如何扩展列表中包含的对象？

Question

I have a List named resourceItems which containes ResourceItem objects.

    public class ResourceItem {
        private Long id;
        private String name;
        public ResourceItem(Long id, String name) {
            this.id = id;
            this.name = name;
        }
        // getters and setters...
    }

    public class SomeClass {
        private List<ResourceItem>  resourceItems = FindAllResourcesWebSerbice();
    }

I would like to extend the objects in the List to include a boolean field named selected.

I've tried several variations of classes that extends ResourceItem (see below) including options using generics, but have not been successful. I would love a solution that uses generics for reuse.

    public class ExtendedResourceItem extends ResourceItem {
        private boolean selected = false;
        public ExtendedResourceItem() {
        }

        public boolean isSelected() {
            return selected;
        }

        public void setSelected(boolean selected) {
            this.selected = selected;
        }
    }

    public class SomeClass {
        private List<ResourceItem>  resourceItems = FindAllResourcesWebSerbice();
        private List<ExtendedResourceItem> extendedResourceItem = resourceItems;
    }

Any help is much appreciated.

Answer 1

Assuming ResourceItem implements equals and hashCode correctly, why not just keep a set of the ones that are selected:

Set<ResourceItem> selectedResourceItems = new HashSet<ResourceItem>();

Then you could add an item to the set when it's selected, remove it when it's deselected, and check to see if the set contains one when you need to know if it's selected.

This solution is very similar to Michael Myers' comment about using a Map<ResourceItem, Boolean> .

Answer 2

You should use composition, not inheritance here. From your description it sounds like ResourceItem is a domain object, but on screen, you should wrap it

class ScreenResourceItem {
    ResourceItem item;
    boolean selected;
}

Create a new list of ScreenResourceItem and put all the wrapped ResourceItem s in it.

Generics and list covariance are not relevant here.

Answer 3

But using your snippet:

private List<ResourceItem> resourceItems = ... \\ some assignment;
private List<ExtendedResourceItem> extendedResourceItems = resourceItems;

is absolutely wrong. Since not any ResourceItem is guarantee to be also ExtendedResourceItem. (resourceItems list can contain objects that are ResourceItem but are NOT ExtendedResourceItem).

I don't see the point you want to reach, but if You want to transform resourceItems into list of resourceItems that have selected attribute, what about this concept:

public class ExtendedResourceItem {
    private ResourceItem item = null;
    private boolean selected = false;

    public ExtendedResourceItem(ResourceItem item, boolean selected) {
        this.item = item;
        this.selected = selected;
    }

    // ... getters and setters
}

public class SomeClass {
    private List<ResourceItem> resourceItems = FindAllResourcesWebSerbice();

    private List<ExtendedResourceItem> extendedResourceItems = new ArrayList<ExtendedResourceItem>();
    for (ResourceItem item: resourceItems) {
        extendedResourceItems.add(new ExtendedResourceItem(item, false));
    }
}

Answer 4

Is it the generics limits that are confusing? Try using the Upper Bounded Wildcards , eg

public class SomeClass {
    private List<ExtendedResourceItem> extendedResourceItems = FindAllResourcesWebSerbice();
    private List<? extends ResourceItem> resourceItems = extendedResourceItems;
}

Alternatively:

public class SomeClass {
    private List<? extends ExtendedResourceItem> extendedResourceItems = FindAllResourcesWebSerbice();
    private List<? extends ResourceItem> resourceItems = extendedResourceItems;
}

More information about wildcards Wildcards and Subtyping .