【对线面试官】 Java 泛型


public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {

    List<T> findAll();

    List<T> findAll(Sort sort);

    List<T> findAllById(Iterable<ID> ids);

    <S extends T> List<S> saveAll(Iterable<S> entities);

    void flush();

    <S extends T> S saveAndFlush(S entity);

    void deleteInBatch(Iterable<T> entities);

    void deleteAllInBatch();

    T getOne(ID id);

    @Override
    <S extends T> List<S> findAll(Example<S> example);

    @Override
    <S extends T> List<S> findAll(Example<S> example, Sort sort);
}


// 传入 需要group by 和 sum 的字段名
public cacheMap(List<String> groupByKeys, List<String> sumValues) {
  this.groupByKeys = groupByKeys;
  this.sumValues = sumValues;
}

private void excute(T e) {
  
  // 从pojo 取出需要group by 的字段 list
  List<Object> key = buildPrimaryKey(e);
  
  // primaryMap 是存储结果的Map
  T value = primaryMap.get(key);
  
  // 如果从存储结果找到有相应记录
  if (value != null) {
    for (String elem : sumValues) {
      // 反射获取对应的字段，做累加处理
      Field field = getDeclaredField(elem, e);
      if (field.get(e) instanceof Integer) {
        field.set(value, (Integer) field.get(e) + (Integer) field.get(value));
      } else if (field.get(e) instanceof Long) {
        field.set(value, (Long) field.get(e) + (Long) field.get(value));
      } else {
        throw new RuntimeException("类型异常,请处理异常");
      }
    }
    
    // 处理时间记录
    Field field = getDeclaredField("updated", value);
    if (null != field) {
      field.set(value, DateTimeUtils.getCurrentTime());
    }
  } else {
    // group by 字段 第一次进来
    try {
      primaryMap.put(key, Tclone(e));
      createdMap.put(key, DateTimeUtils.getCurrentTime());
    }catch (Exception ex) {
      log.info("first put value error {}" , e);
    }
  }
}