接受通用列表作为参数，并根据其类型使用它

Question

我正在为公司开发一个网站，并且使用Spring作为后端。现在有一种情况，我需要两次使用一种Utils方法，但是要使用不同的DAO。

为了避免代码重复，我想知道如何使用Java泛型来使该方法可用于两种情况。该方法仅计算两个DAO共有的字段之一。

使用方法：

SeverityCount calculateSeveritiesCount(List<?> events){

        if(null == events){
            return new SeverityCount();
        }

        if(events.get(1) instanceof EventDAO){
            events = (List<EventDAO>)events;
        }
        else if (events.get(1) instanceof EventsByAreaDAO) {
            events = (List<EventsByAreaDAO>)events;
        }

        Map<String, Long> severityCountMap = events.stream().collect(
            Collectors.groupingBy(
                EventDAO::getSeverity,  //It should be EventDAO or EventsByAreaDAO. both has severity field.
                Collectors.counting())
        );

        return mapper.convertValue(severityCountMap, SeverityCount.class);
    }

活动DAO：

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "events")
public class EventDAO {
    @Id @Column(name = "uid")
    private String uID;

    private String date;

    private String severity;

}

区域DAO：

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "events")
public class EventsByRegionDAO {

     @Id @Column(name = "uid")
    private String uID;

    private String date;

    private String type;

    private String severity;

    private String area;

    private String server;
}

这就是我从服务中调用它的方式：

SeverityCount severitiesCount = Utils.calculateSeveritiesCount(eventsList);  //EventsList could be list of EventDAO or EventsByAreaDAO

Answer 1

您可以将方法更改为

SeverityCount calculateSeveritiesCount(List<? extends SeverityCalculable> events)

哪里 SeverityCalculable

interface SeverityCalculable {
    String getSeverity(); // implemente getter in all subclasses
}

让您所有相关的类都实现此接口。

public class EventDAO implements SeverityCalculable  {

    // ...

    @Override
    public String getSeverity() {
        return this.severity;
    }
}

现在，在您的方法中，删除强制类型转换，它应该变成这样：

 SeverityCount calculateSeveritiesCount(List<? extends SeverityCalculable> events) {

    if(null == events){
        return new SeverityCount();
    }

    Map<String, Long> severityCountMap = events.stream().collect(
        Collectors.groupingBy(
            SeverityCalculable::getSeverity,
            Collectors.counting()
        )
    );

    return mapper.convertValue(severityCountMap, SeverityCount.class);
}