Using mapping() method in Question 65 on OCP Exam 2,....

[schchen2000]

Hi,

I'm trying to get the same thing accomplished by using mapping() instead. The following is the complete code. I'm getting an error and I've spent enough time on it and still couldn't figure out what the error message is saying. Would you mind sharing with me what's that I'm not understanding here? The error came from:

Code: Select all

Map<Grade, List<Student>> grouping = ls.stream().collect(Collectors.groupingBy(x -> x.getGrade(), Collectors.mapping(x -> x.getName(), Collectors.toList())));

For mapping() method, I referenced Oracle website here:

https://docs.oracle.com/javase/8/docs/a ... Collector-

Thanks.

Error message
***************
Schmichaels-MacBook-Pro:Java_Professional_Programs schmichael$ javac Student.java
Student.java:45: error: incompatible types: inference variable T#1 has incompatible bounds
Map<Grade, List<Student>> grouping = ls.stream().collect(Collectors.groupingBy(x -> x.getGrade(), Collectors.mapping(x -> x.getName(), Collectors.toList())));
^
equality constraints: Student
lower bounds: String,U
where T#1,U,T#2,A,R are type-variables:
T#1 extends Object declared in method <T#1>toList()
U extends Object declared in method <T#2,U,A,R>mapping(Function<? super T#2,? extends U>,Collector<? super U,A,R>)
T#2 extends Object declared in method <T#2,U,A,R>mapping(Function<? super T#2,? extends U>,Collector<? super U,A,R>)
A extends Object declared in method <T#2,U,A,R>mapping(Function<? super T#2,? extends U>,Collector<? super U,A,R>)
R extends Object declared in method <T#2,U,A,R>mapping(Function<? super T#2,? extends U>,Collector<? super U,A,R>)
1 error

My code
********

Code: Select all

import java.util.*;

import java.util.stream.*;

public class Student{

        public static enum Grade{A, B, C, D, F}

        private String name;

        private Grade grade;

        public Student(String name, Grade grade){

                this.name = name;

                this.grade = grade;

        }

        public String toString(){

                return name + ":" + grade;

        }

        public String getName(){

                return name;

        }

        public Grade getGrade(){

                return grade;

        }

        public static void main(String... args){

                List<Student> ls = Arrays.asList(new Student("S1", Grade.A), new Student("S2", Grade.A), new Student("S3", Grade.C));

                Map<Grade, List<Student>> grouping = ls.stream().collect(Collectors.groupingBy(x -> x.getGrade(), Collectors.mapping(x -> x.getName(), Collectors.toList())));

                System.out.println(grouping);

        }

}

[Rollebol]

You are grouping Students by grade.
However, you are collecting the student names by grade instead of the Student object itself (the mapping function). The getName() method returns a String.
So the return type should be Map<Grade, List<String>> instead of Map<Grade, List<Student>>.

[schchen2000]

Ah! Thank you so much. You've saved my day.

Schmichael