code: static public LinkedHashMap<String,FeatureList> buildFeatureAtrributeIndex(String attribute,FeatureList list){
  
  		LinkedHashMap<String,FeatureList> featureHashMap = new LinkedHashMap<String,FeatureList>();
  		FeatureList featureList = list.selectByAttribute(attribute);
  		for(FeatureI feature : featureList){
  			String value = feature.getAttribute(attribute);
  			FeatureList features = featureHashMap.get(value);
  			if(features == null){
  				features = new FeatureList();
  				featureHashMap.put(value, features);
  			}
  			features.add(feature);
  		}
  
  		return featureHashMap;
  	}
docs: Build a list of individual features to allow easy indexing and to avoid iterating through large genome gff3 files
  The index for the returned HashMap is the value of the attribute used to build the index
  @param attribute
  @param list
  @return