Given a complete UML class diagram of any Java programm $($i$.$e$.$ the inheritance hierarchy up to and including Object is explicitly repre$-$

sented$),$ consider all class boxes as nodes and all lines as edges of a graph.

In such a diagram, if you draw...

$...$ only $($undirected$)$ extends lines then you get exactly one tree.

$...$ only $($undirected$)$ implements lines then you get a connected graph.

$...$ only $($directed$)$ implements arrows then you get a graph that can contain cycles.

$...$ only $($directed$)$ extends and implements arrows then you get a directed acyclic graph.