Question

Can’t find a satisfactory answer anywhere.

Answer 1

All top-level classes are, by definition, static.

What the static boils down to is that an instance of the class can stand on its own. Or, the other way around: a non-static inner class (= instance inner class) cannot exist without an instance of the outer class. Since a top-level class does not have an outer class, it can’t be anything but static.

Because all top-level classes are static, having the static keyword in a top-level class definition is pointless.

Some code to play around with:

public class Foo {

    public class Bar {
         // Non-static innner class
    }

    public static class Baz {
         // Static inner class
    }
}

public class Example {
    public static void main(String[] args) {
        new Foo(); // this is ok
        new Foo.Baz(); // this is ok
        new Foo.Bar(); // does not compile!

        Foo f = new Foo();
        Foo.Bar bar = f.new Bar(); //this works, but don't do this
    }
}

I put the “but don’t do this” in there because it’s really ugly code design. Instance inner classes should not be visible outside the outer class. They should only be used from within the outer class.