An inner class is referenced using the name of its enclosing class or interface, followed by a period, followed by the inner class name.
For example, if an inner class named Inner is declared inside a class named Outer, it is referenced as Outer.Inner. However, there is a catch in this naming scheme. The actual class name used by the JVM is Outer$Inner. This naming scheme was used to minimize the changes required to the JDK 1.02 virtual machine in order to handle inner classes. Inner classes fall under the same restriction as top-level classes in that they may not take the name of a package.