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1 Introduction to Programming Bertrand Meyer Last revised 5 November 2004 Chair of Softw are Engineering I ntroduction to Programming Lecture 7 2 Lecture 7: References, assignment, and the object structure Chair of Softw are

  1. 1 Introduction to Programming Bertrand Meyer Last revised 5 November 2004 Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 2 Lecture 7: References, assignment, and the object structure Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Announcement: lab sessions 3 � Every Friday (12 November - 17 December 2004) � IFW D31, 12: 00 a.m. - 2: 00 p.m. � During the second hour there will be two assistants answering your questions Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 1

  2. Announcement: classroom exercise 4 � First classroom exercise in next week’s exercise session. � Either on Monday, 22 November 2004, or Tuesday, 23 November 2004. Your assistant will indicate your date. � There is no weekly exercise in this week. Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Reminder: field study 5 � You are encouraged to take part in Marie-Helene Ng Cheong Vee’s field study about learning programm ing. � Just subm it your Eiffel log files regularly. � For any questions contact your assistant. Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Classes and objects 6 � At run time: objects (software machines) � In the program text: classes Each class describes a set of possible run-time objects. Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 2

  3. Object structure 7 An object is made of fields Each field is a value, which is either: � A basic value: integer, character, “real” number... (known as an expanded value) � A reference to another object ( METRO_STOP ) x next upper_ left segment 23.5 ( SEGMENT ) y station lower_ right ( POSITION ) -34.9 ( METRO_STOP ) ( METRO_STATION ) ( POSITION ) Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Cycles in the reference structure 8 O1 name “Almaviva” landlord loved_one O3 O2 name “Figaro” “Susanna” name landlord landlord loved_one loved_one Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Strings are objects 9 O1 ‘A’ name “Almaviva” ‘l’ landlord ‘m’ ‘a’ loved_one ‘v’ ‘i’ ‘v’ ‘a’ The nam e field is a reference field Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 3

  4. Fields reflect attributes of the class 10 class An attribute POSITION feature – Access x: REAL -- Horizontal position y: REAL Another attribute -- Vertical position feature – Element change set_position (xval, yval: REAL) is -- Set coordinates to ( ` xval', ` yval' ). require x_positive: xval > = 0 y_positive: yval > = 0 do Attributes are features of x : = xval the class y : = yval ensure x_set: x = xval y_set: y = yval end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Setting fields (in routines of the class) 11 class POSITION feature – Access x: REAL -- Horizontal position y: REAL -- Vertical position feature – Element change set (xval, yval: REAL) is -- Set coordinates to ( ` xval', ` yval' ). require x_positive: xval > = 0 y_positive: yval > = 0 do x : = xval y : = yval ensure x_set: x = xval y_set: y = yval end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 What you may do 12 size class METRO_STATI ON feature x, y: REAL Station -- Coordinates of metro station size: REAL -- Size of bounding square bounding square upper_left: POSITI ON -- Upper-left position of bounding square adjust_positions is -- Set positions of bounding square do upper_left . set ( x – size/ 2, y + size/ 2 ) ... end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 4

  5. Feature calls 13 � A feature of class POSITION : set (xval, yval: REAL) is ... do ... end � From class POSITION itself: move (dx, dy: REAL) is -- Move by dx horizontally, dy vertically require ... [ Please com plete!] ... do Unqualified call set ( x + dx, y + dy ) ensure ... [ Please com plete!] ... end � From another class, e.g. METRO_STATION Qualified call adjust_positions is do upper_left . set ( x – size/ 2, y + size/ 2 ) ... end upper_left : POSITION Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 The client relation 14 Class METRO_STATION , having a feature upper_left : POSITION (and calls of the form upper_left . set (...) ) is a client of class POSITION Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Client and inheritance, graphically 15 Client Inheritance Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 5

  6. What clients may do 16 class METRO_STATI ON feature x, y: REAL -- Coordinates of metro station size: REAL -- Size of bounding square upper_left: POSITI ON -- Upper-left position of bounding square adjust_positions is -- Set positions of bounding square do upper_left . set ( x – size/ 2, y + size/ 2 ) ... end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 What clients may not do 17 class METRO_STATI ON feature x, y: REAL -- Coordinates of metro station size: REAL -- Size of bounding square upper_left: POSITI ON -- Upper-left position of bounding square NOT PERMITTED! adjust_positions is -- Set positions of bounding square do upper_left . x : = 3 ... end end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Use procedures: 18 upper_left . set (3, upper_left . y ) upper_left . set_x (3 ) Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 6

  7. An object has an interface 19 set x y set_x set_y Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 An object has an implementation 20 set x y set_x set_y Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Information hiding 21 set x y set_x set_y Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 7

  8. The Principle of Uniform Access 22 Features should be accessible to clients in the same way whether implemented by storage or by computation Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Uniform Access: an example 23 balance = list_of_deposits . total – list_of_withdrawals . total list_of_deposits (A1) list_of_withdrawals balance list_of_deposits (A2) list_of_withdrawals Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 The Principle of Uniform Access 24 Features should be accessible to clients in the same way whether implemented by storage or by computation Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 8

  9. Feature categories by role 25 Command Procedure No result Feature Function Computation Returns result Query Attribute Memory Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Feature categories by implementation 26 Procedure No result Routine Computation Returns result Feature Function Memory Attribute Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Feature categories 27 Command Procedure No result No result Routine Computation Returns Feature result Feature Function Computation Returns result Memory Query Attribute Memory Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 9

  10. Entities 28 An entity is a name in the program that denotes possible run-tim e values Some entities are constant Others are variable : � Attributes � Local variables Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Reference game: classes 29 class PERSON create class STATI ON feature make name: STRI NG feature name: STRI NG make (n: STRI NG) friend, enemy: PERSON -- Initialize with name n . station: STATI ON require n / = Void set_friend (f: PERSON) ensure require name = n f / = Void end ensure friend = f indexing set_enemy… description: “List items” set_station… class LI NKABLE feature item: PERSON make (n: STRI NG) right: LI ST_I TEM -- Initialize with name n . end require n / = Void ensure name = n end Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 Reference game: example program 30 create michela.make ("MICHELA") create stephanie.make ("STEPHANIE") stephanie.set_friend (michela) michela.set_enemy (michela) create persons persons.extend (michela) persons.extend (stephanie) persons.i_th (2).item.set_enemy (michela) persons.start persons.forth persons.remove Chair of Softw are Engineering I ntroduction to Programming – Lecture 7 10

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