使用MapReduce技术获取Java8中多个类字段的摘要
以下是学生班级定义:使用MapReduce技术获取Java8中多个类字段的摘要,mapreduce,java-8,Mapreduce,Java 8,以下是学生班级定义: class Student{ String name; int sub1; int sub2; int sub3; // etc...etc... } 我有所有学生的名单。 要求得到sub1、sub2和sub3的平均值。而且,任何学生在任何科目上都能得到最低分和最高分 以下是sub1的代码: IntSummaryStatistics sub1Summary = students.stream().collect(Colle
class Student{
String name;
int sub1;
int sub2;
int sub3;
// etc...etc...
}
我有所有学生的名单。
要求得到sub1、sub2和sub3的平均值。而且,任何学生在任何科目上都能得到最低分和最高分
以下是sub1的代码:
IntSummaryStatistics sub1Summary = students.stream().collect(Collectors.summarizingInt(s -> s.sub1));
是否可以使用单个表达式,或者我必须为每个主题创建IntSummaryStatistics
,并比较每个最低和最高主题值
提前感谢尝试以下内容:
List<Student> students = Arrays.asList(Student.create("A", 60, 55, 35),
Student.create("B", 40, 65, 75),
Student.create("C", 70, 45, 95));
Map<String, IntSummaryStatistics> subjectsSummary = new HashMap<String, IntSummaryStatistics>();
Map<String, List<Integer>> subjectMarks = new HashMap<String, List<Integer>>();
List<Integer> sub1Marks = new ArrayList<>();
List<Integer> sub2Marks = new ArrayList<>();
List<Integer> sub3Marks = new ArrayList<>();
// Input : Student
// Output : Map(Key=SubjectName, Value= List of Marks for students)
Function<Student, Map<String, List<Integer>>> toSubjectsMap = (s) -> {
sub1Marks.add(s.getSub1());
sub2Marks.add(s.getSub2());
sub3Marks.add(s.getSub3());
subjectMarks.put("Sub1", sub1Marks);
subjectMarks.put("Sub2", sub2Marks);
subjectMarks.put("Sub3", sub3Marks);
return subjectMarks;
};
//Function to generate summary stats from the list of integers/marks
Function<List<Integer>, IntSummaryStatistics> generateStatsFunc = (listOfMarks) -> {
return listOfMarks.stream().collect(Collectors.summarizingInt(marks->marks));
};
// 1. Students are mapped to generate Map for subject-->List of Marks
// 2. Doing flatMap on the generate Map to get the Keys from the map(list of subjects)
// 3. Collecting the Keys(Subject Names) of Map to Map(Key=SubjectName,Value=IntSummaryStats for Subject)
students.stream().map(toSubjectsMap).flatMap(subMap -> subMap.keySet().stream())
.collect(() -> subjectsSummary/* Initial Value of Map */, (summary, key) -> {
// Generate the stats from the list of marks for the subject
IntSummaryStatistics st = generateStatsFunc.apply(subjectMarks.get(key));
//Adding the stats for each Subject into the result Summary
summary.put(key, st);
} , (summary1, summary2) -> {
//merging the two maps
summary1.putAll(summary2);
});
System.out.println(subjectsSummary);
你说的任何科目的最低分和最高分是什么意思?是否要作为结果3
IntSummaryStatistics
、一个汇总sub1
、一个汇总sub2
和一个汇总sub3
?或者是sub
s之间的最小相关性?有学生列表。我想得到学生在任何科目(sub1、sub2或sub3)上的最低分数和最高分数。因此,我将只有一个最低和最高的值,整个列表和主题1的平均分数,主题2的平均分数和主题3的平均分数,没有一行,对不起。如果您需要一些阅读检查,或者……您需要编写自己的收集器来汇总所有sub
s,或者为每个sub
s创建一个IntSummaryStatistics
。。看来我必须花5-6个小时来理解stream()块。如果可能,请添加评论。
class Student {
String name;
int sub1;
int sub2;
int sub3;
static Student create(String name, int sub1, int sub2, int sub3) {
// validation
return new Student(name, sub1, sub2, sub3);
}
public String getName() {
return name;
}
public int getSub1() {
return sub1;
}
public int getSub2() {
return sub2;
}
public int getSub3() {
return sub3;
}
private Student(String name, int sub1, int sub2, int sub3) {
this.name = name;
this.sub1 = sub1;
this.sub2 = sub2;
this.sub3 = sub3;
}
@Override
public String toString() {
return name;
}
}