About Question enthuware.ocpjp.v8.2.1802 :

[lenalena]

Please, explain why option 1 is wrong. I can see that options 2 and 3 have wrong syntax, and option 4 is obviously correct, but what would be the "official" reason for option 1 being wrong?

When I tried to compile the code with option 1 I got

Student.java:26: error: incompatible types: inference variable T#1 has incompatible bounds
Map<Integer, List<Student>> grouping = ls.stream().collect( Collectors.groupingBy(Student::getMarks,Collectors.mapping(Student::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

Which I don't quite understand.

[admin]

Student::getName returns a String. This is the same return type that will be used to determine return type of the List returned by Collectors.toList() i.e. List<String>.

Therefore, the return type of Collectors.groupingBy will be Map<Integer, List<String>>.

But this is not compatible with the type of the variable grouping, which is Map<Integer, List<Student>>.
If you change the variable declaration to Map<Integer, List<String>> grouping, it will compile.

HTH,
Paul.

[lenalena]

It's clear now, thank you. The exception given by the compiler actually makes sense.

It's a great explanation, perhaps you could add it to the question... (IMHO).

[dongyingname]

Code: Select all

Map<Integer, List<Student>> grouping = ls.stream().collect(Collectors.groupingBy(s ->s.getMarks()));

Shouldn't the above code return something like {20=[Student("S1", 20),Student("S3", 20)], 30=Student("S3", 30)} instead of {20=[S1:20, S3:20], 30=[S3:30]}?

The values of Map<Integer, List<Student>> are List<Student>s after all.

And shouldn't the answer be

Code: Select all

 Map<Integer, List<Student>> grouping = ls.stream().collect(Collectors.groupingBy(s ->s.getMarks(),Collectors.toList())); 

?

[admin]

Yes, the values are List<Student> but why do you think it will print Student("S1", 20), etc.?
Student implements toString method, and it prints - name+":"+marks;