Can classes and interfaces be unloaded?

An implementation of the Java programming language may unload classes. A class or interface may be unloaded if and only if its defining class loader may be reclaimed by the garbage collector as discussed in JLS 12.6.1.

It is not true that any class may be unloaded when none of it's instances and class objects that represent this class are reachable. Classes and interfaces loaded by the bootstrap loader may not be unloaded.

For more information, please see "12.7 Unloading of Classes and Interfaces" in Java Language Specification 3rd Edition (JLS).