Why would a class be unable to be cast to an interface it implements?

Question

I'm trying to get an application to work on websphere, and eclipse shows I have no errors. Despite that, when I try to start the application I get an error saying the Web application failed to start because org.apache.myfaces.ee6.MyFacesContainerInitializer can't be cast to the interface javax.servlet.ServletContainerInitializer. I did some digging and found that MyFacesContainerInitializer implements ServletContainerInitializer, so I know it's not a problem of this being a subclass that's expected to implement it but doesn't. As far as I can see there's no reason this should fail. What am I missing?

If it helps, here is what I get when I try to run the app.

[ERROR   ] SRVE0015E: Failure to initialize Web application MEFILESWeb
java.util.ServiceConfigurationError: javax.servlet.ServletContainerInitializer: Provider org.apache.myfaces.ee6.MyFacesContainerInitializer could not be instantiated: java.lang.ClassCastException: Cannot cast class org.apache.myfaces.ee6.MyFacesContainerInitializer to interface javax.servlet.ServletContainerInitializer
  at java.util.ServiceLoader.fail(ServiceLoader.java:235)
  at java.util.ServiceLoader.access$100(ServiceLoader.java:192)
  at java.util.ServiceLoader$LazyIterator.next(ServiceLoader.java:381)
  at java.util.ServiceLoader$1.next(ServiceLoader.java:449)
  at com.ibm.ws.webcontainer.webapp.WebApp.initializeServletContainerInitializers(WebApp.java:2058)
  at com.ibm.ws.webcontainer.webapp.WebApp.initialize(WebApp.java:743)
  at com.ibm.ws.webcontainer.webapp.WebApp.initialize(WebApp.java:5779)
  at com.ibm.ws.webcontainer.osgi.DynamicVirtualHost.createRunnableHandler(DynamicVirtualHost.java:249)
  at [internal classes]

As Sheetal said, the problem was that I was missing some files in my WebSphere library. Once they were put in, the problem is gone.

Answer 1

There were some .object and .properties files missing from my websphere folder. When I put them in the project runs fine. The specific files were visible in eclipse under the project folder. This answer will be accepted as soon as it can be, no more answers are necessary.