java中list<entity>中计算entity中类型是BigDecimal的和

可以回答这个问题。可以使用Java 8的Stream API来计算List<Entity>中类型为BigDecimal的和，示例代码如下：

List<Entity> entities = new ArrayList<>();
// 添加实体对象到List中
BigDecimal sum = entities.stream()
.filter(e -> e.getValue() instanceof BigDecimal)
.map(e -> (BigDecimal) e.getValue())
.reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println("BigDecimal类型的和为：" + sum);

其中，Entity是一个自定义的实体类，getValue()方法返回实体对象中的值。

相关问题

public class TzglInvestmentPlanProjectEntity { private String projectId; private BigDecimal investmentTotalMoney; private BigDecimal investmentYearMoney; } 现有两个list List<TzglInvestmentPlanProjectEntity> planProjectList； List<TzglInvestmentPlanProjectEntity> planProjectList2； 两个list中projectId相同的数据，把planProjectList中的investmentTotalMoney和investmentYearMoney赋值给planProjectList2

可以使用循环遍历两个List，找到projectId相同的数据，然后将investmentTotalMoney和investmentYearMoney赋值给planProjectList2中对应的数据。可以按照以下步骤来实现：

for(TzglInvestmentPlanProjectEntity entity1 : planProjectList) {
    for(TzglInvestmentPlanProjectEntity entity2 : planProjectList2) {
        if(entity1.getProjectId().equals(entity2.getProjectId())) {
            entity2.setInvestmentTotalMoney(entity1.getInvestmentTotalMoney());
            entity2.setInvestmentYearMoney(entity1.getInvestmentYearMoney());
            break;
        }
    }
}

这里使用了两个for循环来遍历两个List，如果找到了projectId相同的数据，则将对应的investmentTotalMoney和investmentYearMoney赋值给planProjectList2中对应的数据。注意，如果找到了相同的数据，可以使用break语句来提高效率。

vo类 public class ArealRainfallVo implements Serializable { private static final long serialVersionUID = 1L; @ApiModelProperty(value = "区域编码") private String code; @ApiModelProperty(value = "雨量总和") private BigDecimal value; } Mapper类 public interface WeightBetweenObjectAndRainfallStationMapper extends BaseMapper<WeightBetweenObjectAndRainfallStation> { List<ArealRainfallVo> ListbySumList (List<PointRainfallSumVO> list); } mapper配置类 <mapper namespace="com.ndwp.jboot.bootstrap.mapper.WeightBetweenObjectAndRainfallStationMapper"> <select id="ListbySumList" resultType="com.ndwp.jboot.bootstrap.entity.vo.ArealRainfallVo"> SELECT code, SUM(CASE <foreach collection="list" item = "item"> WHEN rainfall_station_code = '${item.getCode()}' THEN weight * ${item.getValue()} </foreach> END ) AS value FROM bootstrap.weight_between_object_and_rainfall_station GROUP BY code; </select> </mapper> 服务类 public interface IWeightBetweenObjectAndRainfallStationService extends BaseService<WeightBetweenObjectAndRainfallStation> { List<ArealRainfallVo> listAreaRainfall(Date start, Date end); } 服务实现类 @Service public class WeightBetweenObjectAndRainfallStationServiceImpl extends BaseServiceImpl<WeightBetweenObjectAndRainfallStationMapper, WeightBetweenObjectAndRainfallStation> implements IWeightBetweenObjectAndRainfallStationService { @Autowired IDataHistoricRainfallService dataHistoricRainfallService; @Override public List<ArealRainfallVo> listAreaRainfall(Date start, Date end) { List<PointRainfallSumVO> pointRainfallSumVOS = dataHistoricRainfallService.listAllPointRainfallByTime(start, end); return baseMapper.ListbySumList(pointRainfallSumVOS); } } 控制台打印的sql语句能够正常运行，结合代码分析一下为何会返回四个空对象？

根据你提供的代码，我注意到你在mapper配置文件中使用了foreach循环来动态生成SQL语句中的条件部分。然而，你在foreach循环中使用了`${item.getCode()}`和`${item.getValue()}`来获取集合中每个元素的属性值，这是不正确的。

在MyBatis中，`${}`语法用于替换普通的字符串，它不会对参数进行预处理或转义。所以在你的SQL语句中，`${item.getCode()}`和`${item.getValue()}`会被直接替换为对应元素的`getCode()`和`getValue()`方法的返回值，而不是参数值。

为了修复这个问题，你可以改用`#{}`语法来将参数作为预处理参数传递给SQL语句。修改后的mapper配置文件如下所示：

<mapper namespace="com.ndwp.jboot.bootstrap.mapper.WeightBetweenObjectAndRainfallStationMapper">
    <select id="ListbySumList" resultType="com.ndwp.jboot.bootstrap.entity.vo.ArealRainfallVo">
        SELECT code, SUM(CASE
        <foreach collection="list" item="item" separator=" ">
             WHEN rainfall_station_code = #{item.code} THEN weight * #{item.value}
        </foreach> END ) AS value
        FROM bootstrap.weight_between_object_and_rainfall_station
        GROUP BY code;
    </select>
</mapper>

修改后重新运行代码，应该能够正确地获取到数据库中的数据并返回非空对象。