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Generics and supertypes

843793 Member Posts: 41,732 Green Ribbon
edited Apr 30, 2010 8:11AM in Generics

I have an issue with Generics and types, I hope I may find some help here.

I have the following map :
protected static HashMap<String, Action<EndpointClient>> dictionary = 
    new HashMap<String, Action<EndpointClient>>();
In this map I want to put :
protected static final Action<EndpointClient> ACTION_SEND_PRIVATE = new ActionSendPrivateMessage();
protected static final Action<EndpointClientAdmin> ACTION_SEND_TO_ALL = new ActionSendMessageToAll();

dictionary.put(Constants.MSG_SEND_PRIVATE, ACTION_SEND_PRIVATE);
dictionary.put(Constants.MSG_SEND_TO_ALL, ACTION_SEND_TO_ALL);
I have a problem in adding the ACTION_SEND_TO_ALL variable in the map, it says :
The method put(String, Action<EndpointClient>) in the type+
HashMap<String,Action<EndpointClient>> is not applicable for the arguments (String, Action<EndpointClientAdmin>)+

However EndpointClientAdmin extends EndpointClient ...

In java it's possible to to do something like:
List myList = new ArrayList();
and thus reduce the visibility of the class ArrayList.

Why isn't it possible with Generics like this case above ?

Thank you very much ...


  • EJP
    EJP Member Posts: 32,943 Gold Crown
    protected static HashMap<String, Action<EndpointClient>> dictionary = 
    new HashMap<String, Action<EndpointClient>>();
    protected static Map<String, Action<? extends EndpointClient>> dictionary = 
        new HashMap<String, Action<EndpointClient>>();
  • 843793
    843793 Member Posts: 41,732 Green Ribbon
    Thanks again ejp for your help !

    It seems that now in my code I have to put anywhere :
    Action<? extends Client>

    What confuses me is that i can't do something like:
    public Action<? extends Client> getAction(String command) {
    Action<EndpointClient> action = directory.getAction(command);
    (of course EndpointClient is extending Client, both are itnerfaces)

    Or is it a normal thing to have anywhere something like Action<? extends Client>

    Thank you.
  • 843793
    843793 Member Posts: 41,732 Green Ribbon
    edited Apr 9, 2010 3:40PM
    So I replaced by <? extends Client> (EndpointClient and EndpointClientAdmin are both extending Client)
    But I have a new error generated :
    public MessageProcessor(String data, String command, EndpointClient client) {
    // some code
    Action<? extends Client> action = dictionary.getAction(command);
    action.execute(listFields, client);  // Compliation error
    The method execute(LinkedList<String>, capture#3-of ? extends Client) in the type+
    + Action<capture#3-of ? extends Client> is not applicable for the arguments (LinkedList<String>,+
    + EndpointClient)+

    The dictionary is a map containing Action<EndpointClient> and Action<EndpointClientAdmin> elements. I used here Generics because I have in the class Action a function containing a reference to EndpointClient OR EndpointClientAdmin (so the Generics would allow me to avoid some casting)
    protected static HashMap<String, Action<? extends Client>> dictionary = new HashMap<String, Action<? extends Client>>();
    (EndpointClient and EndpointClientAdmin both are interfaces extending Client interface)

    Here is the code of the abstract class Action and two sample subclasses :
    // Action abstract class
    public abstract class Action<C extends Client> {
        public abstract void execute(LinkedList<String> fields, C client);
    // ActionValidateConnection class
    public class ActionValidateConnection extends Action<EndpointClient> {
        public void execute(LinkedList<String> fields, EndpointClient client) {
    // ActionEcho class
    public class ActionEcho extends Action<EndpointClientAdmin> {
        public void execute(LinkedList<String> fields, EndpointClientAdmin client) {
    If I replace
    Action<? extends Client> action = dictionary.getAction(command);
    Action<EndpointClient> action = dictionary.getAction(command);
    It works, of course I'll have an error because getAction returns <? extends Client>, even though EndpointClient extends Client

    Thanks for the help.
  • 843793
    843793 Member Posts: 41,732 Green Ribbon
    It seems you have a Problem in the design...

    In the first code sniplet, suppose the passed command matches a Action<EndpointClientAdmin>, what is it supposed to do with a client of type EndpointClient? It's execute(LinkedList<String> fields, EndpointClientAdmin client) method cant handle this type.
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