Implementation of factory design pattern missing link

Question

I've read the following article in this link.

Please refer to section Class Registration - avoiding reflection only.

In this example I cannot get the meaning of the following code line:

static
{
    ProductFactory.instance().registerProduct("ID1", new OneProduct());
}

The issues that are not clear to me:
1.Where is the method instance() defined?

2.Is instance method should be static, if so how its implementation will look a like?
(return this isn't possible through static method)

*Please stick to the given example unless it is wrong, I'm trying to focus on one factory "recipe".

Answer 1

I am trying to answer your questions below:

1.Where is the method instance() defined?

The method is defined in the class ProductFactory, which is obvious.

2.Is instance method should be static, if so how its implementation will look a like?

Yes, it should be static. Please have a look at the definition of class ProductFactory and instance() method below:

public class ProductFactory extends Factory
{
    private static ProductFactory _instance;
    private HashMap m_RegisteredProducts = new HashMap();

    public static synchronized ProductFactory instance()
    {
        if (_instance == null)
            _instance = new ProductFactory();
        return _instance;
    }

    public void registerProduct(String productID, Product p)    {
        m_RegisteredProducts.put(productID, p);
    }

    public Product createProduct(String productID){
        ((Product)m_RegisteredProducts.get(productID)).createProduct();
    }
}