为什么main方法中无法实例化非静态内部类

No enclosing instance of type close is accessible. Must qualify the allocation with an enclosing instance of type close (e.g. x.new A() where x is an instance of close).

对main方法而言，它是游离于任何类之外的，因此某类的非静态内部类对它而言是不直接可见的，也就无法直接访问，like this:

enclosingInstance = new EnclosingClass();
nestedInstance = enclosingInstance.new NestedClass();