Java 8 List to EnumMap with sums

Question

我有以下类(class):

public class Mark {

    private Long id;

    private Student student;

    private Integer value = 0;

    private Subject subject;

}

public enum Subject {

    MATH,
    CHEMISTRY

}

我必须收到 EnumMap<Subject, Integer> ，其中值是 Mark 中所有值的总和.
List<Mark> 的示例:

Mark(..., value = 1, subject = MATH)
Mark(..., value = 2, subject = MATH)
Mark(..., value = 5, subject = CHEMISTRY)

有了这个值，我必须收到以下 EnumMap :

MATH             -> 3
CHEMISTRY -> 5

我认为应该用 Collectors::groupingBy 来完成, 但我不明白如何获得 EnumMap和他的值(value)。

Answer 1

 markList.stream().collect(
     groupingBy(
         Mark::getSubject,
         () -> new EnumMap<Subject, Integer>(Subject.class),
         summingInt(Mark::getValue)));