Overriding a method with Generic Parameters in Java?

Question

I have an abstract Class Monitor.java which is subclassed by a Class EmailMonitor.java.

The method:

public abstract List<? extends MonitorAccount> performMonitor(List<? extends MonitorAccount> accounts)

is defined in Monitor.java and must be overridden in EmailMonitor.java.

I currently have the method overridden in EmailMonitor.java as follows:

@Override
public List<EmailAccount> performMonitor(List<EmailAccount> emailAccounts) {
    //...unrelated logic
    return emailAccounts;
}

However, this produces the compile time error:

Name clash: The method performMonitor(List<EmailAccount>) of type EmailMonitor has the same erasure as performMonitor(Lis<? extends MonitorAccount> emailAccounts) of type Monitor but does not override it

EmailAccount is a subclass of MonitorAccount, so (in my mind at least) overriding it in this way makes perfect sense. Seeing as the compiler is not happy with my logic though, How should I go about this correctly while still keeping my compile time checks to make sure that all calls to EmailMonitor.performMonitor() receive Lists of EmailAccount rather than some other type of MonitorAccount?

score 38 · Accepted Answer · edited Mar 10 '23 at 11:34

No, it's not overriding it properly. Overriding means you should be able to cope with any valid input to the base class. Consider what would happen if a client did this:

Monitor x = new EmailMonitor();
List<NonEmailAccount> nonEmailAccounts = ...;
x.performMonitor(nonEmailAccounts);

There's nothing in there which should give a compile-time error given your description - but it's clearly wrong.

It sounds to me like Monitor should be generic in the type of account it can monitor, so your EmailMonitor should extend Monitor<EmailAccount>. So:

public abstract class Monitor<T extends MonitorAccount>
{
    ...
    public abstract List<? extends T> performMonitor(
        List<? extends T> accounts);
}

public class EmailMonitor extends Monitor<EmailAccount>
{
    @Override
    public abstract List<? extends EmailAccount> performMonitor(
        List<? extends EmailAccount> accounts)
    {
        // Code goes here
    }
}

You might want to think carefully about the generics in the performMonitor call though - what's the return value meant to signify?

You say 'List extends EmailAccount>'. Now I can't pass EmailAccount through at all; Was that a typo?. I wanted to check that EmailMonitor always receives a list of EmailAccount and always returns a list of EmailAccount that can be used without dynamic casting. posting my solution now :) — Robert Ngetich, Oct 27 '08 at 14:38

score 11 · Answer 2 · edited Jun 17 '15 at 14:49

Here is my own solution. I suspect this is the same thing Jon Skeet was trying to get at... without the typo (see my comment in reply to his answer).

the Monitor.java class:

public abstract class Monitor <T extends MonitorAccount> {
  ...
  public abstract List<T> performMonitor(List<T> accounts);
  ..
}

EmailMonitor.java

public class EmailMonitor extends Monitor<EmailAccount> {
  ...
  public List<EmailAccount> performMonitor(List<EmailAccount> emailAccounts) {
    ..//logic...logic...logic
    return emailAccounts;
  }
  ...
}

In this configuration, EmailMonitor.performMonitor() will always check at compile time that it receives a list of EmailAccount rather than any of my other types FTPAccount, DBAccount, etc... It's much cleaner than the alternative, which would have been receiving/sending a raw list and then having to coerce it the required type resulting in potential runtime type casting exceptions.

EmailMonitor does not extend MonitorAccount – newacct Jan 13 '13 at 20:45 — newacct, Jan 13 '13 at 20:45

Overriding a method with Generic Parameters in Java?

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