Question

I’m following the spring tutorial.

In section “3.2. Add some classes for business logic” an interface ProductManager is created:

package springapp.service;

import java.io.Serializable;
import java.util.List;

import springapp.domain.Product;

public interface ProductManager extends Serializable{

    public void increasePrice(int percentage);

    public List<Product> getProducts();

}

Then a SimpleProductManager implementation class is created:

package springapp.service;

import java.util.List;

import springapp.domain.Product;

public class SimpleProductManager implements ProductManager {

    public List<Product> getProducts() {
        throw new UnsupportedOperationException();
    }

    public void increasePrice(int percentage) {
        throw new UnsupportedOperationException();        
    }

    public void setProducts(List<Product> products) {
        throw new UnsupportedOperationException();        
    }

}

The implementation class adds an extra method setProducts(). Should the interface ProductManager not also have a setProducts method to allow classes which use setProducts to instantiate SimpleProductManager polymorphically. Currently this is not possible –

ProductManager p = new SimpleProductManager();
p.setProducts();

Answer 1

The interface does not include setProducts because the clients of that interface (probably an MVC controller) are not supposed to call it. The interface defines only those operations that clients are supposed to use, rather than defining all of the methods that the implementation may have.

The setProducts method will be accessible to the beans configuration (e.g. using <property name="products">), which allows the products to be statically configured at start up. After that, client code refers to the bean via its restricted interface.

Your p.setProducts() example should never be called in this example, since the products are only configured in the beans config, not by business logic.