Java collections covariance problem

Question

Lets say we have a program which contains such classes:

public interface AbstractItem {
}
public SharpItem implements AbstractItem {
}
public BluntItem imple         


        

Answer 1

Here are a couple of extra ideas. Leave everything the same, but use this:

interface AbstractToolbox {
    public List getItems();
}

This basically says that the abstract class' items are an unknown type, but subclasses can make it concrete. This would require you to call getItems() on a reference of type ExpensiveToolbox or CheapToolbox to be able to retrieve a list that allows you to add items, etc.

ExpensiveToolbox toolbox = new ExpensiveToolbox();
AbstractToolbox absTB = toolbox;

List items1 = absTB.getItems(); //fine
List items2 = absTB.getItems(); //compile error
List items3= toolbox.getItems(); //fine

Alternatively, you could just type AbstractToolbox:

public interface AbstractToolbox {
    public List getItems();
}
public ExpensiveToolbox implements AbstractToolbox {
    public List getItems() { //...
}