ZShape is not static so it requires an instance of the outer class. The simplest solution is to make ZShape and any nested class static if you can. I would also make any fields final or static final that you can as well.
static class Thing will make your program work. As it is, you’ve got Thing as an inner class, which (by definition) is associated with a particular instance of Hello (even if it never uses or refers to it), which means it’s an error to say new Thing(); without having a particular Hello instance in scope. If you declare it as a static class instead, … Read more