Java Collections - Map and List

Question

In Map how do we pass a class as value.

For eg- I have a Student class with two fields as name and department.
Let us say I put a integer type key.

Map<Integer, List<Student>> map = new HashMap<>();

The above does not give me any error but while inserting the values it shows some error, may be I'm doing it wrong.

map.put(1, new Student("name", "dept"));

What is the right way to insert values inside the map?

Answer 1

Avinesh Check this code.

https://code.sololearn.com/csCtb155654V/?ref=app

Answer 2

It means you want to put list of Student in map as a value.

Map<Integer,List<Student>> map = new HashMap<>();

For adding a value in map
List<Student> list = new ArrayList<>();
map.put(0,list);

If you want to put the student as value and not the list of student then

Map<Integer,Student> map = new HashMap<>();

map.put(1,new Student());

Answer 3

Avinesh You have declared a map with the value of list so you cannot directly put a class
To achieve this you need to create a list first then you can put in map like this:-

Map<Integer, List<Student>> map = new HashMap<>();

List<Student> stlist = new ArrayList<>();

Student s = new Student();

slist.add(s);
map.put(1, slist);

Answer 4

I shouldn't have included List, just Student would have done the job.

I was trying to map a student to an integer value so the List was not necessary.

But adding a List would be like mapping many students to a single integer value.

Thanks a lot guys
Ravi Prakash
Sumit Programmer😎😎
And specially 🅰🅹 - ɪ'ᴍ ᴄʀɪᴍɪɴᴀʟʟʏ ɢᴏᴏᴅ! for the code you made for this.

Answer 5

Avinesh You can use Map in different ways according to your requirements. Here are some examples:-

Map<Long, Student> map;
Map<Long, List<Student>> map;
Map<Long, Map<Long, Student>> map;
Map<Long, Map<Long, List<Student>>> map;

Answer 6

🅰🅹 - ɪ'ᴍ ᴄʀɪᴍɪɴᴀʟʟʏ ɢᴏᴏᴅ! Those are some really nice examples, will try those for sure.

Just started digging collections so probably I might come up with more questions.

Answer 7

Map<Integer, Student> map=new HashMap<>();
Try this i think it will work....

Answer 8

https://code.sololearn.com/cH2d2wY5jxfM/?ref=app