Answer 1

Nested classes are divided into two categories.

Static:
A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class.

class OuterClass {
...
static class StaticNestedClass {
...
}
}

Non-static(inner):
An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.

class OuterClass {
...
class InnerClass {
...
}
}