grib/src/main/java/ucar/nc2/grib/collection/PartitionCollectionImmutable.java
public String getFilename(int partno, int fileno) throws IOException {
    Partition part = getPartition(partno);
    try (GribCollectionImmutable gc = part.getGribCollection()) {
      return gc.getFilename(fileno);
    }
  }
public String getFilename(int partno, int fileno) throws IOException {
    Partition part = getPartition(partno);
    try (GribCollectionImmutable gc = part.getGribCollection()) {
      return gc.getFilename(fileno);
    }
  }
debugging