findAll(Pageable) returns wrong sorting order 查找所有(分页)返回错误的排序顺序

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英文:

findAll(Pageable) returns wrong sorting order

问题

为什么PagingAndSortingRepository<City, Long>在调用方法findAll(Pageable)之后返回错误的排序顺序?
在服务层中,我有以下方法,我想通过人口来进行排序:

    public Set<City> findAllPageable(int page, int size, String sortBy) {
        Pageable paging = PageRequest.of(page, size, Sort.by(sortBy));
        return new HashSet<>(repository.findAll(paging).getContent());
    }

这是我期望看到的结果:

    select Population from city order by Population limit 10;
42
167
300
455
503
559
595
682
700
800

而在迭代Set<City>之后,实际结果如下:

682, 42, 300, 700, 559, 595, 800, 167, 455, 503

所有这些数字都是正确的,但顺序是错误的。为什么呢?

英文:

Why PagingAndSortingRepository<City, Long> return wrong sorting order after method findAll(Pageable)?
In service layer I have this method and I'm trying to sort by population:

    public Set<City> findAllPageable(int page, int size, String sortBy) {
        Pageable paging = PageRequest.of(page, size, Sort.by(sortBy));
        return new HashSet<>(repository.findAll(paging).getContent());
    }

This is what I expected to see:

    select Population from city order by Population limit 10;
42
167
300
455
503
559
595
682
700
800

And this is the actual result after iterating the Set<City>:

682, 42, 300, 700, 559, 595, 800, 167, 455, 503

All these numbers are correct but the order is incorrect. Why?

答案1

得分: 2

你不能依赖于在 HashSet 中返回元素的顺序。如果你必须在这里使用一个集合,可以使用 LinkedHashSet,它保证了顺序:

return new LinkedHashSet<>(repository.findAll(paging).getContent());
英文:

You can't rely on the order elements are returned in a HashSet. If you must use a set there, use a LinkedHashSet, which guarantees the order:

return new LinkedHashSet&lt;&gt;(repository.findAll(paging).getContent());

答案2

得分: 1

返回的 HashSet 实现不会保持城市的排序顺序

请使用 TreeSet 实现,并传递一个 Comparator<City>,或让 City 实现 Comparable<City>。我还建议在这种情况下返回 SortedSet<City>

这些元素根据它们的自然顺序排序,或者根据在集合创建时提供的比较器进行排序,具体取决于使用哪个构造函数。

public SortedSet<City> findAllPageable(int page, int size, String sortBy) {
     Pageable paging = PageRequest.of(page, size, Sort.by(sortBy));
     return new TreeSet<>(
         repository.findAll(paging).getContent(), 
         Comparator.comparing(City::getPopulation));
}
英文:

The returned HashSet implementation doesn't maintain the sorting order of the cities.

Use the TreeSet implementation and pass either a Comparator&lt;City&gt; or let City implement Copmarable&lt;City&gt;. I also suggest you to return SortedSet&lt;City&gt; in that case.

> The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

public SortedSet&lt;City&gt; findAllPageable(int page, int size, String sortBy) {
     Pageable paging = PageRequest.of(page, size, Sort.by(sortBy));
     return new TreeSet&lt;&gt;(
         repository.findAll(paging).getContent(), 
         Comparator.comparing(City::getPopulation));
}

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  • 本文由 发表于 2020年8月30日 04:17:19
  • 转载请务必保留本文链接:https://go.coder-hub.com/63651403.html
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