Signatures
HLine paintOn messages in HLine unit test
(main)
line 116: hline1.paintOn(screen)
line 118: hline1.paintOn(screen, 0, 1)
Two declarations for paintOn in HLine.java:
line 45: paintOn(Screen, int, int)
line 58: paintOn(Screen)
   delegates work to first paintOn
JVM uses shape of message to select method
 Signature: method name & types of parameters