist.add( Map.of( "first", "John", "last", "Smith", "age", "32", "city", "Chicago", "State", "IL" ) );
mylist.add( Map.of( "first", "Jane", "last", "Doe", "age", "29", "city", "Memphis", "State", "TN" ) );
mylist.add( Map.of( "first", "Mark", "last", "Roy", "age", "38", "city", "Boston", "State", "MA" ) );
    
var listOfPeople = new MyListOfMap<String,String>( mylist, 
        (person) -> person.get("first") + " " + person.get("last"));
    
var dataModel.put( "people", listOfPeople );

Variable 'people' can be used as a 'summary' string and as a list of hashes

myftltemplate.ftl:

${people}  <#-- evaluates to: John Smith, Jane Doe, Mark Roy -->

and can also be used like

<#list people as person>
  <tr>
    <td>${person.first}</td>
    <td>${person.last}</td>
    <td>${person.age}</td>
    <td>${person.city}</td>
    <td>${person.state}</td>
  </tr>
</#list>

Thanks to @ddekany for the pointers.