Java Classes and Constructors

Question

Hope this doesn't seem like a silly question.

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

What I want to do is every time I add a new identifier to class A to check if there is already an on object of type B with the same identifier in any of the other objects of class A that I have, and if no, create a new object B.

Hope this makes sense. If what I ask is wrong anyway, please be kind enough to explain how this is bad practice or conceptually wrong.

Many thanks.

EDIT: to be more clear, in my case it is not advisable to share a HashMap (ID, someData) for all my objects, for example, let them say that A is a course class, or a directory or a bank, etc. Each Class can be shared by some students / clients, but each Class A can contain different objects of class B.

Answer 1

Good practice, it looks like you are implementing an instance template .

• The ability to search for one or more instances (via the key) of the managed object (s). If there is only one managed entity, then the key is not needed.

Answer 2

What you need to do is collect all objects A so that you can check them. When you get more A objects, it will become more inefficient. It was diverse to structure this; perhaps it would be to use an identifier as a collection.

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

This allows you to guarantee the uniqueness of ID for all A.

Answer 3

To achieve your goal, you can make the HashMap variable a static instead of the instance variable. Therefore, it will be used for all instances of class A.

Answer 4

If I were you, I would create a B_Factory class. This class will be responsible for creating the B objects. The class objects will call the B_Factory methods to add or remove the B object. Inside B_Factory you can use a map to store instances of the B objects.

There must be only one instance of the B_Factory class, which will be introduced in all instances of A.

Answer 5

You should skip all instances of A and use the Hashmaps containsKey method to verify that the new value of B is already stored or not. If you only need to save all B values ​​for each class A only once, you can add a static modifier for your HashMap field.