Are Inner Classes lightweight?

Question

Are inner classes more lightweight than normal classes, or in the end java compiles inner classes just like normal classes?

I know classes in java are not all very lightweight themselves, and they occupy part of the permgen memory, so I'd like to know if it's best to use closure-like functions as inner classes, or if standard classes would also do fine?

Answer 1

Inner classes and anonymous inner classes both compile down to .class files. For example:

class Outer {
     class Inner {

     }

     Object function() {
          return new Object() {

          };
     }
}

Will generate three .class files, Outer.class, Outer$Inner.class, and Outer$1.class. They aren't more "lightweight" than other classes, and (to the best of my knowledge) there's no advantage to using one over the other from a performance perspective. Of course, inner classes and especially anonymous inner classes are really useful in contexts where regular classes are harder to code, but that's a separate issue.