Question

我正在使用Collections.sort根据时间字段以升序对列表进行排序。下面是代码

private String getShipmentInpickingTime(List<Shipments> shipments) {
        logger.info("in getShipmentInpickingTime");
        DateFormat sdf = new SimpleDateFormat("hh:mm");

        Collections.sort(shipments, (o1, o2) -> {
            try {
                if ((!"null".equals(o1.getShipmentinpickingtime())
                        && !StringUtils.isEmpty(o1.getShipmentinpickingtime()))
                        && (!"null".equals(o2.getShipmentinpickingtime())
                                && !StringUtils.isEmpty(o2.getShipmentinpickingtime()))) {
                    return sdf.parse(o1.getShipmentinpickingtime()).compareTo(sdf.parse(o2.getShipmentinpickingtime()));
                }
            } catch (ParseException e) {
                e.printStackTrace();
            }
            int count1 = 0;
            return count1;
        });

此方法引发异常-

java.lang.IllegalArgumentException: Comparison method violates its general contract.

我就这个问题浏览了google-它说的是我正在将较大的对象与较小的对象进行比较。我尝试反转对象的顺序，但是没有运气。

Answer 1

只有其中一个无法解析，您才捕获一次异常。

考虑以下时间的三批装运：
A-12:34
B-34:56
C-空

使用比较器，compare(A, C)将返回0，compare(B, C)将返回0，但是compare(A, B)将返回非零结果，从而违反了传递性的一般约定。

一种简单的方法是使用Comparator.comparing语法分别解析每个元素：

DateFormat sdf = new SimpleDateFormat("hh:mm");

shipments.sort(Comparator.comparing(
                   Shipments::getShipmentinpickingtime, 
                   Comparator.nullsLast(Comparator.comparing(time -> {
                       try {
                           if (!"null".equals(time) && !StringUtils.isEmpty(time)) {
                               return sdf.parse(time);
                           }
                       } catch (ParseException ignoe) {
                           // Not a valid time
                       }
                       return null;
                   })))
              );

Answer 2

您没有提到哪一行代码会引发此错误。我不确定您在这里使用哪个StringUtils，com.sun.deploy.util.StringUtils没有.isEmpty()方法。所以我重新实现了。

以下是您遇到的问题的有效代码：

public static void main(String[] args){
    getShipmentInpickingTime(new ArrayList<Shipments>(){{
        add(new Shipments("12:00"));
        add(new Shipments("12:03"));
        add(new Shipments("12:02"));
    }});
}

private static void getShipmentInpickingTime(List<Shipments> shipments) {
    DateFormat sdf = new SimpleDateFormat("hh:mm");

    System.out.println(shipments.toString());
    Collections.sort(shipments, (o1, o2) -> {
        try {
            if ((!"null".equals(o1.getShipmentPickingTime())
                    && !(o1.getShipmentPickingTime() == null || o1.getShipmentPickingTime().length() < 1))
                    && (!"null".equals(o2.getShipmentPickingTime())
                    && !(o2.getShipmentPickingTime() == null || o2.getShipmentPickingTime().length() < 1))) {
                return sdf.parse(o1.getShipmentPickingTime()).compareTo(sdf.parse(o2.getShipmentPickingTime()));
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return 0;
    });

    System.out.println(shipments.toString());
}

货运类内容：

public class Shipments {

    private String shipmentPickingTime;

    public Shipments(String shipmentPickingTime) {
        this.shipmentPickingTime = shipmentPickingTime;
    }

    public String getShipmentPickingTime() {
        return shipmentPickingTime;
    }

    public void setShipmentPickingTime(String shipmentPickingTime) {
        this.shipmentPickingTime = shipmentPickingTime;
    }

    @Override
    public String toString() {
        return "Shipments{" +
                "shipmentPickingTime='" + shipmentPickingTime + '\'' +
                '}';
    }
}

输出：

[Shipments{shipmentPickingTime='12:00'}, Shipments{shipmentPickingTime='12:03'}, Shipments{shipmentPickingTime='12:02'}] [Shipments{shipmentPickingTime='12:00'}, Shipments{shipmentPickingTime='12:02'}, Shipments{shipmentPickingTime='12:03'}]

如何解决Java Collections.sort（）比较方法违反了其一般合同规定的异常

2 个答案: