About java default package(using Eclipse)

Now, I'm using some methods by using a jar. But these classes I use have no package names. I can use reflection to solve the problem of using these methods. But now I want to override some of the interface and classes. However,others can extends these classes directly without any compilation error on their computers using Elcipse, while I extends directly, I have this compilation error:(classname) cannot be resolved to a type. Is there any chance configuration on my Elcipse is different from others? I'm so confused now...