It might sound like a dumb question, but all anonymous classes must be defined and instantiated within an existing class; therefore, they must be inner classes at the same time.
This is quite true. Your anonymus class could not be implemented outside of other classes, as a seperate class, because as it is anonymus you would not be even able to refer to it in any way.
Additional information: From JLS: An anonymous class is always an inner class (§8.1.3); it is never static (§8.1.1, §8.5.1).
Anonymous classes are types of inner classes. See http://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html
Quoting from there:
"You can also declare an inner class within the body of a method without naming the class. These classes are known as anonymous classes."
来源：https://stackoverflow.com/questions/20777970/are-anonymous-classes-a-subset-of-inner-class