Java, cast HashMap elements to its ancestor

Question

public class Depot extends Item2
{
}

public class Station extends Item2
{
}

private final HashMap<Integer, Depot> depots = new HashMap<>();
private final HashMap<Integer, Station> stations = new HashMap<>();

both depots and stations stores something which has Item2 base. Now I want to pass to a function those:

Item2 item;
canBeAdded(item, depots);
canBeAdded(item, stations);

private boolean canBeAdded (Item2 item, HashMap<Integer, Item2> items)
{

sadly its not good, at canBeAdded it says

"Incompatible types: HashMap <Integer, Depot> cannot be converted to <Integer, Item2>"

oh, cool, and what is this? Or what is its name? Im not expert in Java — John Smith, Oct 03 '16 at 10:10
When using use `` in a generic it means it must be that type. If you use `? extends Item2` it means anything which extends that type. — Peter Lawrey, Oct 03 '16 at 10:13

score 4 · Accepted Answer · edited May 23 '17 at 12:19

4

You could use generic wild card here.

E.g:

private boolean canBeAdded(Item2 item, HashMap<Integer, ? extends Item2> items) {

For more info, Please visit these threads : Java Generics (Wildcards) and When do Java generics require <? extends T> instead of <T> and is there any downside of switching?

edited May 23 '17 at 12:19

Community

1
1

answered Oct 03 '16 at 10:10

Sachin Gupta

7,805
4
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score 1 · Answer 2 · answered Oct 03 '16 at 10:13

Change your signature to this:

private static boolean canBeAdded(Item2 item, HashMap<Integer, ? extends Item2> items) {

HashMap<Integer, Depot> is not a sub class of HashMap<Integer, Item2> but you can specify that any sub class is allowed like that ("wildcard"). Making it less specified it you need to use it though of course.

Java, cast HashMap elements to its ancestor

2 Answers2