Java中 List 集合，通过 Stream 流进行排序总结

一、数据准备

publicclassOrderTest{private String channelCode;private BigDecimal rate;// 省略 getter、setter、toString()、constructor}

        List<OrderTest> orderTestList =newArrayList<>();
        OrderTest z09 =newOrderTest("Z09",newBigDecimal("7.6677"));
        OrderTest B2C =newOrderTest("B2C",newBigDecimal("5.6666"));
        OrderTest Z04 =newOrderTest("Z04",newBigDecimal("4.3137"));
        OrderTest ALL =newOrderTest("ALL",newBigDecimal("4.3137"));
        OrderTest ALL1 =newOrderTest("ALL",newBigDecimal("4.5137"));// 演示多字段排序使用
        OrderTest z091 =newOrderTest("Z09",newBigDecimal("7.6671"));// 演示给 null 值排序用
        OrderTest z092 =newOrderTest("Z09", null);
        OrderTest B2C1 =newOrderTest("B2C",newBigDecimal("5.6666"));
        OrderTest Z041 =newOrderTest("Z04", null);
        orderTestList.add(z09);
        orderTestList.add(z091);
        orderTestList.add(B2C);
        orderTestList.add(Z04);
        orderTestList.add(ALL);
        orderTestList.add(ALL1);

二、单字段排序

2.1、升序

 list.stram().sorted(Comparator.Comparing(YourClass::Class's Field)

System.out.println("----------------------------------------------");
        System.out.println("只按照 channelCode 升序排序：");
        List<OrderTest> channelAsc =
                orderTestList.stream().sorted(Comparator.comparing(OrderTest::getChannelCode)).collect(Collectors.toList());
        channelAsc.forEach(System.out::println);

结果：

2.2、降序

 list.stram().sorted(Comparator.Comparing(YourClass::Class's Field, Comparator.reverseOrder())

System.out.println("----------------------------------------------");
        System.out.println("只按照 channelCode 降序排序：");
        List<OrderTest> channelDesc =
                orderTestList.stream().sorted(Comparator.comparing(OrderTest::getChannelCode, Comparator.reverseOrder())).collect(Collectors.toList());
        channelDesc.forEach(System.out::println);

结果：

二、多字段排序

利用的是 thenComparing()：
升序 thenComparing(YourClass::Class's Field)，
降序 thenComparing(YourClass::Class's Field, Comparator.reverseOrder())。

注意： 使用 thenComparing(YourClass::Class's Field).reversed() 的时候要注意排序要求，
如果先按照 A 字段升序 B 字段升序的话，使用 reversed() 之后的结果是对 A 降序 B 降序。

System.out.println("----------------------------------------------");
        System.out.println("先按照 channelCode 升序，再按照 rate 升序排序：");
        List<OrderTest> channelCodeAscRateAscList =
                orderTestList.stream().sorted(Comparator.comparing(OrderTest::getChannelCode).thenComparing(OrderTest::getRate)).collect(Collectors.toList());
        channelCodeAscRateAscList.forEach(System.out::println);

结果

先按照 channelCode 将序，再按照 rate 升序将序，使用 reversed()：

System.out.println("----------------------------------------------");
        System.out.println("先按照 channelCode 将序，再按照 rate 将序排序，使用 reversed()：");
        List<OrderTest> channelCodeAscRateAscWithReversedList =
                orderTestList.stream().sorted(Comparator.comparing(OrderTest::getChannelCode).thenComparing(OrderTest::getRate).reversed()).collect(Collectors.toList());
        channelCodeAscRateAscWithReversedList.forEach(System.out::println);

结果

三、对 null 值处理

 Comparator.nullsFirst(Comparator.reverseOrder())-- null排在前面，reverseOrder是倒序，升序用naturalOrder
 Comparator.nullsLast(Comparator.reverseOrder())-- null排在后面，reverseOrder是倒序，升序用naturalOrder

orderTestList.add(newOrderTest(("Z09")));
        orderTestList.add(newOrderTest(("B2C")));
        orderTestList.add(newOrderTest(("Z04")));
        System.out.println("----------------------------------------------");
        System.out.println("先按照 channelCode 升序，再按照 rate 降序并且 null 值放前面排序：");
        List<OrderTest> channelCodeAscRateDescNullFirstList = orderTestList.stream().sorted(Comparator.comparing(OrderTest::getChannelCode).thenComparing(OrderTest::getRate, Comparator.nullsFirst(Comparator.reverseOrder()))).collect(Collectors.toList());
        channelCodeAscRateDescNullFirstList.forEach(System.out::println);

结果

四、对排序字段个数不固定的情况，如何排序

需求：排序字段个数不确定，如何实现动态排序？

伪代码示例：

        Comparator<SurgUnArrangeResponse> comparing = null;for(int i =0; i < dictItemList.size(); i++){
            String cd = dictItemList.get(i).getCd();if(i ==0){// 首次排序
                comparing =getComparingByDictCd(cd, null);}else{// 第二次排序及以上
                comparing =getComparingByDictCd(cd, comparing);}}// comparing 比较器，保存了排序的规则，responseList 是被排序的集合
        responseList = responseList.stream().sorted(comparing).collect(Collectors.toList());// getComparingByDictCd 方法private Comparator<SurgUnArrangeResponse>getComparingByDictCd(String dictCd, 
Comparator<SurgUnArrangeResponse> result = null;switch(dictCd){case"age":
                result = comparing == null ? Comparator.comparing(SurgUnArrangeResponse::getAge, Comparator.nullsLast(Comparator.reverseOrder())):
                        comparing.thenComparing(SurgUnArrangeResponse::getAge, Comparator.nullsLast(Comparator.reverseOrder()));break;....}return result;}