class Student {
List<Integer> grades
}我想按年級(jí)對學(xué)生進(jìn)行分組，但是如果每個(gè)學(xué)生只有一個(gè)年級(jí)，則groupingBy可以工作。還有其他蘭巴舞的方法嗎？我希望結(jié)果像 Map<Integer, List<Student>>
2 回答
小怪獸愛吃肉
TA貢獻(xiàn)1852條經(jīng)驗(yàn) 獲得超1個(gè)贊
Map<Integer, List<Student>> collect = students.stream()
.flatMap(student -> student.grades.stream()
.map(grade -> new AbstractMap.SimpleEntry<>(grade,
student)))
.collect(Collectors.groupingBy(entry -> entry.getKey(),
Collectors.mapping(Entry::getValue, Collectors.toList())));
因?yàn)槲覜]有使用Java的Pair tuple實(shí)現(xiàn) AbstractMap.SimpleEntry
回首憶惘然
TA貢獻(xiàn)1847條經(jīng)驗(yàn) 獲得超11個(gè)贊
groupingBy如果您有所幫助，它將完成此工作：您可能會(huì)在Javadoc中為groupingBy找到與此版本等效的版本。
final List<Student> students = ...;
// @formatter:off
students.stream()
.flatMap(student -> student.grades.stream()
.map(grade -> new StudentGrade(student, grade)))
.collect(groupingBy(StudentGrade::getGrade, mapping(StudentGrade::getStudent, toList())));
// @formatter:on
現(xiàn)在，您得到一個(gè)Map<Integer, List<Student>>。過濾重復(fù)項(xiàng)取決于您。
您將需要這些導(dǎo)入：
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;
類StudentGrade很簡單：
class StudentGrade {
private final Student student;
private final Integer grade;
public StudentGrade(Student student, Integer grade) {
this.student = student;
this.grade = grade;
}
public Student getStudent() {return student;}
public Integer getGrade() {return grade;}
}
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