The java.util.EventObject class is the heighest-level class in the event-delegation model class hierarchy.
When a class level object is added it responds to the world
An inner class declared as static is treated as if it were a normal top-level class.
a class level deals with a single object in a particularly group but the world-level method deals with more than one object in a particular group.
There are 2 main types of Hierarchical Inheritance - Single and Multi Level Class A extends Class B - Single Class A extends Class B which in turn extends Class C - Multi level. Actually there is no limit to the number of levels till which you can inherit classes in multi level inheritance. But it is preferable to keep it at around 3 or 4 for ease of maintenance and understanding.
The invert level is the base interior level of a pipe, trench or tunnel; it can be considered the "floor" level. Conversely, the crown level is the highest interior level, and can be considered the "roof" level