You can mimic multiple inheritance by implementing several interfaces. The problem with diamond inheritance Jesper Young mentioned doesn't apply if you have two methods with the same signature, if both methods are completely empty.
You can implement several interfaces and only extend one class. An interface provides a set of method headers; any method using your object under the name of its interfaces can rely on those methods being available, particularly if you have implemented them according to the instructions for the interface.
Read the API documentation for Comparable<T> for an example.
subject: what are the advantages of implementing interface over extending class ???