Java中 List 集合，通过 Stream 流进行排序总结_java list stream 排序

一、数据准备

public class OrderTest {
    private String channelCode;
    private BigDecimal rate;
    // 省略 getter、setter、toString()、constructor
}
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		List<OrderTest> orderTestList = new ArrayList<>();
        OrderTest z09 = new OrderTest("Z09", new BigDecimal("7.6677"));
        OrderTest B2C = new OrderTest("B2C", new BigDecimal("5.6666"));
        OrderTest Z04 = new OrderTest("Z04", new BigDecimal("4.3137"));
        OrderTest ALL = new OrderTest("ALL", new BigDecimal("4.3137"));
        OrderTest ALL1 = new OrderTest("ALL", new BigDecimal("4.5137"));
        // 演示多字段排序使用
        OrderTest z091 = new OrderTest("Z09", new BigDecimal("7.6671"));
        // 演示给 null 值排序用
        OrderTest z092 = new OrderTest("Z09", null);
        OrderTest B2C1 = new OrderTest("B2C", new BigDecimal("5.6666"));
        OrderTest Z041 = new OrderTest("Z04", null);
        orderTestList.add(z09);
        orderTestList.add(z091);
        orderTestList.add(B2C);
        orderTestList.add(Z04);
        orderTestList.add(ALL);
        orderTestList.add(ALL1);
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二、单字段排序

2.1、升序

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

2.2、降序

 list.stram().sorted(Comparator.Comparing(YourClass::Class's Field, Comparator.reverseOrder())
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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);
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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。
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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);
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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);
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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
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orderTestList.add(new OrderTest(("Z09")));
        orderTestList.add(new OrderTest(("B2C")));
        orderTestList.add(new OrderTest(("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);
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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;
		}
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