Nested Collectors in Java 8

I work with demographic data. I have a collection of records for different counties in a state (multiple records per county) that I want to aggregate by county.

I have implemented the following Consumer:

public class CountyPopulation implements java.util.function.Consumer<Population>
{
    private String countyId ;
    private List<Demographic> demographics ;

    public CountyPopulation()
    {
        demographics = new ArrayList<Demographic>() ;
    }

    public List<Demographic> getDemographics()
    {
        return demographics ;
    }

    public void accept(Population pop) 
    {
        if ( countyId == null )
        {
            countyId = pop.getCtyId() ;
        }
        demographics.add( pop.getDemographic() ) ;
    }

    public void combine(CountyPopulation other) 
    {
        demographics.addAll( other.getDemographics() ) ;
    }
}

      

        
        
        
      

    

This CountyPopulation function is used to aggregate data for a specific county using the following code (where "089" is the county ID):

CountyPopulation ctyPop = populations
    .stream()
    .filter( e -> "089".equals( e.getCtyId() ) )
    .collect(CountyPopulation::new, 
             CountyPopulation::accept, 
             CountyPopulation::combine) ;

      

        
        
        
      

    

Now I would like to remove the "filter" and group the records by county before using my aggregator.

Based on your first answers, I understand that it can be done using the Collector.of static function like this:

Map<String,CountyPopulation> pop = populations
    .stream()
    .collect(
        Collectors.groupingBy(Population::getCtyId,
                              Collector.of( CountyPopulation::new,
                                            CountyPopulation::accept, 
                                            (a,b)->{a.combine(b); return a;} ))) ; 

      

        
        
        
      

    

However, this code doesn't work because Collector.of () has a different signature than collect (). I suspect the solution involves modifying the CountyPopulation class so that it implements java.util.function.BiConsumer instead of java.util.function.Consumer, but my attempt to do this did not work and I do not understand why.