Overriding a method with Generic Parameters in Java?

generics, inheritance, java

Solution

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?

Problem

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`?

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