Can one control the identity of objects in Java, and if so, how?

Question

This is a trivial programming question. I am not an expert in Java. Say I use objects of custom classes Company and Employee, in a manner similar to what many RDBMS examples do:

class Employee
{
    Company company;
}

class Company
{
    String name;
}

I need to guarantee that different Company objects have unique names - i.e. no two such objects may have the same name, because from my point of view it makes no sense, and also simply eats memory - if two employees work at IBM, then there is a single Company object with that name, period.

My thoughts right now go along of making Company constructor private - so that the job of allocating Company objects with arbitrary names is delegated to a trusted method - which, suppose, will reject any subsequent attempt to create an object with a name that already exists or return an existing or new object (creating one if necessary).

The problem is, I am not sure how to accomplish this elegantly. One thing that would be nice is not having to do a O(n) lookup every time an Company object with a name is requested - so maybe a hash map or a binary tree is there for my convenience? I would also like to override the way the Company objects are identified - which leads me to this: will I be overriding Object.equals and/or Object.hashCode methods?

Answer 1

Take a look at the flyweight pattern.

What I would do is something like:

// incomplete, but you get the idea hopefully
CompanyFactory
{
    private Map<String, Company> companies;

    public getCompany(final String name)
    {
        Company company;

        company = compaines.get(name);

        if(company == null)
        { 
            company = new Company(name);
            companies.put(name, company);
        }

        return (company);
    }
}

Answer 2

You could override equals and hashCode and store them in a HashMap or HashSet.

Answer 3

One thing that would be nice is not having to do a O(n) lookup every time an Company object with a name is requested - so maybe a hash map or a binary tree is there for my convenience?

That sounds right, yes.

I would also like to override the way the Company objects are identified - which leads me to this: will I be overriding Object.equals and/or Object.hashCode methods?

If you ensure that there are never two instances with the same key value, you actually don't have to do that.