Generics Compile Problem

[petek1976]

Can anyone tell me why this code will not compile? I'm only including
the relevant portions of the code. I don't understand the error from
the compiler:

public class Msg
{
}

public interface MsgListener<T>
{
public void Process(T msg);
}

import java.util.*;
public class MsgRegistration
{
private HashMap<String,MsgListener<? extends Msg>> mcMap = new
HashMap<String,MsgListener<? extends Msg>>();

public <X extends Msg> void doit(String acName, X acMsg)
{
MsgListener<? extends Msg> lcListener= mcMap.get(acName);
lcListener.Process(acMsg);
}

}

MsgRegistration.java:10: Process(capture of ? extends Msg) in
MsgListener<capture of ? extends Msg> cannot be applied to (X)
lcListener.Process(acMsg);

Why can't it be applied to X since X extends Msg?

 

[Daniel Pitts]

Can anyone tell me why this code will not compile? I'm only including
the relevant portions of the code. I don't understand the error from
the compiler:

public class Msg
{
}

public interface MsgListener<T>
{
public void Process(T msg);
}

import java.util.*;
public class MsgRegistration
{
private HashMap<String,MsgListener<? extends Msg>> mcMap = new
HashMap<String,MsgListener<? extends Msg>>();

public <X extends Msg> void doit(String acName, X acMsg)
{
MsgListener<? extends Msg> lcListener= mcMap.get(acName);
lcListener.Process(acMsg);
}

}

MsgRegistration.java:10: Process(capture of ? extends Msg) in
MsgListener<capture of ? extends Msg> cannot be applied to (X)
lcListener.Process(acMsg);

Why can't it be applied to X since X extends Msg?

The problem is that "lcListener" is of type "? extends Msg", which
might mean its a "Y" which extends Msg. X objects wouldn't be valid in
that case...
What you want is MsgListener<Msg>
Actually, it looks like you don't even want generics in this case. if X
is a subclass of Msg, you can just use polymorphism.

 

[Lew]

Method and variable names should start with a lower-case letter for readability.

- Lew