Java classes and constructors

Question

I hope this won't sound like a stupid question.

I have a class A and a class B. Now the only thing B has is a String ID. Now I can have multiple objects A that each can have 0 or more objects of type B. These are contained in a HashMap(ID,someData) each class A has.

What I want to do is each time I add a new ID in class A to check whether there already is a on object of Type B with the same ID in any of the other class A objects I have, and if not, create a new B object.

Hope this makes sense. If what I ask is wrong is some way, please be kind enough to explain how is this a bad practice or conceptualy wrong.

Thanks a lot.

EDIT:To be more clear,in my case, it is not desireable to share the HashMap(ID,someData) for all my objects as for example let's say A is a Course class,Or Catalogue, or Bank etc. Each A class may share some students/clients but each A class may contain different class B objects.

Luchian Grigore · Accepted Answer · 2012-03-05 10:00:21Z

4

It's not bad practice, it seems like you're implementing the Instance Manager Pattern.

Ability to look up one or more instances (via a key) of the managed object(s). If only one managed object, then a key is not necessary.

answered Mar 5, 2012 at 10:00

Luchian Grigore

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Peter Lawrey · Accepted Answer · 2012-03-05 10:00:51Z

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The thing you need to do this is to have a collection of all A objects so you can check them all. As you get more A objects this will become more inefficient. A diiferent was to structure this might be to use the ID as you collection.

Map<ID,/* class with A and someData *> map = ...

This allows you to ensure uniqueness of ID across all A.

answered Mar 5, 2012 at 10:00

Peter Lawrey

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Murat Derya Özen · Accepted Answer · 2012-03-05 10:01:47Z

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To achieve your purpose, you can make the HashMap a static variable, instead of an instance variable. Therefore, it will be shared among all instance of Class A.

answered Mar 5, 2012 at 10:01

Murat Derya Özen

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kgiannakakis · Accepted Answer · 2012-03-05 10:05:26Z

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If I were you I would have created a B_Factory class. This class would be responsible for creating the B objects. A class objects would call B_Factory methods to add or delete a B object. Inside B_Factory, you could use a Map to store the instances of B objects.

There should be only one instance of B_Factory class, which will be injected in all A instances.

answered Mar 5, 2012 at 10:05

kgiannakakis

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gyurix · Accepted Answer · 2015-08-16 08:22:44Z

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You should loop through all your A instances and use Hashmaps containsKey method to check that the new B value is already stored or not. If you only need to store all the B values to every A class only once, you can add a static modifier for your HashMap field.

answered Aug 16, 2015 at 8:22

gyurix

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