Messages define a standard interface to objects. Objects may have different internal implementations as long as the message interface is maintained.

Example: Vector (send v x)

• Vector type 1: x is stored.

  ( 187., 111.)(0,0) ( 29., 0.) ( 200., 111.) ( 0., 82.) ( 29., 29.) ( 10., 92.) x ( 101., 81.) x ( 102., 53.) y ( 29., 97.) (5,-1) 64.

• Vector type 2: r and theta are stored. x is computed as r * cos(theta)

( 187., 111.)(0,0) ( 29., 0.) ( 200., 111.) ( 0., 82.) ( 29., 29.) ( 10., 92.) x ( 58., 43.) ( 51., 43.) ( 31., 97.) (5,-6) 27. ( 144., 86.) r ( 144., 51.) theta ( 138., 84.) (-3,-1) 29. ( 138., 56.) (-1,0) 29.

The two kinds of vectors appear the same to the outside world.

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