带有和的Java 8 List到EnumMap

Question

我有以下课程:

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>,其中值是来自的所有值的总和Mark.
示例List<Mark>:

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

有了这个价值,我必须收到以下内容EnumMap:

数学 - > 3
化学 - > 5

我认为应该这样做Collectors::groupingBy,但我无法理解如何获得EnumMap和他的价值.

Answer 1

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