Java: Creating an Instance of a Subclass from the Superclass in a static method

Question

I am trying to create a new Instance of a subclass from my Super Class. This is my super class

public abstract class Worker {

    String world;

    protected abstract void onLoad(Scanner read);

    public static Worker load(Scanner read) {
        // I want to create the instance of my sub class here and call it w
        w.onLoad(read);
        return w;
    } 

    public void setWorld(String world) {
        this.world = world;
    }

}

And this is my subclass

public class Factory extends Worker {

    @Override
    protected onLoad(Scanner read) {
        setWorld(read.readline());
    }

}

And this is what I want to do with those classes.

public class MainClass{

    public List<Factory> loadFactories() {
        List<Factory> facts = new ArrayList<Factory>();
        Scanner read = new Scanner(new FileInputStream("factory.txt"));

        while(read.hasNextLine()) {
            Factory f = (Factory)Factory.load(read);
            facts.add(f);
        }

        read.close();
        return facts;
    }

}

Is there any way I can do this without starting over? Thanks for any help.

A good start if you want Factory to be a subclass of Worker is to write: class Factory extends Worker. — assylias
– assylias, Commented Nov 11, 2012 at 21:22
@user1816686 It is difficult to understand what you are trying to achieve. Factory.load(read) actually calls Worker.load(read) which returns a NoInputWorker which you assign to a Factory... Some pieces of the puzzle are still missing... — assylias
– assylias, Commented Nov 11, 2012 at 21:33

thedayofcondor · Accepted Answer · 2012-11-11 23:01:26Z

2

Is this what you want?

public static Worker load(Scanner read) {
    Factory w=new Factory();
    w.onLoad(read);
    return w;
}

Edit:

public class MainClass {

    public List<Factory> loadFactories() throws FileNotFoundException, InstantiationException, IllegalAccessException {
        final List<Factory> facts = new ArrayList<Factory>();
        final Scanner read = new Scanner(new FileInputStream("factory.txt"));

        while (read.hasNextLine()) {
            final Factory f = Worker.load(read, Factory.class);
            facts.add(f);
            final Pipeline p = Worker.load(read, Pipeline.class);
        }

        read.close();
        return facts;
    }

    static public class Factory extends Worker {

        @Override
        protected void onLoad(final Scanner read) {

        }

    }

    static public class Pipeline extends Worker {

        @Override
        protected void onLoad(final Scanner read) {

        }

    }

    static public abstract class Worker {

        String world;

        protected abstract void onLoad(Scanner read);

        public static <T extends Worker> T load(final Scanner read, final Class<T> t) throws InstantiationException, IllegalAccessException {
            final T w = t.newInstance();
            w.onLoad(read);
            return w;
        }

        public void setWorld(final String world) {
            this.world = world;
        }

    }
}

edited Nov 11, 2012 at 23:01

answered Nov 11, 2012 at 22:04

thedayofcondor

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8 Comments

Martin Wilson Over a year ago

Not sure why this has been down voted- it's the only answer if you take the question at face value!

thedayofcondor Over a year ago

To me it look like a perfect example of Factory design pattern! I cannot possibly interpret it in another way. Of course the main method should be Worker f = Worker.load(read);

ChrisixStudios Over a year ago

Yes but not exactly. Lets say I have another Worker class called Pipeline and I want to tell if it is a Pipeline or a Factory (without an enum).

thedayofcondor Over a year ago

like if (w instanceof Pipeline)... ?

thedayofcondor Over a year ago

Factory f = (Factory)Worker.load(read); could lead to problems - if you add the Pipeline class. A typical use is to return Worker, without letting the user know what class you are ACTUALLY passing back (allowing you to change it later)

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