Rollerskate Example

Here is an example of how to draw a class diagram and and object diagram to model the following example scenario of people rollerskating.

Here's what we want to model:

There should be people who can rolerskate about a skating rink
The rink should have a maximum capacity, and should be able to have more people join in
We should be able to tell if no one is skating
These people should all have names
Everyone skating should have on two skates
skates have four wheels and a color
the skates also have a color

Here is what the class diagram could look like:

Now imagine that somewhere in the middle of our computation, the a rollerskating rink has ten skaters, one of whom is named Ellen. Ellen is wearing one green skate and one blue skate. Here is what the object diagram would look like: