具有接口上限的设置器，但没有通配符的获取器

Question

As recommended by Sonar rule RSPEC-1452 ,

Generic wildcard types should not be used in return types

How can I define setter method of a property that accepts a List<? extends Item> List<? extends Item> , but getter method that returns List<Item> ?

Consider the below contract of Cart interface:

interface Cart {
    public List<Item> getItems();
}

interface Item {
    public int getCost();
}

Clients of this contract do not need to know about any known implementations of Cart or Item interfaces, eg

void getValue(Cart cart) {
    int sum = cart.getItems().stream().mapToInt(Item::getCost).sum();
}

However, the library that implements the Cart interface obviously has to deal with concrete classes. Assume it (or its Builder ) also has to expose a setItems method so that another internal class can set the entire list of items. Upper bound of Interface allows to pass a List<DefaultItem> to setItems method.

static class DefaultCart implements Cart {

    private List<Item> items;

    public void setItems(List<? extends Item> items) {
        this.items = new ArrayList<Item>(items);
    }

    @Override
    public List<Item> getItems() {
        return items;
    }
}

I am not happy with the above setItems method as it creates another copy of the list. What is a better way of dealing with this?

Answer 1

How can I define setter method of a property that accepts a List<? extends Item> List<? extends Item> , but getter method that returns List<Item> ?

You can't, because List<? extends Item> List<? extends Item> is not a subclass of List<Item> , ie they are not assignment compatible with each other.

For explanation, see: Is List<Dog> a subclass of List<Animal>

What you can do, is to make your class generic, so you don't use wildcard:

static class DefaultCart<E extends Item> implements Cart {

    private List<E> items;

    public void setItems(List<E> items) {
        this.items = items;
    }

    @Override
    public List<E> getItems() {
        return items;
    }
}