Java error: Found interface ... but class was expected

Question

I am getting a strange runtime error from my code:

"Found interface [SomeInterface] but class was expected"

How can this happen? How can an interface get instantiated?

Update: (In response to some answers) I am compiling and running against the same set of libraries, but I am using Guice to inject a Provider for this particular Interface.

The problem went away when I bound an implementation to the interface (seems like the @ImplementedBy annotation was not enough).

I was more interested in the mechanics through which Guice managed to actually instantiate an interface.

Answer 1

It sounds like you did

class MyClass extends SomeInterface

when it should actually be

class MyClass implements SomeInterface

Am I right?

EDIT: Oh, you say it's a runtime error and not a compile-time error? Let me look around a bit...

EDIT 2: It looks like Jared has the correct answer. Anyway, trying to extend an interface would actually give a "no interface expected here" message at compile time, not a "found interface but class was expected" error.

Answer 2

This happens when your runtime classpath is different than your compile time classpath.

When your application was compiled, a class (named SomeInterface in your question) existed as a class.

When your application is running at compile time, SomeInterface exists as an interface (instead of a class.)

This causes an IncompatibleClassChangeError to be thrown at runtime.

This is a common occurence if you had a different version of a jar file on the compile time classpath than on the runtime classpath.

Answer 3

Most likely the code was compiled against a class in a library, which was then changed to an interface in the version you run against.