按数组列表分组 [英] groupingby list of arrays

问题描述

我得到了需要分组的对象数组的列表.数组包含不同类型的对象. 这是一个示例:

I get a list of object arrays that I need to group. The arrays contain different types of objects. Here is an example:

List<Object[]> resultList = query.getResultList(); // call to DB

// Let's say the object array contains objects of type Student and Book
// and Student has a property Course.
// Now I want to group this list by Student.getCourse()
Map<String, Object[]> resultMap = resultList.stream().collect(Collectors.groupingBy(???));

我可以为Collectors.groupingBy()方法提供什么? Student对象位于对象数组的索引0处.

What do I provide to the Collectors.groupingBy() method ? The Student object is at index 0 in the object array.

推荐答案

groupingBy在默认情况下会为您提供Map<String, List<Object[]>>，因为您将根据学生的课程价值对数组进行分组.

groupingBy will by default give you a Map<String, List<Object[]>> in your case, because you will group arrays based on their student's course value.

因此，您需要按数组中学生的课程值进行分组.因此，您将应用的功能将是:

So you need to group by the course value of the student in the array. The function you will apply will hence be:

o -> ((Student)o[0]).getCourse()

因此，按实现分组的内容为:

thus the grouping by implementation becomes:

Map<String, List<Object[]>> resultMap = 
    resultList.stream().collect(groupingBy(o -> ((Student)o[0]).getCourse()));

顺便说一句，您可能希望使用一个类来键入数据并避免进行强制转换，否则可能会在运行时引发异常.

As an aside, you may want to use a class to have typed data and to avoid the cast, that could possibly throw an exception at runtime.

您还可以在数据库级别执行分组.

You could also perform the grouping by at the database level.

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