### Question:
AddDistrictKeys implements Func1<List<District>, List<District>> { public AddDistrictKeys(int year) { mYear = year; } AddDistrictKeys(int year); @Override List<District> call(List<District> districts);  }### Answer:
@Test public void testAddDistrictKeys() { List<District> districtList = new ArrayList<>(); districtList.add(mDistrict); districtList = mKeyAdder.call(districtList); assertNotNull(districtList); assertEquals(districtList.size(), 1); assertEquals(districtList.get(0), mDistrict); assertEquals(mDistrict.getYear().intValue(), mYear); assertEquals(mDistrict.getKey(), DistrictHelper.generateKey(mDistrict.getAbbreviation(), mYear)); }