Question

I have to read a binary file. I have to read two type of object “Company” and “Person”. Those object are represented by the two classes: Company and Person. Those classes are derived from “User” classes.
When I read this file I have two put the two object “Company” and “Person” into two different HashMap. My professor told me I have to use the keyword instanceof but I don’t understand how use it.

            try
    {
    ObjectInputStream reader=new ObjectInputStream(new FileInputStream(fname));

        while(reader.available()>0)
        {
            User obj=reader.readObject();
                            if(obj istanceOf Company)
                            ...
                else if(obj istanceOf Person)

                     }



    }
    catch(FileNotFoundException ffe)
    {
        System.err.println("Error: the file was not found!");
    }
    catch(IOException ioe)
    {
        ioe.printStackTrace();
    }

is it right?

Answer 1

Try something like this:

if (obj instanceof Company) {
    Company company = (Company) obj;
    companies.put(someId, company);
} else if (obj instanceof Person) {
    Person person = (Person) obj;
    persons.put(someId, person);
}

In the above code, obj is the Object you read from the file, assuming that you don’t know its type. Also I’m assuming that you have a Map of companies called companies and a Map of persons called persons, and in either case you must have some sort of identifier (I’m calling it someId) to use as a key for storing the object in the corresponding map.