reference does not work

[nimo frey]

I have this:

Person p = getPersonByGroup().get("men");

// Way 1: unfortunately, this does not create a new Person referenced by key "men"
if(p==null) p = new Person();


Map<String, Person> g =  getPersonByGroup();
// Way 2: this works	
if(g.get("men")==null) g.put("men", new Groups());

So why does way 2 works and way 1 not ??

[Jesper de Jong]

With way 1, you are creating a new Person object and you make the variable p refer to that new object, if p was null (because the entry in the map with the key "men" wasn't in there).

Just making variable p refer to something else does not change anything in the map. There is no link between variable p and the entry in the map.

[nimo frey]

okay thanks,


I have thought that the "link" is achieved by this:

// link getPersonByGroup().get("men") with p:
Person p = getPersonByGroup().get("men");

but I was wrong.

So the only solution is way 2.

[Campbell Ritchie]

That latter code snippet gets out of your Map what you earlier put in under "men".

I presume you are getting the Map as a return value from the getPersonByGroup() method, in which case that method is very inappropriately named. Why isn't the Map a field of the class?

[nimo frey]

Sorry I have write the method not completly:

The map "personByGroup" is a field of a Object called Users:

public class Users{

Map<String, List><Persons>> personByGroup;
//getter/setter
}

and p is obtained by this reference:

Person p = this.users.getPersonByGroup().get("men"); 

So you see the Map is a field of the class Users.

[Campbell Ritchie]

That shows how confusing it is if you don't post real code. And I still don't like personByGroup as a field name.

[Sean Clark]

Hey,
I think you might be best to post both the Person and User classes so we can see how they link. That's just confusing.