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In the following illustration, all arrows represent subclass–superclass relations:
*--------*
| Object | <-- Root
*--------*
^ ^
*--------* *---------* | Abstract classes
| | | tend to be in
*---* *-----* | the upper layers
| | | | |
*---* *-----* |
^^^ ^ ^ |
*--------*|*--------* | *------* |
| | | | | |
*---* *---* *---* *-----* *-----* |
| | | | | | | | | | .
*---* *---* *---* *-----* *-----* .
^ ^ ^ ^ ^ ^ ^ ^ ^ ^
*-* *-* *-* *-* *-* *-* *--* | *--* |
| | | | | | | | | | Final classes
*--* *--* *--* *--* *--* *--* *--* *--* *--* *-- must appear only
| | | | | | | | | | | | | | | | | | <--- as leaves
*--* *--* *--* *--* *--* *--* *--* *--* *--*