I have an interface defined as
interface ListItem {
public String toString();
public String getUUID();
}
And a class (BrowseItem) implementing that interface. When I try:
ArrayList<ListItem> = (method returning ArrayList of type BrowseItem)
I get an incompatible type error (found ArrayList<BrowseItem>, require ...<ListItem>)
Am I approaching this wrong?
Java generics are not covariant.
See (among many other questions on SO):
Solutions:
Change the return type of the problematic method. That is, change
List<listItem> = (method returning List of type browseItem)
// to
List<listItem> = (method returning List of type listItem)
Use wildcard covariance (I think that's what this is called):
List<? extends listItem> = (method returning List of type browseItem)
Be aware that you cannot add items to the list if you take this route.
N.B. it is generally good practice to declare list types as List<T> and not ArrayList<T>. The pseudocode above reflects this.
your example doesnt work, but you can use
ArrayList<? extends ListItem> list = (method returning ArrayList of type browseItem)
this should work.
ArrayList<listItem> is not equal to ArrayList<browseItem>
They are strictly type safe
you can make use of ?
