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?

Answer 1

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 abtract 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?