使用Java8的Stream进行多字段排序以及剔除非空字段排序

使用 Stream的sort() 方法，并结合 Comparator 类 进行排序，主要解决以下两个问题：

一、当某个字段为空时，怎么进行排序？—使用 Comparator.nullsFirst() 或者 Comparator.nullsLast( ) 进行非空的剔除

List list = collect.stream().filter(StreamUtil.distinctByKey(AdminCertificateVo::getAdminId)).distinct()
                .sorted(Comparator.comparing(AdminCertificateVo::getIssueDate,Comparator.nullsFirst(String::compareTo)).reversed())
                .sorted(Comparator.comparing(AdminCertificateVo::getState,Comparator.nullsFirst(Integer::compareTo)).reversed())
                .collect(Collectors.toList());

这段代码中，按照issueDate和state降序，如果为null则排在结尾。

二、如果比较两个字段？—使用 thenComparing( )

List collect4 = userList.stream().sorted(Comparator.comparing(User::getAddress, Comparator.nullsFirst(String::compareTo)).
                 thenComparing(User::getPhone, Comparator.nullsFirst(String::compareTo)).reversed()).collect(Collectors.toList());
         System.out.println("先根据地址正序排序,再根据电话逆序排序,null的在后面: ");
         System.out.println(JSON.toJSONString(collect4));
         System.out.println("=========================");
         System.out.println();

运行结果：

先根据地址正序排序,再根据电话逆序排序,null的在后面: 
[{"address":"NY","age":28,"userName":"Durant"},{"address":"Miami","age":29,"phone":"55555","userName":"Wade"},{"address":"LA","age":36,"phone":"11111","userName":"Bryant"},{"address":"Houston","age":39,"phone":"33333","userName":"Harden"},{"age":37,"phone":"22222","userName":"James"}]
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