Java split list based on element

Aiden :

So I've got a class Standard with properties and constructor:

String studentName;
String category;

Standard(studentName, category)

I've also got a List of Standard objects like so:

[S1,S2,S3,S4]

where

S1 = Standard("ABC","1");
S2 = Standard("DEF","2")
S3 = Standard("IJK","1")
S4 = Standard("LMN","2")

I would like to group Standard objects with same category into a Map(String,List<Standard>) with key as category and value as List of Standard objects like so:

Map<String,List<Standard>> m = {1=[S1, S3], 2=[S2,S4]};

Now I would like to know whether this can be done using Java 8 streams.

Andronicus :

You can use groupingBy collector:

Map<String, List<Standard>> map = standards.stream()
    .collect(Collectors.groupingBy(Standard::getCategory));

Assuming you have getCategory method implemented.