This mapping is not valid.

1. @ManyToMany is for entities
2. @ElementCollection is for mapping basic and embeddable types.

In your case, you should use @ElementCollection if you need a List<String> or use @ManyToMany if you need a List<StringWrapper> where StringWrapper is a Child entity to wraps the String value you need.

Hi vlad,

Sorry I was busy with my pc repair cos of harddisk crash.
And now I am back but still stuck at this piece of work.

Could you kindly explain what is String value ?

Do you mean String subjName which I have used in my controller which I tried to get the Parameter values ?

And the turn the List<String> into a List<Child> to List<subjName> ?

So, do I now do the mapping at this new entity or at the tutor entity and subject entity ?

Hope you could clarify.

Thanks again.

Just turn the List<String> into a List<Child> and it will work.

Just read the bidirectional @OneToMany section and do the mapping like that.

So,

1. The Parent entity will have a List<Child> using mappedBy.
2. The Child will map the Parent using @ManyToOne.

It's just a typical one-to-many table relationship based on a FK.

Hi vlad,

I did as you said by following the example in your book.

But, I am getting an error message from IDE :







Is there anything that I have not been following correctly ?

This doesn't look right and that's why you get the Exception.

If you had a List<Subject>, you'd call:



If you had a Subject instance, you'd call:



Now, back to your code:



This part is useless:



And this is how you add a list of Subjects:



I think you should read Effective Java from Joshua Bloch first. Then learn Hibernate, Spring, REST, etc.

Hi vlad,

From what I have studied before, I can only do the above like what you said when I can convert List<String>subjects into a List<Subject> subjlist.

So, this is the part that I am stuck cos I googled everywhere just can't find anything that allow me to do the conversion.

Just implement a Subject constructor that populates the fields based on the encoded String value.

Hi vliad,

I would like to clarify the above.

So, now I am using a constructor in my Subject class with a field variable String subject.

Therefore, in my Tutor class, I can't write this below as per your example right ?

public List<Subject>getSubjects(){
return subjNames;
}

public void addSubject(Subject subject){
subjNames.addAll(subjNames);
//subjNames.add((Subject) list);
((Subject) subjNames).setTutor(this);
//((Subject) list).setTutors(this);
}

public void removeSubject(Subject subject){
subjNames.remove(subject);
subject.setTutor(null);
}

cos with the above code, I am getting a java.lang.NullPointerException
at model.Tutor.addSubject(Tutor.java:184)
at controller.tutorController.doPost(tutorController.java:75)

In this case how should I write the add and remove portion ?

What's the meaning for this line of code?